-
B1 Mod 11.08.doc Issue No * Page 1
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
CONTENTS
1 FIRE
PROTECTION......................................................................
1-1
1.1 FIRE DETECTION AND WARNING
.................................................... 1-1 1.1.1
Thermal Switch Detectors
............................................. 1-1 1.1.2 Continuous
Loop (Fire Wire) Detectors ......................... 1-2 1.1.3 Dual
Loop System
......................................................... 1-4 1.1.4
Pressure-Type Sensor
.................................................. 1-4 1.1.5
Thermocouple System
.................................................. 1-4
1.2 FIRE ZONES
.................................................................................
1-5 1.2.1 Hot And Cool Zones
...................................................... 1-6 1.2.2
Fireproof Bulkheads
...................................................... 1-6 1.2.3
Engine Fire Prevention
.................................................. 1-6 1.2.4
Cockpit and Cabin Interiors.
.......................................... 1-7
1.3 SMOKE DETECTION
......................................................................
1-7 1.3.1 Carbon Monoxide Detectors
.......................................... 1-7 1.3.2 Photoelectric
Smoke Detectors. .................................... 1-8 1.3.3
Ionization Type Smoke Detector. ..................................
1-8
1.4 FIRE EXTINGUISHING
....................................................................
1-9 1.4.1 Extinguishing System
.................................................... 1-10 1.4.2
Directional Flow Control Valves (2 Way Valves) ............ 1-12
1.4.3 Fire Extinguishant Container
......................................... 1-12 1.4.4 Toilet
Compartment Systems ........................................ 1-13
1.4.5 Warnings And Indications
.............................................. 1-13 1.4.6 Hand Held
(Portable) Fire Extinguishers ....................... 1-14
1.5 SYSTEM TESTS
............................................................................
1-14
-
B1 Mod 11.08.doc Issue No * Page 2
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
PAGE INTENTIONALLY
BLANK
-
B1 Mod 11.08.doc Issue No * Page 1-1
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
1 FIRE PROTECTION As fire is one of the most dangerous threats
to an aircraft, manufacturers and operators will install a variety
of overheat, fire and smoke detection devices as well as
extinguishing equipment. Different types of aircraft require
different levels of equipment, depending on the demands of the
relevant airworthiness authorities. 1.1 FIRE DETECTION AND WARNING
Overheat and fire protection systems on larger, modern aircraft do
not rely on observation by crew members as a primary method of fire
detection. An ideal fire protection system will include as many as
possible of the following features: 1. Must not cause false
warnings under any circumstances. 2. Rapid indication of the fire
and its accurate location. 3. Accurate indication that the fire is
out 4. Indication that the fire has re-ignited 5. Continuous
indication for the duration of the fire 6. Means of testing the
detection system electrically, from the cockpit. 7. Detectors that
are proof against oil, water, vibration and high temperatures. 8.
Detectors that are light and easily located throughout the aircraft
9. Detector circuitry that depends on basic power supplies only.
10. Minimum electrical current draw during 'stand-by' when not
indicating. 11. Each system should indicate, via a cockpit light,
showing location and an
audible alarm system. 12. A separate system for each engine. The
size of modern airliners makes the detection of heat, fire and
smoke difficult, so most have detection systems installed in the
most likely places that they may occur. These can include the
engine bays, electrical compartments, cargo holds, hot air duct
runs and wheel bays. All these areas must be separately indicated
to the flight deck, so that the crew can take suitable action.
1.1.1 Thermal Switch Detectors This system is a spot-type system
that uses a number of thermally activated switches to warn of fire
(Figure 1). They are mounted in parallel and are connected in
series with the warning light. This parallel connection allows the
remainder of the system to work even when one indicator has failed.
The indicators operate using a bi-metallic thermoswitch that closes
when heated to a high temperature and, just as importantly, goes
open circuit again when the heat is removed. A thermoswitch spot
detector is shown in Figure 2
-
B1 Mod 11.08.doc Issue No * Page 1-2
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
Thermoswitch Type Fire Detection System Figure 1
Thermoswitch Spot Detector
Figure 2 1.1.2 Continuous Loop (Fire Wire) Detectors This method
permits more complete coverage of a fire hazard area than any type
of spot-type of temperature detectors. The continuous loop uses the
principle of capacitance and resistance to indicate a rise in
temperature at any point along the length of the detector loop. The
commonest type has a stainless steel or Inconel outer tube, an
inner pure nickel wire surrounded by ceramic beads wetted by
eutectic salt. The effect of this design is that a rise in
temperature causes a sharp fall in electrical resistance, as well
as a rise in capacitance.
-
B1 Mod 11.08.doc Issue No * Page 1-3
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
Once the detection unit senses this effect, anywhere along the
wire, it will cause an overheat warning to be generated. This
continuous loop system is often referred to as a 'firewire'
system.The advantage of a firewire system is that a loop can cover
the complete powerplant, (Figure 3) within its cowling so that an
overheat or fire will be detected quickly no matter where it
starts. The firewire will also re-set the control box to remove the
warning when the temperature falls below the limit temperature.
Fire Wire Layout Figure 3
Firewire elements are attached to the airframe structure with
quick release clips approximately 6 apart and 4 from the end
fittings. The element is supported in clips with a rubber grommet
to prevent rubbing and to help damp out vibrations. (Figure 4).
Care is taken to eliminate strain on the element as excessive
bending could result in work hardening of the capillary.
Fire Wire Clips and Connections Figure 4
-
B1 Mod 11.08.doc Issue No * Page 1-4
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
1.1.3 Dual Loop System Most aircraft use the dual loop system of
indication. Each sensing circuit has dual sensing loops. Each Loop
A and Loop B is independent of each other. When the loop selector
switch is set to BOTH, both loops must detect a fire condition
before the warning system is activated. If only one loop detects a
fire the associated loop fault light will illuminate. If the
selector is switched to a single loop (A or B) full fire warnings
will activate if the selected loop senses a fire condition.
Pressing the loop test button simulates a fire condition on the
respective loop. This is done by earthing the inner electrode of
the loop which functionally checks the system and checks the
continuity of the loop. 1.1.4 Pressure-Type Sensor The pressure
type detection system uses a continuous loop for the detection
element. This loop is made from sealed stainless steel tube that
contains an element which absorbs gas when it is cold but releases
the gas when it is heated. This tube is connected to a pressure
switch that will close when the pressure reaches a pre-determined
level. The commonest make of this type of system is the
Systron-Donner system which uses a centre titanium centre wire and
the expansion of both helium and hydrogen gas to give the two-stage
warnings. Whilst the firewire system actuates when any part of the
loop reaches the limit temperature, the pressure type system will
actuate in two different ways. If a localised fire occurs, the
hydrogen gas is released and its pressure closes the pressure
switch which will set off the warning system, however, if the
temperature over a larger area rises to a lower level than a fire
warning the helium expands and closes the pressure switch to
activate the system warning. 1.1.5 Thermocouple System The
thermocouple warning system operates on a different principle from
the thermal switch system. A thermocouple depends on the rate of
temperature rise and will not give a warning when an engine slowly
overheats or a short circuit developes. The system normally
consists of the thermocouples, a relay box and a warning system.
Figure 5 shows a typical thermocouple. The thermocouple is
constructed of two dissimilar metals such as chromel and
constantin. The point where these metals are joined will be exposed
to the heat and is called the hit junction. There is also a
reference junction, which is insulated and enclosed in a dead air
space. A metal sheath surrounds the thermocouple for protection
without obstructing the air flow to the hot junction.
-
B1 Mod 11.08.doc Issue No * Page 1-5
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
If the temperature rises rapidly the thermocouple produces a
voltage because of the temperature difference between the hot
junction and the reference junction. If both junctions are heated
at the same rate as is normal with a gradual rise in engine
temperature, no voltage will be produced.
Thermocouple Figure 5
If there is a fire situation the hot junction will heat much
more quickly than the reference junction. The voltage produced will
activate the detector circuit and the warning signals will be sent
to the cockpit warning panels. 1.2 FIRE ZONES On light aircraft,
the only protection against fire is a stainless steel or titanium
bulkhead (firewall), dividing the engine bay from the cabin and the
rest of the aircraft. Larger aircraft have the complete engine
cowlings isolated from the airframe/wing assemblies and, in
addition, aircraft cowlings can be divided into a number of 'fire
zones', each one usually having its own warning and extinguishing
system. The types of zone dictate what type of protection that they
receive, for example, light aircraft have piston engines and hence,
due to the high flow of air through the bay, have no fire
protection and depend on isolating the engine of fuel to put out
any fire. The example has four zones around the engine only two of
which have firewires and extinguishing.
-
B1 Mod 11.08.doc Issue No * Page 1-6
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
1.2.1 Hot And Cool Zones Engines are usually split into hot and
cool zones (Figure 6). The hot zone comprises the combustion
chamber turbines and exhaust areas, the cool zone comprises the
intake, compressors and accessory drives.
Engine Fire Zones Figure 6
1.2.2 Fireproof Bulkheads These prevent fire from spreading to
other areas. Auxillary power units and tail mounted engines are
normally contained within such bulkhead compartments separating
them from the rest of the airframe. The engine pylons also contain
a firewall to separate the engine from the wing. These are made
from titanium or stainless steel and all joints are sealed with
fireproof sealants 1.2.3 Engine Fire Prevention There are a number
of techniques used to help prevent a fire occurring around engines.
These are, the use of flameproof or flame resistant materials, use
of bonding strips to prevent arcing, drainage of spilt fuel/oil and
efficient cooling. All pipes which carry fuel, oil or hydraulic
fluids are made fire resistant and all electrical components and
connections are made flame proof.
-
B1 Mod 11.08.doc Issue No * Page 1-7
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
It is essential that a fire staring in any zone is contained
within that zone and is not allowed to spread to any other part of
the aircraft. The engine cowlings form a natural container but they
are usually made from light alloy and would not contain a ground
fire for long. In flight however cooling airflows through the
cowlings provide sufficient cooling to render the cowlings
fireproof. The fireproof bulkheads and any cowling that has no
cooling air flow are usually made from titanium or stainless steel.
1.2.4 Cockpit and Cabin Interiors. All wool, cotton and synthetic
fabrics used in interior trim are treated to render them flame
resistant. Tests conducted have shown that whilst the foam used in
seat cushions is flammable, if covered with a flame-resistant
fabric, there is little danger of fire from accidental contact with
a cigarette, for example. Fire protection for the aircraft interior
is usually provided by hand-held extinguishers. Various types are
available including, Water, CO2 and Dry Powder. Each type is best
used on one kind of fire but may be used on other kinds. It is best
to be sure which is safe to use on which type of fire. 1.3 SMOKE
DETECTION A smoke detection system monitors certain areas of the
aircraft for the presence of smoke, which is could be indicative of
a fire condition. These may include, cargo and baggage compartments
and the toilets of transport category aircraft. A smoke detection
system is used where the type of fire anticipated is expected to
generate a substantial amount of smoke before temperature changes
are sufficient to actuate a heat/fire detection system. 1.3.1
Carbon Monoxide Detectors The presence of Carbon Monoxide (CO), or
Nitrous Oxides (N2O), is dangerous to flight crew and passengers
alike and may indicate a fire condition as it is a by product of
combustion. Detection of the presence of either or both of these
gases could be the earliest warning of a possible dangerous
situation. Carbon Monoxide is very dangerous, firstly due to the
minute amount required to cause loss of attention and headaches,
(this is approximately 2 parts in 10,000). It is colourless,
odourless, tasteless and a non-irritant. Carbon Monoxide detectors
are usually used in cabin and cockpit areas. The detector is
usually a small card with a transparent pocket containing silica
gel crystals that have been treated with a chemical, which changes
colour to green or black when they come into contact with carbon
monoxide.
-
B1 Mod 11.08.doc Issue No * Page 1-8
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
1.3.2 Photoelectric Smoke Detectors. Air from the monitored
compartment is drawn through the detector chamber and a light beam
is shone on it. A photoelectric cell installed in the chamber
senses the light that is refracted by the smoke particles. The
photocell is installed in a bridge circuit that measures any
changes, in the amount of current that it conducts. Figure 7 shows
a typical photoelectric smoke detector.
Photo Electirc Smoke Detector Figure 7
When there is no smoke in the chamber air, no light is refracted
and the photocell produces a reference current. When smoke is in
the chamber air, some of the light is refracted and sensed by the
photocell. Its conductivity changes, changing the amount of
current. These changes in current are amplified and used to
initiate a smoke warning signal. 1.3.3 Ionization Type Smoke
Detector. A small amount of radioactive material is mounted on the
side of the detector chamber. This material bombards the oxygen and
nitrogen molecules in the air flowing through the chamber and
ionizes it to the extent that a reference current can flow across
the chamber through the ionised gas to an external circuit.
-
B1 Mod 11.08.doc Issue No * Page 1-9
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
Ionisation Type Smoke Detector Figure 8
Smoke flowing through the chamber changes the level of
ionisation and decreases the current. When the current reduces to a
specific level the external circuit initiates a smoke warning
signal. Figure 8 shows a typical ionisation smoke detectot.
Flame Detectors This system uses a photoelectric cell to detect
a sharp rise in light, such as that from a flame in a closed bay.
1.4 FIRE EXTINGUISHING There are a variety of aircraft and ramp
extinguishing agents. Their use depends upon several variables such
as location, proximity to personnel, environment, possible sources
of fire, etc. There are integral extinguishing systems on board the
aircraft as well as hand held extinguishers
-
B1 Mod 11.08.doc Issue No * Page 1-10
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
1.4.1 Extinguishing System Aircraft that have an integral fire
extinguisher system have a system similar to the arrangement shown
in Figure 9. There are a number of pressurised bottles with
extinguishant inside and each bottle has two explosive cartridges,
(squibs), which can be fired from the flight deck. Each bottle can
feed either the port or starboard engines through a crossfeed. The
extinguishant is fed through a series of pipelines and valves to
the outlet nozzles and tubes. In some aircraft, fixed systems may
also be provided for the protection of landing gear wheel bays and
baggage compartments. These systems may be independent of each
other. They may be fully automatic or require the air crew to
initiate them when a fire is indicated.
Basic Aircraft Extinguishing System
Figure 9 On multi-engine aircraft there may be one extinguisher
bottle provided for each engine or one bottle may feed 2 engines
(Figure 10). There is always usually a facility for cross feeding
to another engine should the need arise.
-
B1 Mod 11.08.doc Issue No * Page 1-11
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
Dual Container System Figure 10
Two bottles giving either two 'shots', to a single engine or,
one 'shot' each to either engine (Figure 11). The bottle condition
is indicated either through a pressure gauge on each bottle, or a
red/green sectioned gauge showing red when the bottle is empty or
its pressure is low as well as a discharge indication on the
associated fire control panel I the cockpit.
Typical 2 Shot System
Figure 11
-
B1 Mod 11.08.doc Issue No * Page 1-12
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
There may also be pop up indicators to indicate that the squib
has been fired. A pressure switch may also be fitted which gives an
electrical indication to the cockpit control panel when the
pressure drops to a pre-determined level. Each bottle will have
protection against overpressure using a 'rupture disc', which fails
if the bottle pressure becomes excessive due to overheating. 1.4.2
Directional Flow Control Valves (2 Way Valves) These valves are non
return valves designed for use in a crossfeed system to allow the
contents of one or several extinguishers to be directed into any
one engine (or compartment). The valves prevent the reverse flow of
the extinguishant into the other bottle or engine. 1.4.3 Fire
Extinguishant Container Figure 12 shows a typical extinguishant
container. The cartridge is electrically ignited which drives the
cartridge cutter into the disc which on rupture releases the
extinguishant. The strainer prevents any of the broken disc from
entering the distribution system.
Fire Extinguisher Bottle Figure 12
-
B1 Mod 11.08.doc Issue No * Page 1-13
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
The safety plug is connected by a pipeline to a red indicator
disc on the outside of the compartment. If the gas pressure
increases due to an increase in the compartment temperature that
the bottle is located in, the fusable safety plug melts at a
pre-determined temperature and the bottle contents are discharged
overboard. As the bottle discharges overboard it blows out the red
indicator. The gauge shows the pressure of the extinguishant in the
container. 1.4.4 Toilet Compartment Systems Small, automatic units
will often be found in the toilet waste bins, where they will
discharge themselves when a heat source is sensed in the region of
75 degrees centigrade. A fusible type plug will melt allowing the
contents to discharge. Most aircraft with this system fitted do not
generate any indications to the cockpit or attendants panel if the
system was activated. Some systems have a visible temperature strip
which can be checked before each flight, or by the cabin crew in
flight. 1.4.5 Warnings And Indications Once a fire has been
detected in the engine bay (or compartment being sensed) a signal
is generated by the firewire element and this signal is sent to a
control unit. The control unit processes the signal and sends a
signal to the cockpit CWP, associated power lever handle, and the
fire control panel. The CWP red Fire warning caption light
illuminates for the affected engine (or compartment) as well as the
master warning lights and audio warnings. The Affected power lever
handle and fire extinguisher handle on the overhead console also
illuminate red. To activate the extinguishant, the red fire handle
is pulled to arm the system and then the squib button is pressed to
fire the bottle. If after the bottle contents have exhausted and
the fire indication remains, the second squib button is pressed to
fire the contents of the other bottle into the same affected engine
(or compartment). Some aircraft activate the extinguishers
differently. The bottle may be fired by pressing the affected fire
button on the fire panel. If the fire remains a cross feed switch
is activated which opens a crossfeed valve and the same fire button
is repressed to fire the other bottles contents into the same
affected system. Once discharged an amber DISCH caption on the fire
control panel will indicate when the corresponding bottle is empty.
These captions are usually electrically activated Whatever the
method of operation of the extinguisher system, the same basic
principle applies. The contents of each bottle, can be cross fed
into the affected area, that is on fire.
-
B1 Mod 11.08.doc Issue No * Page 1-14
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
1.4.6 Hand Held (Portable) Fire Extinguishers Each aircraft must
carry portable fire extinguishers for use by the cabin crew in the
event of a fire. These are positioned in various places within the
cabin with easy access to the crew. The amount and location depends
on the type of aircraft and its size. Halon extinguishers contain a
gas that interrupts the chemical reaction that takes place when
fuels burn. These types of extinguishers are often used to protect
valuable electrical equipment since they leave no residue to clean
up. Halon extinguishers have a limited range, usually 4 to 6 feet.
The initial application of Halon should be made at the base of the
fire, even after the flames have been extinguished Carbon Dioxide
fire extinguishers disperses the gas quickly, these extinguishers
are only effective from 3 to 8 feet. The carbon dioxide is stored
as a compressed liquid in the extinguisher; as it expands, it cools
the surrounding air. The cooling will often cause ice to form
around the horn where the gas is expelled from the extinguisher.
They are primarily used to extinguish electrical fires in the cabin
and cockpit. The CO2 can be aimed at the fire and discharged using
a trigger.
A dry powder fire extinguisher use compressed nitrogen to expel
a dry powder such as sodium bicarbonate or potassium bicarbonate.
They can be used on most fires but should never be used on the
flight deck, due to lack of visibility and interference with some
electrical equipment caused by the powder.. Water extinguishers are
also fitted to some aircraft and should be used to put out fires in
ordinary combustibles, such as wood and paper. The hand held
extinguishers are subject to periodic maintenance. The extinguisher
is checked for its weight. This is stamped on the neck of the
bottle and indicates its charged weight. If the weight is below the
set limits, it is to be replaced. 1.5 SYSTEM TESTS All
extinguishing systems have a method of testing their
serviceability. This can vary from weighing the complete cylinder
off-aircraft, (which will have the correct 'full' weight marked on
it), through to the bottle having a gauge with safe and
low-pressure sectors marked on it. Figure 13 shows an engine
extinguisher with a fitted gauge. Other more sophisticated systems
have internal pressure switches fitted to the bottle, which will
notify the flight deck of the loss of bottle pressure, (or
discharge), via a warning light, magnetic indicator etc.
-
B1 Mod 11.08.doc Issue No * Page 1-15
JAR 66 CATEGORY B1
MODULE 11.08 FIRE PROTECTION
Regardless of the system, all bottles and squibs have a life,
after which they have to be removed and returned to the
manufacturer for maintenance.
Fire Bottle With Pressure Gauge
Figure 13
Fire System Test Switch A test switch is available for each
system. When pressed all warning lights and audio warnings are
checked. If a light fails to illuminate it will normally indicate a
bulb filament failure.
Fire Wire Loop Test A test switch on the cockpit fire panel is
available to test each sensing element loop. When selected the
continuity of each circuit is checked. If the system is serviceable
the Loop caption(s) will illuminate. If the caption(s) do not
illuminate there is a fault in the system.
Squib-Test . A squib test button is available to check the
continuity of the discharge heads for each of the fire extinguisher
bottles. When pressed a squib warning light or magnetic indicator
will illuminate if the system is serviceable. No illumination means
that there is a fault in the system. The current used during the
squib test is at a much lower value than that required to fire the
squib.
-
B1 Mod 11.08.doc Issue No * Page 1-16
JAR 66 CATEGORY B1 )
MODULE 11.08 FIRE PROTECTION
.
PAGE INTENTIONALLY
BLANK