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SYSTEM LAY-OUT 3111 Rigid tanks 3112 Rigid Metal Tanks 4113 Flexible Fuel Tanks 5114 Tank Coverings 5115 Attachments and Fittings 6116 Integral Fuel Tanks 7117 Tank Numbering 10118 Water Draining 10
SUPPLY SYSTEMS 1
12 ENGINE FUEL FEED 1121 Design Requirements of an Aircraft Fuel Feed System 1122 Engine Fuel Feed (Multi Tank and Booster Pumps) 2123 Engine Fuel Feed (Collector Tanks) 2124 Engine Fuel Feed (Collector Tanks) ErrorBookmark not defined 125 Engine Fuel Feed (Fuel Cells) 3
The following example of a system layout is for a typical large commercial twinaircraft The number of tanks and system complexity will vary from aircraft toaircraft Clearly a four-engine aircraft will have more components than a twinThe figure shows a typical fuel cell layout
Typical Fuel Cell Layout
Figure 1NOTE For additional range some operators will install centre tanks these areoffered as optional on most single isle and wide bodied aircraftFuel Tanks
Fuel tanks normally fall into three categories of construction
bull Rigid
bull Flexible
bull Integral
bull
111 RIGID TANKS
These are normally made from metal or plastic material they are fitted internallywhere space permits Flexible fuel tanks have an advantage over rigid tanksbecause they can be shaped and fitted into odd shaped spaces where rigid tankscannot be fitted In general flexible tanks are lighter and easier to handle andstore than rigid tanks Integral fuel tanks are of rigid construction because theyare part of the airframe structure They are not independent items like the othertanks
Whatever the construction method fuel tanks should be shaped so that almost all
the fuel is available to the engine Awkward pockets which prevent fuel fromleaving the tank are undesirable and are avoided if possible
Fuel tanks are made in shapes and sizes to fit the spaces available in eachparticular airframe and therefore the size and shape of the fuel tanks will not bethe same for all aircraft Metal fuel tanks are constructed from aluminium alloystainless steel or tinned steel and they are riveted welded or soldered togetherThe tank is a light structure which is strengthened by the use of internal stiffenersangle pieces and by incorporating baffles to give strength and which arenecessary in large tanks to reduce the effects of fuel surge caused when theaircraft manoeuvres Secure attachment of a rigid tank within the airframe maybe achieved by built-in padded cradles and padded metal straps The cradle isshaped to match the contours of the tank and the straps secure the tank to itscradle Each tank will have the brackets strap guides and fittings to match theaircraft structure into which the tank is to be fitted
It must be stressed that very few aircraft over 5700 kg would utilise metal rigidtanks except when long range tanks are fitted in the cargo hold ie commercialIATA LD6 containers etc
Flexible fuel tanks may be constructed with thin and very flexible walls (called bagtanks) or they may be made of thicker less flexible material These tanks aremade in shapes to fit particular spaces in the aircraft structure and their flexibilityenables the tanks to be folded and inserted through a small aperture whichwould not allow a rigid tank of similar capacity to be fitted Because flexible tankscan be made in shapes to suit most of the space available a greater fuel capacityis made available to a particular aircraft when flexible tanks are used Someaircraft fuel systems are designed to include rigid flexible and external fuel tanksso that the greatest possible fuel load is carried
The compartment for a flexible fuel tank is made as smooth as possible on the
inside and projecting joints are covered to prevent chafing the tank materialBefore a tank can be fitted the compartment must be properly cleaned out and allswarf and loose items removed
After a flexible tank has been inserted into the tank compartment the tank iscarefully unfolded and the various external fittings are aligned Usually the wallsof the more flexible tanks are attached to the compartment walls by a type ofpress-stud fitting When filled with fuel the tank expands to contact the walls ofthe tank compartment so that the weight of the fuel is carried by the aircraftstructure and not by the tank Because the load is not carried by the tank flexingof the aircraft structure does not impose harmful loads upon the tank materialFlexible fuel tanks are resilient like an inner tube and because they are resilient
the tanks can withstand a considerable amount of distortion or shock loading If aflexible tank is not completely full it is unlikely to burst on a crash impact
114 TANK COVERINGS
Protective Covering
A protective covering may be fixed to the outside of a flexible fuel tank Thecovering is not special to type and similar covering materials are used to protectdifferent types of tank The protective covering usually consists of several layersof fabric or fabric and rubber which are cemented to the material of the tank withadhesives When a tank is fitted with a protective cover it in general becomesstiff enough to support its own weight and retain its shape However when thevarious metal fittings are added the tank will sag and it needs support whenfitted
Some tanks which do not have protective covers are reinforced by nylon fabricor net This type of reinforcement does not stiffen the tank which remains veryflexible and limp This type of tank cannot support its own weight and is the typewhich is sometimes called a lsquobag tankrsquo
These coverings have been developed to reduce the magnitude of a fuel leak iffor any reason the fuel tank is pierced or ruptured The self-sealing covering isusually made from layers of cellular rubber with an overall protective cover ofglass fabric or nylon fabric on the outside This type of rubber is a material that isimmediately affected by contact with fuel If a tank leaks the cellular rubberswells on contact with the fuel and forces its way into the puncture to block thehole and reduce or stop the leak Unfortunately minor leaks may remainundiscovered for some time until the self-sealing cover begins to swell and bulgeon the outside
115 ATTACHMENTS AND FITTINGS
To complete a flexible fuel tank provision must be made for attaching fuel systemcomponents and for joining each tank into the fuel system The fuel tank isconstructed with moulded connectors and apertures of an appropriate size andposition but because of the flexible nature of the material each aperture needs tobe reinforced before a system component can be fitted Each aperture isstrengthened and stiffened by fitting a metal attachment ring The attachmentrings are sometimes called lsquostud ringsrsquo or lsquobolt ringsrsquo
An integral fuel tank is a fuel-proofed space in the aircraft structure which is filledwith fuel and provided with the appropriate fittings and connections for fuel feedfuel transfer air lines vents and fuel pumps required at that particular position inthe fuel system Connections and fittings cause few problems but sealing andfuel proofing the aircraft structure is the vital element which decides the successof the integral tank An integral wing tank is usually an area of a mainplanebetween the front and rear spars and bounded by the external skin which coversthe wing structure The tank area is sealed and fuel proofed during assemblySpecial sealants are used under controlled conditions and the skin attachmentsstructures rivets and bolts are assembled whilst the sealant is wet Dryassemblies cannot be adequately sealed afterwards
Before assembly all the structural parts that become integral fuel tanks arecleaned to a particular specification the clean parts are immediately coated witha special sealant and assembled wet It is important that the joints are finished(rivets closed or bolts tightened) before the sealant sets This first coating ofsealant is called the lsquointerfayrsquo and it should bond with all parts of the joint Afterthe joint is tightened it is necessary to remove the surplus sealant that has beensqueezed out as the joint closed After cleaning the work a neat coating ofsealant is applied at the edges of the joint this coating is called the fillet (see thefigure) and it should be strong enough to cope with any flexing between the parts A final brush-on coat of sealant is applied to overlap the joint and fillet Interfay
fillet and the brush-on coat are part of the standard treatment for sealing integralfuel tank structures and all use a similar sealant As an aid to quick productionthe joint can be covered by a barrier coating of a quicker drying substance Thebarrier-coating material is not the same as the sealant used for jointing and it willnot prevent or cure leaks The barrier-coat becomes tack-free in a relatively shorttime and it is applied over partially cured sealants to reduce the possibility ofcontamination from swarf when work must continue in the area of an uncured joint To extend the leak-free life of the integral fuel tank take great care whenhandling or working on the skin area which covers the integral fuel tank
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The following example of a system layout is for a typical large commercial twinaircraft The number of tanks and system complexity will vary from aircraft toaircraft Clearly a four-engine aircraft will have more components than a twinThe figure shows a typical fuel cell layout
Typical Fuel Cell Layout
Figure 1NOTE For additional range some operators will install centre tanks these areoffered as optional on most single isle and wide bodied aircraftFuel Tanks
Fuel tanks normally fall into three categories of construction
bull Rigid
bull Flexible
bull Integral
bull
111 RIGID TANKS
These are normally made from metal or plastic material they are fitted internallywhere space permits Flexible fuel tanks have an advantage over rigid tanksbecause they can be shaped and fitted into odd shaped spaces where rigid tankscannot be fitted In general flexible tanks are lighter and easier to handle andstore than rigid tanks Integral fuel tanks are of rigid construction because theyare part of the airframe structure They are not independent items like the othertanks
Whatever the construction method fuel tanks should be shaped so that almost all
the fuel is available to the engine Awkward pockets which prevent fuel fromleaving the tank are undesirable and are avoided if possible
Fuel tanks are made in shapes and sizes to fit the spaces available in eachparticular airframe and therefore the size and shape of the fuel tanks will not bethe same for all aircraft Metal fuel tanks are constructed from aluminium alloystainless steel or tinned steel and they are riveted welded or soldered togetherThe tank is a light structure which is strengthened by the use of internal stiffenersangle pieces and by incorporating baffles to give strength and which arenecessary in large tanks to reduce the effects of fuel surge caused when theaircraft manoeuvres Secure attachment of a rigid tank within the airframe maybe achieved by built-in padded cradles and padded metal straps The cradle isshaped to match the contours of the tank and the straps secure the tank to itscradle Each tank will have the brackets strap guides and fittings to match theaircraft structure into which the tank is to be fitted
It must be stressed that very few aircraft over 5700 kg would utilise metal rigidtanks except when long range tanks are fitted in the cargo hold ie commercialIATA LD6 containers etc
Flexible fuel tanks may be constructed with thin and very flexible walls (called bagtanks) or they may be made of thicker less flexible material These tanks aremade in shapes to fit particular spaces in the aircraft structure and their flexibilityenables the tanks to be folded and inserted through a small aperture whichwould not allow a rigid tank of similar capacity to be fitted Because flexible tankscan be made in shapes to suit most of the space available a greater fuel capacityis made available to a particular aircraft when flexible tanks are used Someaircraft fuel systems are designed to include rigid flexible and external fuel tanksso that the greatest possible fuel load is carried
The compartment for a flexible fuel tank is made as smooth as possible on the
inside and projecting joints are covered to prevent chafing the tank materialBefore a tank can be fitted the compartment must be properly cleaned out and allswarf and loose items removed
After a flexible tank has been inserted into the tank compartment the tank iscarefully unfolded and the various external fittings are aligned Usually the wallsof the more flexible tanks are attached to the compartment walls by a type ofpress-stud fitting When filled with fuel the tank expands to contact the walls ofthe tank compartment so that the weight of the fuel is carried by the aircraftstructure and not by the tank Because the load is not carried by the tank flexingof the aircraft structure does not impose harmful loads upon the tank materialFlexible fuel tanks are resilient like an inner tube and because they are resilient
the tanks can withstand a considerable amount of distortion or shock loading If aflexible tank is not completely full it is unlikely to burst on a crash impact
114 TANK COVERINGS
Protective Covering
A protective covering may be fixed to the outside of a flexible fuel tank Thecovering is not special to type and similar covering materials are used to protectdifferent types of tank The protective covering usually consists of several layersof fabric or fabric and rubber which are cemented to the material of the tank withadhesives When a tank is fitted with a protective cover it in general becomesstiff enough to support its own weight and retain its shape However when thevarious metal fittings are added the tank will sag and it needs support whenfitted
Some tanks which do not have protective covers are reinforced by nylon fabricor net This type of reinforcement does not stiffen the tank which remains veryflexible and limp This type of tank cannot support its own weight and is the typewhich is sometimes called a lsquobag tankrsquo
These coverings have been developed to reduce the magnitude of a fuel leak iffor any reason the fuel tank is pierced or ruptured The self-sealing covering isusually made from layers of cellular rubber with an overall protective cover ofglass fabric or nylon fabric on the outside This type of rubber is a material that isimmediately affected by contact with fuel If a tank leaks the cellular rubberswells on contact with the fuel and forces its way into the puncture to block thehole and reduce or stop the leak Unfortunately minor leaks may remainundiscovered for some time until the self-sealing cover begins to swell and bulgeon the outside
115 ATTACHMENTS AND FITTINGS
To complete a flexible fuel tank provision must be made for attaching fuel systemcomponents and for joining each tank into the fuel system The fuel tank isconstructed with moulded connectors and apertures of an appropriate size andposition but because of the flexible nature of the material each aperture needs tobe reinforced before a system component can be fitted Each aperture isstrengthened and stiffened by fitting a metal attachment ring The attachmentrings are sometimes called lsquostud ringsrsquo or lsquobolt ringsrsquo
An integral fuel tank is a fuel-proofed space in the aircraft structure which is filledwith fuel and provided with the appropriate fittings and connections for fuel feedfuel transfer air lines vents and fuel pumps required at that particular position inthe fuel system Connections and fittings cause few problems but sealing andfuel proofing the aircraft structure is the vital element which decides the successof the integral tank An integral wing tank is usually an area of a mainplanebetween the front and rear spars and bounded by the external skin which coversthe wing structure The tank area is sealed and fuel proofed during assemblySpecial sealants are used under controlled conditions and the skin attachmentsstructures rivets and bolts are assembled whilst the sealant is wet Dryassemblies cannot be adequately sealed afterwards
Before assembly all the structural parts that become integral fuel tanks arecleaned to a particular specification the clean parts are immediately coated witha special sealant and assembled wet It is important that the joints are finished(rivets closed or bolts tightened) before the sealant sets This first coating ofsealant is called the lsquointerfayrsquo and it should bond with all parts of the joint Afterthe joint is tightened it is necessary to remove the surplus sealant that has beensqueezed out as the joint closed After cleaning the work a neat coating ofsealant is applied at the edges of the joint this coating is called the fillet (see thefigure) and it should be strong enough to cope with any flexing between the parts A final brush-on coat of sealant is applied to overlap the joint and fillet Interfay
fillet and the brush-on coat are part of the standard treatment for sealing integralfuel tank structures and all use a similar sealant As an aid to quick productionthe joint can be covered by a barrier coating of a quicker drying substance Thebarrier-coating material is not the same as the sealant used for jointing and it willnot prevent or cure leaks The barrier-coat becomes tack-free in a relatively shorttime and it is applied over partially cured sealants to reduce the possibility ofcontamination from swarf when work must continue in the area of an uncured joint To extend the leak-free life of the integral fuel tank take great care whenhandling or working on the skin area which covers the integral fuel tank
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The following example of a system layout is for a typical large commercial twinaircraft The number of tanks and system complexity will vary from aircraft toaircraft Clearly a four-engine aircraft will have more components than a twinThe figure shows a typical fuel cell layout
Typical Fuel Cell Layout
Figure 1NOTE For additional range some operators will install centre tanks these areoffered as optional on most single isle and wide bodied aircraftFuel Tanks
Fuel tanks normally fall into three categories of construction
bull Rigid
bull Flexible
bull Integral
bull
111 RIGID TANKS
These are normally made from metal or plastic material they are fitted internallywhere space permits Flexible fuel tanks have an advantage over rigid tanksbecause they can be shaped and fitted into odd shaped spaces where rigid tankscannot be fitted In general flexible tanks are lighter and easier to handle andstore than rigid tanks Integral fuel tanks are of rigid construction because theyare part of the airframe structure They are not independent items like the othertanks
Whatever the construction method fuel tanks should be shaped so that almost all
the fuel is available to the engine Awkward pockets which prevent fuel fromleaving the tank are undesirable and are avoided if possible
Fuel tanks are made in shapes and sizes to fit the spaces available in eachparticular airframe and therefore the size and shape of the fuel tanks will not bethe same for all aircraft Metal fuel tanks are constructed from aluminium alloystainless steel or tinned steel and they are riveted welded or soldered togetherThe tank is a light structure which is strengthened by the use of internal stiffenersangle pieces and by incorporating baffles to give strength and which arenecessary in large tanks to reduce the effects of fuel surge caused when theaircraft manoeuvres Secure attachment of a rigid tank within the airframe maybe achieved by built-in padded cradles and padded metal straps The cradle isshaped to match the contours of the tank and the straps secure the tank to itscradle Each tank will have the brackets strap guides and fittings to match theaircraft structure into which the tank is to be fitted
It must be stressed that very few aircraft over 5700 kg would utilise metal rigidtanks except when long range tanks are fitted in the cargo hold ie commercialIATA LD6 containers etc
Flexible fuel tanks may be constructed with thin and very flexible walls (called bagtanks) or they may be made of thicker less flexible material These tanks aremade in shapes to fit particular spaces in the aircraft structure and their flexibilityenables the tanks to be folded and inserted through a small aperture whichwould not allow a rigid tank of similar capacity to be fitted Because flexible tankscan be made in shapes to suit most of the space available a greater fuel capacityis made available to a particular aircraft when flexible tanks are used Someaircraft fuel systems are designed to include rigid flexible and external fuel tanksso that the greatest possible fuel load is carried
The compartment for a flexible fuel tank is made as smooth as possible on the
inside and projecting joints are covered to prevent chafing the tank materialBefore a tank can be fitted the compartment must be properly cleaned out and allswarf and loose items removed
After a flexible tank has been inserted into the tank compartment the tank iscarefully unfolded and the various external fittings are aligned Usually the wallsof the more flexible tanks are attached to the compartment walls by a type ofpress-stud fitting When filled with fuel the tank expands to contact the walls ofthe tank compartment so that the weight of the fuel is carried by the aircraftstructure and not by the tank Because the load is not carried by the tank flexingof the aircraft structure does not impose harmful loads upon the tank materialFlexible fuel tanks are resilient like an inner tube and because they are resilient
the tanks can withstand a considerable amount of distortion or shock loading If aflexible tank is not completely full it is unlikely to burst on a crash impact
114 TANK COVERINGS
Protective Covering
A protective covering may be fixed to the outside of a flexible fuel tank Thecovering is not special to type and similar covering materials are used to protectdifferent types of tank The protective covering usually consists of several layersof fabric or fabric and rubber which are cemented to the material of the tank withadhesives When a tank is fitted with a protective cover it in general becomesstiff enough to support its own weight and retain its shape However when thevarious metal fittings are added the tank will sag and it needs support whenfitted
Some tanks which do not have protective covers are reinforced by nylon fabricor net This type of reinforcement does not stiffen the tank which remains veryflexible and limp This type of tank cannot support its own weight and is the typewhich is sometimes called a lsquobag tankrsquo
These coverings have been developed to reduce the magnitude of a fuel leak iffor any reason the fuel tank is pierced or ruptured The self-sealing covering isusually made from layers of cellular rubber with an overall protective cover ofglass fabric or nylon fabric on the outside This type of rubber is a material that isimmediately affected by contact with fuel If a tank leaks the cellular rubberswells on contact with the fuel and forces its way into the puncture to block thehole and reduce or stop the leak Unfortunately minor leaks may remainundiscovered for some time until the self-sealing cover begins to swell and bulgeon the outside
115 ATTACHMENTS AND FITTINGS
To complete a flexible fuel tank provision must be made for attaching fuel systemcomponents and for joining each tank into the fuel system The fuel tank isconstructed with moulded connectors and apertures of an appropriate size andposition but because of the flexible nature of the material each aperture needs tobe reinforced before a system component can be fitted Each aperture isstrengthened and stiffened by fitting a metal attachment ring The attachmentrings are sometimes called lsquostud ringsrsquo or lsquobolt ringsrsquo
An integral fuel tank is a fuel-proofed space in the aircraft structure which is filledwith fuel and provided with the appropriate fittings and connections for fuel feedfuel transfer air lines vents and fuel pumps required at that particular position inthe fuel system Connections and fittings cause few problems but sealing andfuel proofing the aircraft structure is the vital element which decides the successof the integral tank An integral wing tank is usually an area of a mainplanebetween the front and rear spars and bounded by the external skin which coversthe wing structure The tank area is sealed and fuel proofed during assemblySpecial sealants are used under controlled conditions and the skin attachmentsstructures rivets and bolts are assembled whilst the sealant is wet Dryassemblies cannot be adequately sealed afterwards
Before assembly all the structural parts that become integral fuel tanks arecleaned to a particular specification the clean parts are immediately coated witha special sealant and assembled wet It is important that the joints are finished(rivets closed or bolts tightened) before the sealant sets This first coating ofsealant is called the lsquointerfayrsquo and it should bond with all parts of the joint Afterthe joint is tightened it is necessary to remove the surplus sealant that has beensqueezed out as the joint closed After cleaning the work a neat coating ofsealant is applied at the edges of the joint this coating is called the fillet (see thefigure) and it should be strong enough to cope with any flexing between the parts A final brush-on coat of sealant is applied to overlap the joint and fillet Interfay
fillet and the brush-on coat are part of the standard treatment for sealing integralfuel tank structures and all use a similar sealant As an aid to quick productionthe joint can be covered by a barrier coating of a quicker drying substance Thebarrier-coating material is not the same as the sealant used for jointing and it willnot prevent or cure leaks The barrier-coat becomes tack-free in a relatively shorttime and it is applied over partially cured sealants to reduce the possibility ofcontamination from swarf when work must continue in the area of an uncured joint To extend the leak-free life of the integral fuel tank take great care whenhandling or working on the skin area which covers the integral fuel tank
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Fuel tanks are made in shapes and sizes to fit the spaces available in eachparticular airframe and therefore the size and shape of the fuel tanks will not bethe same for all aircraft Metal fuel tanks are constructed from aluminium alloystainless steel or tinned steel and they are riveted welded or soldered togetherThe tank is a light structure which is strengthened by the use of internal stiffenersangle pieces and by incorporating baffles to give strength and which arenecessary in large tanks to reduce the effects of fuel surge caused when theaircraft manoeuvres Secure attachment of a rigid tank within the airframe maybe achieved by built-in padded cradles and padded metal straps The cradle isshaped to match the contours of the tank and the straps secure the tank to itscradle Each tank will have the brackets strap guides and fittings to match theaircraft structure into which the tank is to be fitted
It must be stressed that very few aircraft over 5700 kg would utilise metal rigidtanks except when long range tanks are fitted in the cargo hold ie commercialIATA LD6 containers etc
Flexible fuel tanks may be constructed with thin and very flexible walls (called bagtanks) or they may be made of thicker less flexible material These tanks aremade in shapes to fit particular spaces in the aircraft structure and their flexibilityenables the tanks to be folded and inserted through a small aperture whichwould not allow a rigid tank of similar capacity to be fitted Because flexible tankscan be made in shapes to suit most of the space available a greater fuel capacityis made available to a particular aircraft when flexible tanks are used Someaircraft fuel systems are designed to include rigid flexible and external fuel tanksso that the greatest possible fuel load is carried
The compartment for a flexible fuel tank is made as smooth as possible on the
inside and projecting joints are covered to prevent chafing the tank materialBefore a tank can be fitted the compartment must be properly cleaned out and allswarf and loose items removed
After a flexible tank has been inserted into the tank compartment the tank iscarefully unfolded and the various external fittings are aligned Usually the wallsof the more flexible tanks are attached to the compartment walls by a type ofpress-stud fitting When filled with fuel the tank expands to contact the walls ofthe tank compartment so that the weight of the fuel is carried by the aircraftstructure and not by the tank Because the load is not carried by the tank flexingof the aircraft structure does not impose harmful loads upon the tank materialFlexible fuel tanks are resilient like an inner tube and because they are resilient
the tanks can withstand a considerable amount of distortion or shock loading If aflexible tank is not completely full it is unlikely to burst on a crash impact
114 TANK COVERINGS
Protective Covering
A protective covering may be fixed to the outside of a flexible fuel tank Thecovering is not special to type and similar covering materials are used to protectdifferent types of tank The protective covering usually consists of several layersof fabric or fabric and rubber which are cemented to the material of the tank withadhesives When a tank is fitted with a protective cover it in general becomesstiff enough to support its own weight and retain its shape However when thevarious metal fittings are added the tank will sag and it needs support whenfitted
Some tanks which do not have protective covers are reinforced by nylon fabricor net This type of reinforcement does not stiffen the tank which remains veryflexible and limp This type of tank cannot support its own weight and is the typewhich is sometimes called a lsquobag tankrsquo
These coverings have been developed to reduce the magnitude of a fuel leak iffor any reason the fuel tank is pierced or ruptured The self-sealing covering isusually made from layers of cellular rubber with an overall protective cover ofglass fabric or nylon fabric on the outside This type of rubber is a material that isimmediately affected by contact with fuel If a tank leaks the cellular rubberswells on contact with the fuel and forces its way into the puncture to block thehole and reduce or stop the leak Unfortunately minor leaks may remainundiscovered for some time until the self-sealing cover begins to swell and bulgeon the outside
115 ATTACHMENTS AND FITTINGS
To complete a flexible fuel tank provision must be made for attaching fuel systemcomponents and for joining each tank into the fuel system The fuel tank isconstructed with moulded connectors and apertures of an appropriate size andposition but because of the flexible nature of the material each aperture needs tobe reinforced before a system component can be fitted Each aperture isstrengthened and stiffened by fitting a metal attachment ring The attachmentrings are sometimes called lsquostud ringsrsquo or lsquobolt ringsrsquo
An integral fuel tank is a fuel-proofed space in the aircraft structure which is filledwith fuel and provided with the appropriate fittings and connections for fuel feedfuel transfer air lines vents and fuel pumps required at that particular position inthe fuel system Connections and fittings cause few problems but sealing andfuel proofing the aircraft structure is the vital element which decides the successof the integral tank An integral wing tank is usually an area of a mainplanebetween the front and rear spars and bounded by the external skin which coversthe wing structure The tank area is sealed and fuel proofed during assemblySpecial sealants are used under controlled conditions and the skin attachmentsstructures rivets and bolts are assembled whilst the sealant is wet Dryassemblies cannot be adequately sealed afterwards
Before assembly all the structural parts that become integral fuel tanks arecleaned to a particular specification the clean parts are immediately coated witha special sealant and assembled wet It is important that the joints are finished(rivets closed or bolts tightened) before the sealant sets This first coating ofsealant is called the lsquointerfayrsquo and it should bond with all parts of the joint Afterthe joint is tightened it is necessary to remove the surplus sealant that has beensqueezed out as the joint closed After cleaning the work a neat coating ofsealant is applied at the edges of the joint this coating is called the fillet (see thefigure) and it should be strong enough to cope with any flexing between the parts A final brush-on coat of sealant is applied to overlap the joint and fillet Interfay
fillet and the brush-on coat are part of the standard treatment for sealing integralfuel tank structures and all use a similar sealant As an aid to quick productionthe joint can be covered by a barrier coating of a quicker drying substance Thebarrier-coating material is not the same as the sealant used for jointing and it willnot prevent or cure leaks The barrier-coat becomes tack-free in a relatively shorttime and it is applied over partially cured sealants to reduce the possibility ofcontamination from swarf when work must continue in the area of an uncured joint To extend the leak-free life of the integral fuel tank take great care whenhandling or working on the skin area which covers the integral fuel tank
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Flexible fuel tanks may be constructed with thin and very flexible walls (called bagtanks) or they may be made of thicker less flexible material These tanks aremade in shapes to fit particular spaces in the aircraft structure and their flexibilityenables the tanks to be folded and inserted through a small aperture whichwould not allow a rigid tank of similar capacity to be fitted Because flexible tankscan be made in shapes to suit most of the space available a greater fuel capacityis made available to a particular aircraft when flexible tanks are used Someaircraft fuel systems are designed to include rigid flexible and external fuel tanksso that the greatest possible fuel load is carried
The compartment for a flexible fuel tank is made as smooth as possible on the
inside and projecting joints are covered to prevent chafing the tank materialBefore a tank can be fitted the compartment must be properly cleaned out and allswarf and loose items removed
After a flexible tank has been inserted into the tank compartment the tank iscarefully unfolded and the various external fittings are aligned Usually the wallsof the more flexible tanks are attached to the compartment walls by a type ofpress-stud fitting When filled with fuel the tank expands to contact the walls ofthe tank compartment so that the weight of the fuel is carried by the aircraftstructure and not by the tank Because the load is not carried by the tank flexingof the aircraft structure does not impose harmful loads upon the tank materialFlexible fuel tanks are resilient like an inner tube and because they are resilient
the tanks can withstand a considerable amount of distortion or shock loading If aflexible tank is not completely full it is unlikely to burst on a crash impact
114 TANK COVERINGS
Protective Covering
A protective covering may be fixed to the outside of a flexible fuel tank Thecovering is not special to type and similar covering materials are used to protectdifferent types of tank The protective covering usually consists of several layersof fabric or fabric and rubber which are cemented to the material of the tank withadhesives When a tank is fitted with a protective cover it in general becomesstiff enough to support its own weight and retain its shape However when thevarious metal fittings are added the tank will sag and it needs support whenfitted
Some tanks which do not have protective covers are reinforced by nylon fabricor net This type of reinforcement does not stiffen the tank which remains veryflexible and limp This type of tank cannot support its own weight and is the typewhich is sometimes called a lsquobag tankrsquo
These coverings have been developed to reduce the magnitude of a fuel leak iffor any reason the fuel tank is pierced or ruptured The self-sealing covering isusually made from layers of cellular rubber with an overall protective cover ofglass fabric or nylon fabric on the outside This type of rubber is a material that isimmediately affected by contact with fuel If a tank leaks the cellular rubberswells on contact with the fuel and forces its way into the puncture to block thehole and reduce or stop the leak Unfortunately minor leaks may remainundiscovered for some time until the self-sealing cover begins to swell and bulgeon the outside
115 ATTACHMENTS AND FITTINGS
To complete a flexible fuel tank provision must be made for attaching fuel systemcomponents and for joining each tank into the fuel system The fuel tank isconstructed with moulded connectors and apertures of an appropriate size andposition but because of the flexible nature of the material each aperture needs tobe reinforced before a system component can be fitted Each aperture isstrengthened and stiffened by fitting a metal attachment ring The attachmentrings are sometimes called lsquostud ringsrsquo or lsquobolt ringsrsquo
An integral fuel tank is a fuel-proofed space in the aircraft structure which is filledwith fuel and provided with the appropriate fittings and connections for fuel feedfuel transfer air lines vents and fuel pumps required at that particular position inthe fuel system Connections and fittings cause few problems but sealing andfuel proofing the aircraft structure is the vital element which decides the successof the integral tank An integral wing tank is usually an area of a mainplanebetween the front and rear spars and bounded by the external skin which coversthe wing structure The tank area is sealed and fuel proofed during assemblySpecial sealants are used under controlled conditions and the skin attachmentsstructures rivets and bolts are assembled whilst the sealant is wet Dryassemblies cannot be adequately sealed afterwards
Before assembly all the structural parts that become integral fuel tanks arecleaned to a particular specification the clean parts are immediately coated witha special sealant and assembled wet It is important that the joints are finished(rivets closed or bolts tightened) before the sealant sets This first coating ofsealant is called the lsquointerfayrsquo and it should bond with all parts of the joint Afterthe joint is tightened it is necessary to remove the surplus sealant that has beensqueezed out as the joint closed After cleaning the work a neat coating ofsealant is applied at the edges of the joint this coating is called the fillet (see thefigure) and it should be strong enough to cope with any flexing between the parts A final brush-on coat of sealant is applied to overlap the joint and fillet Interfay
fillet and the brush-on coat are part of the standard treatment for sealing integralfuel tank structures and all use a similar sealant As an aid to quick productionthe joint can be covered by a barrier coating of a quicker drying substance Thebarrier-coating material is not the same as the sealant used for jointing and it willnot prevent or cure leaks The barrier-coat becomes tack-free in a relatively shorttime and it is applied over partially cured sealants to reduce the possibility ofcontamination from swarf when work must continue in the area of an uncured joint To extend the leak-free life of the integral fuel tank take great care whenhandling or working on the skin area which covers the integral fuel tank
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
These coverings have been developed to reduce the magnitude of a fuel leak iffor any reason the fuel tank is pierced or ruptured The self-sealing covering isusually made from layers of cellular rubber with an overall protective cover ofglass fabric or nylon fabric on the outside This type of rubber is a material that isimmediately affected by contact with fuel If a tank leaks the cellular rubberswells on contact with the fuel and forces its way into the puncture to block thehole and reduce or stop the leak Unfortunately minor leaks may remainundiscovered for some time until the self-sealing cover begins to swell and bulgeon the outside
115 ATTACHMENTS AND FITTINGS
To complete a flexible fuel tank provision must be made for attaching fuel systemcomponents and for joining each tank into the fuel system The fuel tank isconstructed with moulded connectors and apertures of an appropriate size andposition but because of the flexible nature of the material each aperture needs tobe reinforced before a system component can be fitted Each aperture isstrengthened and stiffened by fitting a metal attachment ring The attachmentrings are sometimes called lsquostud ringsrsquo or lsquobolt ringsrsquo
An integral fuel tank is a fuel-proofed space in the aircraft structure which is filledwith fuel and provided with the appropriate fittings and connections for fuel feedfuel transfer air lines vents and fuel pumps required at that particular position inthe fuel system Connections and fittings cause few problems but sealing andfuel proofing the aircraft structure is the vital element which decides the successof the integral tank An integral wing tank is usually an area of a mainplanebetween the front and rear spars and bounded by the external skin which coversthe wing structure The tank area is sealed and fuel proofed during assemblySpecial sealants are used under controlled conditions and the skin attachmentsstructures rivets and bolts are assembled whilst the sealant is wet Dryassemblies cannot be adequately sealed afterwards
Before assembly all the structural parts that become integral fuel tanks arecleaned to a particular specification the clean parts are immediately coated witha special sealant and assembled wet It is important that the joints are finished(rivets closed or bolts tightened) before the sealant sets This first coating ofsealant is called the lsquointerfayrsquo and it should bond with all parts of the joint Afterthe joint is tightened it is necessary to remove the surplus sealant that has beensqueezed out as the joint closed After cleaning the work a neat coating ofsealant is applied at the edges of the joint this coating is called the fillet (see thefigure) and it should be strong enough to cope with any flexing between the parts A final brush-on coat of sealant is applied to overlap the joint and fillet Interfay
fillet and the brush-on coat are part of the standard treatment for sealing integralfuel tank structures and all use a similar sealant As an aid to quick productionthe joint can be covered by a barrier coating of a quicker drying substance Thebarrier-coating material is not the same as the sealant used for jointing and it willnot prevent or cure leaks The barrier-coat becomes tack-free in a relatively shorttime and it is applied over partially cured sealants to reduce the possibility ofcontamination from swarf when work must continue in the area of an uncured joint To extend the leak-free life of the integral fuel tank take great care whenhandling or working on the skin area which covers the integral fuel tank
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
An integral fuel tank is a fuel-proofed space in the aircraft structure which is filledwith fuel and provided with the appropriate fittings and connections for fuel feedfuel transfer air lines vents and fuel pumps required at that particular position inthe fuel system Connections and fittings cause few problems but sealing andfuel proofing the aircraft structure is the vital element which decides the successof the integral tank An integral wing tank is usually an area of a mainplanebetween the front and rear spars and bounded by the external skin which coversthe wing structure The tank area is sealed and fuel proofed during assemblySpecial sealants are used under controlled conditions and the skin attachmentsstructures rivets and bolts are assembled whilst the sealant is wet Dryassemblies cannot be adequately sealed afterwards
Before assembly all the structural parts that become integral fuel tanks arecleaned to a particular specification the clean parts are immediately coated witha special sealant and assembled wet It is important that the joints are finished(rivets closed or bolts tightened) before the sealant sets This first coating ofsealant is called the lsquointerfayrsquo and it should bond with all parts of the joint Afterthe joint is tightened it is necessary to remove the surplus sealant that has beensqueezed out as the joint closed After cleaning the work a neat coating ofsealant is applied at the edges of the joint this coating is called the fillet (see thefigure) and it should be strong enough to cope with any flexing between the parts A final brush-on coat of sealant is applied to overlap the joint and fillet Interfay
fillet and the brush-on coat are part of the standard treatment for sealing integralfuel tank structures and all use a similar sealant As an aid to quick productionthe joint can be covered by a barrier coating of a quicker drying substance Thebarrier-coating material is not the same as the sealant used for jointing and it willnot prevent or cure leaks The barrier-coat becomes tack-free in a relatively shorttime and it is applied over partially cured sealants to reduce the possibility ofcontamination from swarf when work must continue in the area of an uncured joint To extend the leak-free life of the integral fuel tank take great care whenhandling or working on the skin area which covers the integral fuel tank
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Generally speaking large commercial aircraft have three tanks in each winginner fuel tank outer fuel tank and a surge tank On some aircraft the fuel tanksare referred to as fuel cells A centre tank is sometimes available as a standardoptionEach fuel tank has additional space for 2 expansion of the fuel without spillage
into the surge tank Removable access panels are provided in the lower wingsurface The centre tank if fitted is accessible through manholes in the rearspan
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Aircraft manufacturers number fuel tanks in which case the philosophy will befrom left to right nose to tail
Fuel Tank Layout
Figure 7
118 WATER DRAINING
Water drain valves are provided at low points of each tank All valves may beopened with standard tools and the outer seal of the valve is replaceable without
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
A typical system has a water scavenge system fitted in the optional centre tank
Two jet pumps using tappings on the tank pumps for motive power collect waterfrom low points and discharge it towards the fuel pump inlet Removable accesspanels are provided in the lower wing surface The optional centre tank isaccessible through two manholes in the rear spar
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
121 DESIGN REQUIREMENTS OF AN AIRCRAFT FUEL FEED SYSTEM
On an aircraft a fuel system should be designed to comply with manyrequirements as laid down in Joint Airworthiness Requirements An example ofthese requirements is as follows
1 Each fuel system should be constructed and arranged to ensure a flow of fuelat a rate and pressure to ensure proper functioning of the engine for each
likely operating condition2 The fuel system must allow the supply of fuel to each engine through a
system independent of the system supplying fuel to any other engine
3 The system design should be such that it is not possible for any pump to drawfuel from two or more tank simultaneously unless means are provided toprevent the introduction of air into the system
4 If fuel can be pumped from one tank to another in flight the fuel tank ventsand transfer system must be designed so that no structural failure can occurbecause of over-filling
5 Integral tanks must have facilities for interior inspection and repair
6 Fuel tanks must be designed located and installed so that no fuel is releasedin or near the engines in sufficient quantities to start a fire in otherwisesurvivable crash conditions
7 Pressure cross-feed lines passing through crew passenger or cargocompartments shall either be enclosed in a fuel and vapour proof enclosureventilated and drained to the outside OR consist of a pipe without fittings androuted or protected against accidental damage
8 The system shall incorporate means to prevent the collection of water and dirtor the deposition of ice or other substances from satisfactory functioning ofthe system
9 Lines which can be isolated from the system by means of valves or fuelcocks shall incorporate provision for the relief of excess pressure due toexpansion of the fuel
10 Each fuel tank filler connection must be marked with type of fuel and beprovided with a bonding point and drain discharging excess fuel
11 There must be a fuel strainer at each fuel tank outlet or for the boosterpump(s)
12 Each fuel line must be designed installed and supported to prevent excessivevibration and allow a reasonable degree of deformation and stretching withoutleakage
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Another multi tank system is the use of fire cells In normal conditions eachengine is supplied from one pump in the optional centre tank or both pumps in thetank of its own wing Any one pump can supply the maximum demand of oneengine A cross-feed pipe controlled by a double motor actuated spherical plugvalve allows both engines to be fed from one side or all the fuel to be used byone engine The valve is mounted on the rear spar in the centre section
Two plug-in ac driven booster pumps supplied from different busbars are fittedin each tank Each pump has a suction inlet On each side the two pumps in thewing tank and one pump in the centre tank (when fitted) deliver fuel via a built innon-return valve into a single pipe The pumps in the wing tanks are fitted with
pressure relief sequence valves that ensure that when all pumps are running thecentre tank pumps will deliver fuel preferentially No sequence valves areprovided on a two tank version aircraft
In each wing tank the pumps are located in a collector box The box is fed bygravity through flap non-return valves This ensures that the system can continueto supply fuel under negative lsquogrsquo or transient manoeuvres A bypass is providedat the pumps to permit gravity feed
Air release valves are fitted to the feed lines
The supply of fuel to each engine can be shut off by an engine LP valve mountedon the front spar This is a spherical plug valve driven by a double motor
actuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are routed separately
Controls and indications for pumps and crossfeed valves of the feed system arelocated on the overhead panel In normal operation all wing pumps will remainon throughout the flight If a centre tank is fitted switching of pumps is automaticIf there are no malfunctions no action is required during flight
The engine LP valves are controlled by operating the engine fire handles
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Pumps employed in aircraft fuel systems differ in size shape output etcHowever regardless of type and any special features they may have they alloperate on the same principle and consist of very similar components
Each tank is normally provided with two fuel pumps They are all identical andinterchangeable These pumps are installed in the canister assemblies to enablereplacement without de-fuelling the tank
The fuel pumps are centrifugal pumps driven by 115 volts three phase motors
The output of each pump is about 250-300 litres per minute Maximum fuelpressure at zero flow is about 38 psi
Each pump includes a non-return and a by-pass valve
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
These are another method of transferring fuel around an aircraft fuel systemThey use fuel bled from the booster pump which is continually fed through thecentral nozzle into a venturi The depression created in the venturi draws fuelfrom the surrounding tank in through the filter then up through the venturi tubeand either into the next fuel tank or straight to the collector box
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Sequence valves are fitted to give an automatic transfer from one tank to anotherthe following example is for an aircraft with pumps in the centre tank inner tankand outer tank
The valve limits the fuel pressure of the outer tank pumps from 38 psi to 175 psiThis is to give priority to the inner tank fuel pumps for structural reasons
When the inner tanks are empty the engines will be automatically supplied fromthe outer tanks So the outer fuel pumps run continuously
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The example at figure 13 shows the fuel tank split into two cells at rib 15 Toenable transfer to take place two transfer valves are fitted in this instance at rib15 Operation of these valves is actuated by a signal from low level sensorsshown just inboard of rib 2
135 LP VALVE
LP ValveFigure 19
The LP shut off valve enables isolation of the fuel system in the event of fire andengine maintenance ie engine removal Located at the top of the pylon on theoutside of the front wing spar it will be controlled normally be operation of the firehandles and activated by either a pair of electric motors or mechanically asshown above
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The cross feed valve enables fuel to be fed to any engine from any tankNormally of a spherical type construction with two 28 VDC electric motorsmounted on a differential gearbox One motor only will drive the valve at anytime the other motor is a back up The cross feed valve would normally be fitted
on the rear spar as shown in the figure
14 APU FUEL FEED
The feed to the APU is taken from the left engine feed but may be taken from theright engine feed when the cross feed valve is open
The tank booster pumps can supply fuel to the APU at the required pressure Forstarting the APU without electrical power available for the tank pumps a separatepump is provided that can be operated from the aircraft batteries and is mountedin the feed line on the rear spar of the centre section
The supply of fuel to the APU can be shut off by a valve mounted on the rear sparof the centre section It is a spherical plug valve driven by a double motoractuator To provide the maximum integrity the two actuators are supplied fromdifferent busbars and the cables are located in separated routes
The feed pipe emerges from the top of the tank and passes through thepressurised fuselage in a drained and vented shroud that extends to the APU firewall
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Fuel jettison systems are fitted to a number of large commercial aircraft to allowthe jettisoning of fuel in an emergency thus reducing weight so as to preventstructural damage when landing
Fuel jettison systems are often fitted after the installation of a centre tankbecause of the extra fuel weight
The system illustrated is from a wide-bodied twin fitted with multi tanks andbooster pumps The jettison pipe is branched off the feed pipe between the inner
tank fuel pump and the inner tank shut off valve
A check valve is installed to separate the outer tanks during jettisoning Thefunction of this check valve is to prevent the dumping of the outer tanks fuel The jettison pipe runs inside the wing tanks through the ribs into the outer tankswhere the jettison valves are installed These valves are fitted to the bottom ofthe tank
Jettison SystemFigure 22
Because of electrical emergency situations the valve will be driven by two 28VDC electric motors The motors are mounted from the outside and are attachedto the bottom of the tank through a gearbox and in many instances are
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The outlet of the jettison pipe is normally at the end of the flap track fairing andfitted with an anti corona device to avoid vaporisation of the fuel A normaltransfer rate will be in the region of 30-350 litres per minute
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The jettison operation is controlled from a jettison panel located either on a flightengineers station or from an overhead panel on a two crew configuration
Normally the panel is protected by a quick release cover
In the following example two switches are provided to operate the jettison valve
i A primary switch for motor number one
ii A guarded secondary switch for motor number two
Fuel Jettison Control
Figure 25
The position of the right and left-hand jettison valve is monitored by two magneticindicators showing green cross-line when the valve is closed and in-line whenthe valve is open As is common with this type of indicator it will show ambercross-line to indicate transit or malfunction
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
An air vent is fitted to the top of each tank to allow free flow of air in and out of thetank as the fuel level rises and falls This is known as inwards and outwardsventing and is required to prevent over pressurisation of the tanks as the fuellevel rises and depressurisation as it falls
162 VENTING DUE TO HEAT
Another important aspect of an aircraft vent sub-system is that it must be able tocope automatically with any expansion andor contraction of the fuel As the fuelexpands due to heat the vent must allow air and sometimes fuel to escape toatmosphere via vent pipes Conversely the sub-system must allow air into thetanks during contraction of the fuel when the outside air temperature (OAT) isdecreasing
163 UNPRESSURISED SYSTEM VENTING
This is a very simple method of venting tanks which requires only that fuel tankvent orifices be connected to a vent pipe gallery which leads to atmosphere
directly Venting of this type is found mainly in small aircraft some helicoptersand aircraft with low flight ceilings
The disadvantages of ldquoopen orificerdquo or ldquoopen ventedrdquo tanks are that they aresubject to fuel venting during manoeuvres they limit the maximum ceiling of theaircraft due to the fact that fuel boils at the low ambient atmospheric pressurefound at altitude danger of cavitation in fuel supply lines if fuel should boilincreased rate of evaporation (REID VAPOUR PRESSURE) leading to a greaterfire risk
REID VAPOUR PRESSURE (RVP) ndash the rate at which fuel gives off vapour
Obviously there are many inherent problems with the open vented system It is
for many reasons that most aircraft fuel systems are pressurised
164 PRESSURISED FUEL TANKS
On most large aircraft the fuel tanks are vented through a pipe connected to thesurge vent tank The vent pipes are sized to prevent tank overpressure in theevent of a refuel cut off failure
In the example shown the centre tank vent pipe is connected to the left-handsurge vent tank
The inner and outer tank pipes are connected to the relevant side surge vent
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The centre tank vent pipe ends inside the surge vent tank at the top The innerand outer vent pipes end about 3 centimetres above the bottom of the surge venttank
These ends are arranged so that any fuel overflowing into the surge vent tank isdrawn back into the wing tanks by suction as long as one or more fuel pumps arerunning On some aircraft fuel pumps are fitted to pump the fuel back to thetanks from the surge tank and will be activated by a float switch
Each vent tank is vented to atmosphere via the NACA valve
This valve ensures tank pressurisation during flight and allows the fuel to flow outin the event of a high level cut-off failure during refuelling
On some aircraft a frangeable disc is fitted in the surge tank to prevent structuraldamage caused by over pressure A flame arrester is also fitted in the NACAintake in case of ground fires
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The use of the centre wing box as a tank on some aircraft has made it necessaryto protect this area against leaking fuel A vapour seal is installed around theforward and lower part of the tank The space between the tank sink and thevapour seal is ventilated with air coming from the air conditioning system The airis directed to the outside through several small outlets If the tank has a fuel leakthe vent air line will collect this fuel and drain it through these outlets
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Cross-feed valves and transfer valves enable the transfer of fuel from one tank toanother or from any fuel tank to any engine Some transfer valves will have anautomatic function others will require more crew input
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Providing the mode selector is set to ldquoAUTOrdquo the transfer valves (fuel cells) willautomatically function The valves will be signalled to open by means of low levelsensors in the inner cell Both valves will open when either tank quantity falls tothe appropriate level Once open they will be latched open The valves areautomatically closed at the next refuel operation The ECAMEFIS systems page(Fuel) will display valve operation there is no direct control from the overheadpanel
The lsquoXrsquo feed valves would normally be closed in flight To open the valve thepush button switch on the overhead panel would have to be pushed on lsquoOPENrsquo
A light indicates position
18 THREE MAN CREW
Older aircraft will have a fuel control panel on the flight engineerrsquos panel Thisexample is taken from a wide-bodied twin No in flight transfer from tank to tankis possible but fuel can be fed from any tank to any engine The valves in thisinstance are not called ldquoTRANSFER VALVESrdquo but ldquoTANK SHUT OFF VALVESrdquo
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Each centre tank shut-off valve is controlled by a three-position selector ON AUTO and SHUT
The normal position is AUTO In this position the valves are open when the tankis full and automatically shut when the tank is empty
This is indicated by magnetic indicators showing-
bull Green in-line when the valves are open
bull Green cross-line when the valves are shut
bull Amber during the transit or when there is a failure
The inner tank and outer tank shut off valves are each controlled by a rotaryselector
They are marked by an engraved line to show the selected position
In-line for open
Cross-line for closed
Inside of the rotary selector are disagreement lights They illuminate during thetransit of the valve The light extinguishes when the valve has reached theselected position but it will remain on when the valve has a different position tothe switch
19 CROSS FEED
There is a fifth rotary selector situated just between the other four
This is the selector for the cross-feed valve
As this valve is provided with two electrical motors the selector has two differentengravings marked I and II
In-line lsquoIrsquo means the valve is opened by motor lsquoIrsquo and in-line lsquoIIrsquo means that the
valve is opened by motor lsquoIIrsquo Cross-line means the valve is closed The knob alsoincludes a disagreement light The function is identical to the lights of the otherselectors
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The type of indicators and warnings will be dependent on the technology level of theaircraft (analogue or glass cockpit) The first example is the indication and warningsfor a wide-bodied twin (analogue) with a three-man crew
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
A modern aircraft will use thermistors to send signals through amplifiers toactuate warnings sequencing etc Older aircraft may use float switches asshown in the following diagram
Low Level SensingFigure 37
Float operated switches are of a magnetic type similar to the one shown aboveand are designed to isolate the electrical mechanism from the fuel tank for safetyreasons Upward movement of the float brings the armature closer to the magnetand at a predetermined fuel level it has sufficient influence to attract the magnetwhich results in operation of the micro switch As the fuel level and the float fallthe attraction of the armature is eventually overcome by the combined forces ofthe counterweight and the micro switch spring and the counterweight fallschanging the micro switch circuit
Whether they are float switches or thermistors their functions are as follows
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
4 Under full level sensing5 Level sensing for calibration (Fuel Trim only)
1101 HIGH LEVEL SENSING
High level sensing is installed to prevent an overfilling of the fuel tanks When thefuel washes around the respective sensor the
bull associate refueldefuel valve closes
bull blue FULL light on the fuelling panel comes on
The high level signal from the inner and outer tanks could be used for
computation purposes in the fuel quantity computer when refuelling in AUTOMODE
1102 OVERFLOW SENSING
If during refuelling the high level shut off system fails fuel enters the adjacentvent tank and washes around the overflow sensor This is indicated by the amberFULL light on the refuel panel
1103 LOW LEVEL SENSING
Low level sensing is divided into
bull outer tank low level and
bull innercentre tank low level sensing
If the outer tank LO LVL sensor is exposed to air the associated amber LO LVLlight comes on
The innercentre tank low level sensing have only in the AUTO MODE a function(ref fuel pump control)
1104 CALIBRATION SENSING (FUEL TRIM ONLY)
Calibration sensors are installed in centre tanks inner tanks and trim tank Theygive a signal at a predetermined filling level in the trim tank for accuracy test ofthe fuel quantity indication during refuelling For the trim tank the calibrationsensor switching level is corrected by the stabiliser position
1105 UNDER FULL LEVEL SENSING
When the fuel quantity drops in either outer tank below a certain level themaximum flight speed (VMO) becomes reduced in order to protect the wingstructure The sensor signals are sent to the ADC (Air Data Computer)
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
111 FUEL QUANTITY SYSTEM MEASUREMENT AND INDICATION
The system has the following tasks
1 Measuring of the fuel quantity in the tanks
2 Indicating of the fuel quantity on
bull The fuel quantity indicator
bull The pre-selector
bull The ECAM system fuel page
bull ECAMEFIS
3 Controlling of automatic refuelling
4 Fuel quantity messaging to the flight management computer
The system comprises
1 fuel quantity computer
2 capacitance probes
3 capacitance index compensator
4 cadensicon sensor
5 attitude sensor
6 THS position detector
7 associated indicator in the flight compartment
112 PRINCIPLE OF CAPACITANCE GAUGING
A capacitor is an electrical device which stores electrical charge The amount ofcharge it can hold depends upon three physical properties of the capacitor itselfnamely
a The surface area of the plates
b The size of the gap between the plates
c The insulating material (dielectric) between the plates
In a fuel tank ldquocapacitor stackrdquo two of the above are fixed ie the area of theplates and the gap between them The only variable is the dielectric which in afuel tank is either fuel or air or both The amount of charge held in the capacitorwhen the tank is full will be of a preset value As the fuel level falls the dielectricwill gradually change to air and the amount of charge stored will reduce Thischange in capacitance is sensed by a signal conditioner and the change in fuel
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Each tank has installed a group of probes arranged so that a minimum of oneprobe is immersed at all times the number of probes will vary from aircraft toaircraft The following example is from a wide-bodied twin fitted with a fuel trimsystem
The number of probes is
bull 6 in each outer tank
bull 6 in each inner tank
bull 4 in the centre tank
The probes of each group are wired in parallel and connected to a summingadapter located on the wing rear spar The probe level signals are sent to thefuel quantity computer
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The signals from the capacitance probes in each tank are sent via adapters to thefuel quantity computer The computer calculates the fuel quantity To increasethe measuring accuracy further signals enter the computation
bull capacitance index compensator balances different fuel types
bull Condensicon sensor
Senses while refuelling the
Density
Dielectric constant of running fuel
bull Attitude Sensor
Senses on ground and in flight the attitude of the aircraft to the
Roll axis (longitudinal)
Pitch axis (lateral)
The attitude signal computation depends on the AIRGRND signal (wingbending direction)
bull THS Position Detector
Senses the THS position steady and gives its signal to the fuel quantitycomputer for correction of trim tank fuel measurement
Fuel Quantity Indicator
The fuel quantity of the tanks is normally displayed in 10 kg steps Power supplyand the indication signals are delivered by the fuel quantity computer To avoidtransmission errors the indicator sends feedback signals to the computer Theindicator is also used for test purposes In the test mode the indicator displays
different number codes
The examples shown are from an aircraft with a two-man crew The refuellingsystem will be looked at later The aircraft is a twin with a centre tank an innerand an outer tank
Note The LO LVL lights in the indicator receive their signals from the outer tankLO LVL sensing circuit
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
As you will be aware as any liquid flows through a pipeline it will produce Static Electricity If this static electricity were allowed to discharge in the presence ofaviation fuel vapour an explosion would result with possible catastrophic resultsTo therefore minimise the explosion risks the following guidelines must befollowed
Refuelling a small aircraft is no more complex than filling the family car Onelimitation is that on some aircraft it is not possible to fly the aircraft with all theseats occupied with full baggage allowance when the tanks are full This meansthat if the aircraft is to be flown fully loaded it may be necessary to re-fuel to lessthan full to keep the aircraft within its weight limits
As the aircraft become more complex the refuelling exercise has to be carriedout with more care If the aircraft is small but has say two tanks in each wingand the fuel load is to be three quarters full then it may be the rule for that aircraftthat the inner tanks have to be filled to the top first and the remainder put into the
outer tanks This puts less bending load on to the wing spars
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
When we get to larger aircraft there are several further problems to consider Notonly must the aircraft be filled laterally in the correct order but if the aircraft hasthe fin tailplane and rear fuselage tanks mentioned earlier it must be refuelled inthe correct order longitudinally as well to ensure the aircraft stability ismaintained
Modern large aircraft utilise pressure refuelling which has replaced open line refuelling on most aircraft with high fuel capacities The time taken to fill a Boeing747 through a normal hose and nozzle system would take hours With pressurerefuelling a large diameter hose is rigidly connected to a coupling in the aircraft
and fuel under pressure of about 40 psi is pumped into the aircraft tanks Toassist this operation most aircraft can have the total fuel load pre-set at the pointof connection so that the aircraft stops the refuelling at the correct time Theillustrations show the location and layout of a typical Boeing 777 refuellingpanel
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Fuel flows from the refuel adapters into the refueljettison manifold When therefuel valves open fuel flows from the manifold into the fuel tanks A flow tube atthe end of each refuel valve decreases the exit force of the fuel The flow tubealso puts the fuel in different parts of the tank
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
As each tank reaches full the high level sensor signals the refuel valve to close
to stop fuel flow When all refuel flow ceases fuel that is left in the refueljettisonmanifold goes through the manifold drain valves and into the main tanks Themanifold has two vacuum relief valves These valves permit air into the manifoldwhen the fuel leaves via the manifold drain valves
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
If a refuel system failure prevents the refuel valves from closing fuel goes into thesurge tanks If the fuel gets to the level of the surge tank float switches the switchcloses and all refuel valves are closed
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
Defuelling a pressure type fuel system is almost the reverse of the refuellingprocedure A de-fuel bowser would be connected to the single fuel point couplingand using a combination of both the bowserrsquos suction pump and the aircraftrsquos ownfuel supply booster pumps selected tanks can have their contents returned to thebowser
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
The weight of the fuel is a large percentage of an aircraftrsquos total weight and thebalance of the aircraft in flight changes as the fuel is used These conditions addto the complexity of the design of an aircraft fuel system In small aircraft the fueltank or tanks are located near the centre of gravity so the balance changes verylittle as the fuel is used In large aircraft fuel tanks are installed in every availablelocation and fuel valves allow the flight engineer to keep the aircraft balanced byscheduling the use of the fuel from the various tanks High performance military jets and more modern civil aircraft will use a fully automatic fuel schedulingsystem to reduce the workload on the flight crew
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks
In supersonic flight the aerodynamic centre of pressure moves aft thus changing
the longitudinal stability This is compensated in the Concord by moving thecentre of gravity by shifting fuel as necessary between the fuel tanks in the finand the wings as shown in the previous diagram the front and rear tanks areTrim tanks and the centre section contains the main tanks