Ethiopian Aviation Academy

04/08/23 aircraft instrumnets 1

ETHIOPIAN AVIATION

ACADEMY

AVIATION MAINTENANCE

TECHNICIAN SCHOOL


BAC THREE: BASIC

INSTRUMENTS

INSTRUCTOR: TSEGAY TEKLU B.tech

EE,specialized in f.control and opt.


Course OutlineI. Introduction To Aircraft Instruments

A. Historical BackgroundB. Aircraft instruments in general.C. Requirements and Standards

II. Instrument Display, Panels and LayoutsA. Instrument classification as to their

application and operating principle.B. DisplaysC. Instrument Panels and LayoutsD. Instrument MountingsE. Illumination of Instruments and Instrument

Panels 3aircraft instrumnets


Course Outline

III. Instrument Elements and Mechanisms

A. Instrument Elements

B. Instrument Mechanisms

C. Temperature Compensation of Instrument Mechanisms.

D. Magnetic indicators of flow lines.

4aircraft instrumnets


Course Outline

IV. Synchros and ServomechanismsA. Synchros

B. Servomechanisms

C. Problems associated with synchros and servomechanisms

V. Engine and Airframe InstrumentsA. Pressure Measuring Instruments

B. Temperature Measuring Instruments

C. Gyros – Attitude Indication System



Course Outline

VI. Navigation InstrumentsA. Review of Magnetic Properties

B. Magnetic Compass

C. Construction

D. Magnetic compass errors

E. Compass swinging procedure



Course Objective

• Describe the historical development of aircraft instruments.

• Identify the first instruments used on aircrafts and their purpose.

• Identify why standards and requirements are needed for aircraft instruments

• Describe what an airworthiness requirement is and who sets it.



Course Objective (cont….)

• Describe the requirements regarding location, visibility and grouping of aircraft instruments.

• Identify the different types of instrument grouping categories and the indicators in each category.

• Identify the different types of instrument displays used on aircraft.

• List the different types of panels used on the aircraft.



Course Objectives (cont…)• Describe a servo mechanism and its

applications in aircraft instruments.• Identify the different types of pressure and

temperature measuring instruments used.• Describe the principle of operations in

pressure and temperature indications.• Define a gyro and describe its characteristics

and application as an attitude indicator.• Identify and describe the operation of a

magnetic compass.9aircraft instrumnets


Course objective (cont…)

• Identify and list the different elements that make up an instrument.

• Differentiate between each of the elements of an instrument.

• List the types of mechanisms used on aircraft instruments.

• Describe what temperature compensation means and identify the types of temperature compensations used on aircraft instruments. 10aircraft instrumnets


Text Book

• Aircraft Instruments, 2nd Edition By EHJ Pallett

Historical Background Chapter 1: Requirements and Standards Chapter 2: Instrument elements and mechanisms Chapter 3: Instrument displays, panels and layouts Chapter 5: Primary Flight Instruments Chapter 6: Heading Indicating Instruments Chapter 9: Synchronous Systems Chapter 11: Measurement of Temperature Chapter 12: Measurement of Pressure

11aircraft instruments


References

• Aircraft Instruments and AvionicsBy Max F. Henderson

• Avionics FundamentalsBy Jeppesen, Sanderson Training Products

• Airframe and Power plant Mechanics Airframe Handbook, AC65-15A

• Navy Electricity and Electronics Training SeriesModule 15 – Principles of Synchros, Servos, and Gyros



Course Duration

• Theory – 40 hours, and • Practical – 40 hours.• Total course duration – 80 hours.• The course period is from September

01,2008 to September 05,2008.



I. INTRODUCTION TO

AIRCRAFT INSTRUMENTS

A. HISTORICAL BACKGROUND


A. Historical Background• The first successful airplanes flown by the

pioneers were “stick and string”, with temperamental engines.

• The airplane was maneuvered by the pilot (without cockpit) lying,sitting or crouching in the open

• Instruments designed specifically for use in an airplane were non-existent.



Cont’d

• It is a little difficult to specify in what sequence instruments were introduced into airplanes.Magnetic compass – to fly from point A to B.Fuel gauge – a glass sight gauge to monitor amount

of petrol in the tankClock – to calculate speed from a time/distance

method, as an aid to navigation.



History – World War I• Shortly before the WW I, attention was

given to the development of instruments for use on military and naval aeroplanes, and the first principles of air navigation were emerging with designs for instruments specially adapted for the purpose.

• Consequently more instruments were designed:AltimeterAirspeed , andRPM indicator and oil pressure indicator; the first

engine instruments. 17aircraft instrumnets


History – World War I• During World War I, the accuracy of the

instruments was improved, investigation of new principles and realization to design instruments to withstand vibration, acceleration, temperature change & so on.

• Attitude indication system was introduced using:Fore-aft-level – a specially constructed glass tube

containing a liquid which moved up and down against a graduated scale,

The cross level – specially adapted version of the simple spirit level. 18aircraft instrumnets


History – cont…

• To improve the “blind flight”, a need to replace the natural horizon reference and

integrate the information from the cross-level and the fore-aft level instruments.

Stable indication of heading which is not affected by acceleration and turning maneuvers of the airplane.

• The outcome was two additional gyroscopic instruments:Gyro horizon, and Directional gyro.



I. INTRODUCTION TO

AIRCRAFT INSTRUMENTS

B. REQUIREMENTS AND STANDARDS


ICAO & CAA

• The international operation of civil aircraft requires international recognition,

• that aircraft do comply with their respective national airworthiness requirements.

• International standards of airworthiness are laid down by the ICAO.

• These standards do not replace national regulations,• But define the complete minimum international

standard required for airworthiness certification.



B. Requirements & Standards • Modern aircraft and their equipments,

the environmental conditions under which they operate • require conformity of design,

• development and subsequent operation with established requirements and standards.

These standards and requirements are called airworthiness requirements.

• The countries are responsible for the formulation and control of airworthiness requirements

AND • the recommended standards to which raw materials,

instruments and other devices should be designed and manufactured in their regions.



Standards / Specifications

• The purpose of specifications or standards is:- to ensure conformity with the required production

processes. to set standard for quality of the product. to ensure reliability when performing the intended

function.



Cont…

• With regard to aircraft instruments, the standards provide:-definitions,constructional requirements,Dimensions,calibration data,accuracy required under varying environmental

conditions, andmethods of testing e.t.c.



Federal Aviation

Administration• The FAA has many regulations that

concern the installation of instruments in certificated aircraft.

• The FAA Regulations that are related with instruments are:FAR Part 23FAR Part 43FAR Part 91



FAR 23

• Certain instrument requirements are part of these Airworthiness Standards. FAR 23 1381

• INSTRUMENT LIGHTS FAR 23 1541

• MARKINGS AND PLACARDS FAR 23 1543

• INSTRUMENT MARKINGS AND GENERAL REQUIREMENT FAR 23 1549

• POWER PLANT INSTRUMENTS FAR 23 1553

• FUEL QUANTITY INDICATOR



FAR 43 and 91

• FAR 43 Appendix AAppliance major repairs

• FAR 43 Appendix DScope and detail of items to inspect for 100 hour and

annual inspections

• FAR 91.9Powered civil aircraft with standard category U.S.

Airworthiness Certificates; instrument and equipment requirement.



Aircraft instruments in general

Introduction• Flying under IFR is inconceivable unless the aircraft is

equipped with instruments indicating its flight attitude and heading .

• HOW is Flying safety obtained? with accurate indications and reliable operation of the aircraft

instruments ,and their correct reading by the pilot personnel .• The aircraft instruments are designed ,

to check the current flight parameters , As well as the performance of aircraft power plants and individual

systems.

aircraft instruments 28


Requirements – Location, Visibility, and

Grouping of Instruments• Instruments shall be located to be read easily by the flight crew.• Illumination of instruments should be sufficient but,

not glaring into the pilot’s eyes.

• Flight, navigation and power-plant instruments shall be, plainly visible to the pilot from his station.

• All flight instruments shall be grouped on the instrument panel, symmetrically on the vertical plane of vision of the pilot.

• All power plant instruments shall be grouped together to be readily seen by the crew members and in a manner to minimize confusion.

• Instrument panel vibration must not impair seriously, the accuracy of the instruments or damage them.



Instrument classifications

• Functionally, aircraft instruments may be divided in to three classifications:

1. Flight and navigation instruments Enable the pilot to fly an aircraft and solve navigation

problems. As a rule are arranged in the center of the instrument

panel. are air speed indicator, altimeter ,rate of climb indicator,

turn indicator, gyro- horizone,compass,accelerometers, clock,machmetere,e.t.c.


Instrument classification …cont’d

2.Engine control instruments Are instruments which inform the pilot about engine

temprature,lubricant condition, engine power ,or thrust indications, fuel capacity and consumption, fuel and oil gauges of exhaust gas temperature , e.t.c

Usually are located in the right hand section of the instrument panel.


Cont…3.Instruments checking the action of the aircraft individual

systems :- High altitude and oxygen equipments(airflow,altitude and differential

pressure indicators, cabin pressure warning units, oxygen sets e.t.c) Instruments of pneumatic and hydraulic systems pressure gages of the

landing gear, flaps, and brake control systems e.t.c.). Instruments indicating the position of the aircraft’s structure

components ( trailing edge flap position indicator, tail plane and intake spike position indicators e.t.c)

Test instrumentation and signaling equipments(VH recorder VHG recorder automatic angle of attack indicating system e.t.c)



Instrument location in the cockpit

• Flight and navigation instruments should be placed in front of the pilot /copilot seat.

• The engine power determining instruments are placed in b/n the pilot/copilot panel forward electronic panel).

• Miscellaneous instruments of large fleet are placed on the forward over head and aft overhead panels.


Boeing 767



Boeing 747 Cockpit



Boeing 777-200



Aircraft instruments classification as to their

principle of operations1. Diaphragm actuated-pressure instruments:- depend on pressure difference for

their operation (ASI,mach meter, pressure gauges, rate of climb indicators).2. Aneroid-actuated pressure instruments embodying the absolute pressure

measurement principles(barometeric altimeters, correctors , altitude sensors)3. Gyroscopic instruments employing the properties of gyro with two and three

planes of freedom (turn indicators, gyro-horizon, vertical gyro e.t.c )4. Electric instruments acting on the principle that non-electric quantities are

measured by the electric method (exhaust gas thermometers ,oil thermometers , fuel and pressure gauges e.t.c).

5. Magnetic compass employing the principle of freely suspended magnet which aligns itself with the earth’s magnetic meridian .

6. Mechanical instruments which in action follow the mechanics laws (clock, accelerometers, bimetallic thermometers e.t.c).

7. Optical instruments whose action is based on the optics laws (sighting telescopes, ____________)

•


Requirements – duplicate instruments

• In aircraft involving two pilot operation, it is necessary for each pilot to have his/her own flight and navigation instruments (pitot static

and gyroscopic instruments,) two independent operating systems, where failure in one

will not impair the operation of the other.



Flight instruments

• Main types :Pitot-static instrument.Gyroscopic instruments.Magnetic instruments.


Pitot/Static Systems• Pitot pressure, or impact air pressure,

• is sensed through an open-end tube pointed directly into the relative wind flowing around the aircraft.

• The pitot tube connects to pressure operated flight instruments such as ASI.

• Three basic pressure-operated instruments found in most aircraft instrument panels are: the sensitive altimeter, ASI, and vertical speed indicator (VSI).

• All three receive pressures sensed by the aircraft pitot-static system.

• The static ports supply pressure to the ASI, altimeter, and VSI.

• The pitot port supplies pressure only to the ASI.


Schem of pitot-static ports




Schematic diagram of pitot-static

tube


Static Pressure• The instruments depend

• upon accurate sampling of the ambient still air atmospheric pressure to determine the height and speed of movement of the aircraft through the air, both horizontally and vertically.

• This pressure, called static pressure, is sampled at one or more locations outside the aircraft.

• The pressure of the static air is also sensed at a flush port where the air is not disturbed.

• On some aircraft, air is sampled by static ports on the side of the electrically heated pitot-static head,

• Other aircrafts pick up the static pressure through flush ports on the side of the fuselage or the vertical fin.


Requirements – pitot static system• The system should be air tight (except vents)• The system shall be provided with heated pitot-pressure probe

to prevent malfunctioning due to icing.• Sufficient moisture traps shall be installed to ensure positive

drainage.• If alternate or emergency system is installed, selector valve

should be clearly marked to indicate which system is in use.



Requirement …Cont’d• These ports are in locations proven by flight tests to be in

undisturbed air, and• they are normally paired, one on either side of the aircraft. • This dual location prevents lateral movement of the aircraft

from giving erroneous static pressure indications. • The areas around the static ports may be heated with electric

heater elements to prevent ice forming over the port and blocking the entry of the static air.

47



Pitot/Static Instruments…cont’d

• Sensitive Altimeter• A sensitive altimeter is an aneroid barometer that

measures the pressure of the ambient air and displays it in terms of feet or meters above a selected pressure level.


Principle of Operation• A sensitive altimeter is one with an adjustable barometric scale allowing

the pilot , to set the reference pressure from which the altitude is measured.

• The sensitive element in a sensitive altimeter is a stack of Evacuated, and corrugated capsules.

• The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them.

Results in thickness changes as the air pressure changes.

• Stacking several aneroid increases the dimension change as the pressure varies over the usable range of the instrument.



.• Altimeter Errors

A sensitive altimeter is designed to indicate standard changes from standard conditions,

but most flying involves errors caused by nonstandard conditions and Thus the pilot must be able to modify the indications to correct the errors. There are two types of errors: mechanical and inherent.

• Mechanical errors A preflight check to determine the condition of an altimeter consists of

setting the barometric scale to the local altimeter setting. The altimeter should indicate the surveyed elevation of the airport. If the indication is off by more than 75 feet from the surveyed elevation,

the instrument should be referred to a certificated instrument repair station for recalibration.

Differences between ambient temperature and/or pressure causes an erroneous indication on the altimeter.


YAW MOTION

PITCH UP & DOWN MOTION

ROLL MOTION


Gyroscopic systems


Gyroscopic Systems• GYROSCOPE :is a device which consists of weighted wheel or rotor

which spins at high speed and is held in an arrangement of hinged mounting rings called gimbals.

• Has three degrees of rotational freedoms.1. Spinning freedom about an axis perpendicular through its center (axis of

spin X1X).

2. Tilting freedom about a horizontal axis at right angle to the spin axis (axis of tilt Y1Y).

3. Veering freedom about a vertical axis perpendicular to both the spin and tilt axes (axis of veer Z1Z).

• The degrees of freedom are obtained by mounting the rotor in two concentrically pivoted rings, called inner and outer gimbals rings.

• The whole assembly is called gimbals system of free or space gyroscope

aircraft instrumnets 55


Elements of gyroscope


Gyroscopic Systems…cont’d• A spinning gyroscope possesses two characterstics to be used

in aircrafts: Rigidity and precession.

• Rigidity is the property that resists any force tending to change the plane of rotation of its rotor.

• This property depends on three factors1. Mass of the rotor2. The speed of rotation3. The distance at which the mass acts from the center i.e the radius of

gyration.• Precession is the angular change in direction of the plane of rotation

under the influence of an applied force.• The change in direction takes place not always inline with the applied

force but always 90 –degree in the direction of rotation.



Gyro system…cont’d• Precession depends on three factors.

1. The strength and direction of the applied force.

2. The moment of inertia of the rotor.

3. The angular velocity of the rotor.

• N:B The greater the force the greater is the precession The greater the moment of inertia and angular velocity, the smaller is the

rate of precession.

• Precession continues until the plane of rotation is inline with the plane the applied force and, until the direction of rotation and applied force coincide.


Gyroscopic attitude instruments• Gyro horizon.

• Attitude director indicator.

• Directional gyro.

1. Gyro horizon.

• This flight instrument shows the relationship between the pitch and roll axes of the aircraft and ,a vertical line through the center of the earth.

• It gives the pilot a stable reference so he can control the aircraft wings level.

• And Tells nothing about the direction in which the nose of the aircraft is pointing.


Gyroscopic instrument (gyro

horizon)


Gyroscopic instr…cont’d• Attitude and heading instruments operate on the principle

of rigidity.

• For these instruments, the gyro remains rigid

in its case and the aircraft rotates about it.

• Rate indicators, such as turn indicators and turn

coordinators, operate on the principle of precession.

• In this case, the gyro processes (or rolls over) proportionate to the rate the aircraft rotates about one or more of its axes.

62


Gyro horizon• A symbol indicating the wings of the plane is mounted inside the

instrument case to show the relationship between the airplane and the horizon. The brown color represented the horizon, but now it is a disc with a line

representing the horizon and both pitch marks and bank-angle lines. The top half of the instrument dial and horizon disc is blue, representing the

sky; and the bottom half is brown, representing the ground. A bank index at the top of the instrument shows the angle of bank marked on

the banking scale with lines that represent 10°, 20°, 30°, 45°, and 60°.


Gyro horizon


Cont….

• ILS + GYRO HORIZON = FLIGHT DIRECTOR INDICATOR (FDI)


Requirements – gyroscopic instruments

• Instruments may be vacuum or electrically operated, but instruments shall be provided with two independent sources of power. a means of selecting either power source. a means of indicating that power supply is working

satisfactorily. failure on one source should not interfere with the

operation of the other.



Attitude director indicator• Is portion of the an automatic flight control system that provides cues to

the pilot so that he can fly at the command of the flight control system.

• Has command bars driven by the computer.

• When the flight director commands a climb, the command bars move up and the pilot must raise the nose of the

aircraft to place the wing tips on the command bar.

• The symbolic aircraft doesn’t move ,but since the command bar and the horizon card connected to the gimbals behind the aircraft move ,it appears to the pilot that the symbolic aircraft has moved to answer the command).



Turn and bank indicator

• It is the first aircraft flight instrument to use gyroscope as detecting element.

• In conjunction with magnetic compass it made a valuable contribution to the art of flying with out reference.

• Was considered as essential primary blind flying instrument for all types of aircraft.



Rate gyroscope

• Used to detect rate of turn of the aircraft.• The gyroscope defers from these employed in

directional gyros and gyro horizons in two respects:- It has one gimbal ring and, it has spring connected b/n the gimbal ring and casing to

restrain movement about the longitudinal axis Y1Y.

• The rate-of-turn is actuated by the gimbal ring and a magnifying system the design of which varies b/n manufacturers.


Bank indication

• This instrument is used indicate weather the aircraft is correctly banked for the particular turn.

• For its correct operation ,it depends on the gravitational and centrifugal forces.

• Two principal mechanical methods may be employed

1. One utilizing a gravity weight and pointer .

2. A ball in a curved liquid-filled glass tube.




Turn coordinator

• The gyroscope in this instrument is spring-restrained and , Is mounted so that the axis is at about 30-degree with

respect to the aircraft’s to the longitudinal axis, thusmaking the gyroscope sensitive to banking of the

aircraft and turning.



Cont….

• ILS + GYRO HORIZON = FLIGHT DIRECTOR INDICATOR (FDI)


FDI and Gyro Horizon



Requirements – gyroscopic instruments

• Instruments may be vacuum or electrically driven, • but instruments shall be provided with two

independent sources of power,with a means of selecting either power source a means of indicating that power supply is whether

working satisfactorily or not.A means that failure on one source should not interfere

with the operation of the other.



Power Sources

• Aircraft and instrument manufacturers have designed redundancy in the flight instruments, so that any single failure will not deprive the pilot of the ability to

safely conclude the flight.

• Gyroscopic instruments are crucial for instrument flight;

• therefore, they are powered by separate electrical or pneumatic sources.



Air driven (gyro) heading indicator


Pneumatic Systems

• Pneumatic gyros are driven by a jet of air impinging on buckets cut into the periphery of the wheel.

• On many aircrafts this stream of air is obtained , by evacuating the instrument case with a vacuum source and, allowing filtered air to flow into the case through a nozzle to spin

the wheel.



Venture Tube Systems

• Aircraft that do not have a pneumatic pump to evacuate the

instrument case can use venture tubes mounted on the outside of the aircraft, similar to the system shown in Figure below.

• Air flowing through the venture tube speeds up in the narrowest part and, according to Bernoulli’s principle, the pressure drops.

• This location is connected to the instrument case by

a piece of tubing.



Venture

tube system

aircraft instruments 85


Vacuum Pump Systems

• Steel-vane air pumps have been used for many years to evacuate the instrument cases.

• The vanes in these pumps are lubricated by a small amount of engine oil metered into the pump and discharged with the air.

• In some aircraft the discharge air is used to inflate rubber deicer boots on the wing and empennage leading edges.

• To keep the oil from deteriorating the rubber boots, it must be removed with an oil separator like the one in THE FIGURE BELOW.



Vaccum pump system …cont’d• The vacuum pump moves a greater volume of air needed

to supply the instruments with the suction required, The suction-relief valve is installed in the inlet side of the Pump, This spring-loaded valve draws-in just enough air to maintain the required low pressure inside

the instruments, as is shown on the suction gauge in the instrument panel.

• Filtered air enters the instrument cases from a central air Filter ,

• As long as aircraft fly at relatively low altitudes, enough

air is drawn into the instrument cases to spin the gyros at a

sufficiently high speed, why enough air…?



Vacuum pump systems…

schematics.



Vacuum pump systems…

schematics


Powerplant instruments

• Shall be mounted in such a manner that they may be read appropriately by both of the crew members.Tachometer: to measure the rotational speed of crankshaft

or compressor.Cylinder head temperature indicator to indicate the

temperature of the hottest cylinder.Carburetor intake air temp indicator.Oil temp indicator to show the oil inlet and/outlet temp.



Cont…EGT indicator to show whether the turbine or exhaust gas

temp is maintained within its limits.Fuel pressure indicator- to show fuel supply pressure and

as means of low pressure warning.Oil pressure indicator- to indicate a lubricating system

pressure and as a warning of low oil pressure.Fuel quantity indicator – to indicate in gallons or

equivalent units the quantity of usable fuel.• Indicators shall be calibrated to read zero when unusable fuel is

remaining.



Auxiliary instruments

• They are:-

• Clock( electrically powered with their lead ahead of the master switch and have always power regardless of position of the master switch) Outside air temperature indicator (bimetallic type and electrically

operated and reads changes in resistance of coil of nickel wire due to change in temperature).

Pressure indicators (oil, fuel, hydraulic pressure e.t.c, Three methods to measure these pressures, pressure capsules

(diaphragms), high pressure with Burdon tube mechanism) . fuel quantity indicators (magnetic coupling system, variable resistor

types fuel quantity indicator, capacitance type fuel quantity indicator).


System Instrument Or Auxiliary Instruments

• Provide the pilot with the information on: the status of the aircraft subsystem such as pneumatic

system, oxygen, etc. the operation of the various components such as the

landing gear and the flaps.Examples are:

• Hydraulic pressure gauge.

• Oxygen pressure gauge

• Water quantity gauge

• Voltmeter

• Ammeter93

aircraft instruments


II. INSTRUMENT DISPLAYS,

PANELS AND LAYOUTS

A.DISPLAYS


Man-Machine Loop System• In flight an airplane and its operating crew form a man-machine system

loop.• Depending on the size and type of aircraft, this loop could be fairly simple

or very complex.• The function of the crew within the loop is that of CONTROLLER and

the extent of the control function is governed by the simplicity or otherwise the machine as an integrated whole.

• Depending upon the control function the pilots’ duty can be that of : a controller – if the pilot is flying the aircraft manually, or a monitor – if autopilot is flying the aircraft.



Inst

Feedback

Pilot

aircraft

Man-Machine Closed-loop systemController/monitor controlled



Cont…• Instruments play a vital role as they are the means of

communicating data between systems and the controller/pilot.

• As a result the content and form of the data displayed is very important. The most common forms of data display applied to aircraft instruments

are:

1. Quantitative – In which the variable quantity being measured is presented in terms of a numerical value and by the relative position of a pointer or index .

2. Qualitative – In which the information is presented in symbolic or pictorial form.



1. Quantitative Displays

• There are three principal methods:i. The circular scale or clock type.

ii. Straight scale

iii. Digital, or counter

I. Circular Scale • May be considered as the classical method of

displaying information in quantitative form .• It has clock type presentation or display.



Circular scale• The main components in a circular scale quantitative display

are: Scale base Scale marks Scale spacing Scale length Pointer or index



Scale Base

• Also known as Graduation circle.• It refers to the line which may be actual or

implied ,running from end to end of scale and from which the scale marks and line of travel of the pointer are defined.



Scale marks

• Also called graduation marks.

• Are the marks which constitute the scale of the instrument.

• The number of marks should be chosen carefully:Too few marks dividing the scale, vital information may be

lost and reading errors may occur.Too many marks, time will be wasted since speed of

reading decreases as the number of markings increases.




Scale marks - rules• The scales will be divided so that the marks represent units of

1, 2 or 5 or decimal multiples thereof.

• The size of the marks to be numbered are generally the largest while those in between are shorter and usually all of the same length.



Scale spacing• This is governed by physical laws related to the quantity to be

measured.

• Hence, there cannot be complete uniformity between all quantitative displays.

• In general, have two groups:Linear – scales with evenly spaced marks, orNon - linear – non-evenly spaced marks

• Square law type (airspeed), or

• Logarithmic type (rate of altitude changes)



Linear and non-linear scales

linear

logarithmic

Square law


ALTITUDE METERE


Numbering• The sequence of numbering always increases in a clockwise

direction, except for instruments having a centre zero.

• As in the case of marks, numbering is always in steps of 1, 2 or 5 or decimal multiples thereof.

• The numbers may be marked on the dial either inside or outside the scale base.

•Scale length• The distance between the centers of the marks indicating the

minimum and the maximum values of the chosen range of the measurement and measured along the scale base .



Numbering… Cont’d• Governing factors in the choice of scale length for a particular

range are:

• the size of the instrument the accuracy with which it is needed to be read. the conditions under which it is to be observed.

• Theoretically, the length of a scale designed for observing at a distance of 30 in and capable of being read to 1% of the total indicated quantity, should be about 2 inches regardless of the shape.



Scale length - standardization

• But to retain legibility of aircraft instruments at different conditions, the following standard have been adopted: Instruments displaying information which is to be read

accurately and at frequent intervals have scales about 7 inch in length fitting into standard 31/4 in cases, and

Instruments requiring only occasional observation, or from which only approximate readings are required, have shorter scales and fit into smaller cases.



High Range Long-Scale Display

• For the measurement of some quantities such as turbine engine rev/min, airspeed and altitude, high measuring ranges are involved with the result that very long scales are required.

• To overcome these problems, we useConcentric scalesFixed and rotating scalesCommon scale, triple pointerSplit pointer



High Range Long-Scale Display

Common scale, triple pointers

Concentric scales

Split pointer

Fixed and rotating scales



Concentric scales

• To accommodate a lengthy scale, we split it into two concentric scales – the inner is made a continuation of the outer.

• The presentation is using two interconnected pointers of different sizes to avoid mis-reading

• Found in turbine engine rev/min indicators.



Fixed and rotating scales• A single pointer rotates

against a circular scale and drives a second scale instead of a pointer.

• The rotating scale is visible through the aperture in the main dial.

• Commonly used on airspeed indicators.



Common scale, triple pointers• Three concentric pointers of

different sizes register against a common scale.

• Common in altimeters where the large pointer indicating hundreds, the intermediate pointer thousands and the small pointer tens of thousands.

• Disadvantage – interpretation of a reading is difficult.



Split pointer• Uses an inner and outer scale

and two different sized pointers – appearing as one.

• The two pointers rotate together and after completing a revolution, the tip of the longer pointer will be covered and only the shorter pointer continues its movement to register against the inner scale.

Applied to airspeed measurement114aircraft instrumnets


Angle of observation

• The angle at which an instrument is to be observed affects the choice of the correct scale length, and case size

• A standard is laid down that no part of an instrument should be obscured by the instrument case when observed at angles up to 300 from the normal.

• A method adopted is the fitting of instrument mechanisms inside square cases.



Parallax error• Observing an instrument at an angle results in errors due to

parallax.

• The magnitude of error due to parallax is dependant on:The angle at which the data is observed.The clearance distance between the pointer and the dial

plate.

• It could be solved by using ‘platform scale’ where the scale marks are set out on a circular platform which is secured to the main dial plate so that it is raised to the same level as the tip of the pointer.



Platform scale



Scale range and operating range

• Instrument scale lengths and ranges usually exceed the operating range of the system with which the instrument is associated thus leaving part of the scale unused.

• This is done to improve the accuracy with which readings may be observed.



Reading accuracy

Equal scale length and operating range Scale range exceeding operating range119aircraft instrumnets


Straight Scales• In addition to the circular scale presentation, a quantitative

display may also be of the straight scale (vertical or horizontal) type.

• In the field of aircraft instruments, there are very few applications of the straight scale and pointer

display, B/C they are not suitable for monitoring of the majority of quantities

to be measured, but the moving tape or thermometer type is utilized.

• Advantages:To economize the panel space Improved observational accuracy.Can be installed horizontally or vertically.

• Sequence of numbering is from bottom to top or left to right.120aircraft instrumnets


Comparison of scales



Digital Display• It is also called veeder counter.

• Data are presented in the form of letters and numbers, known as alpha-numeric display.

• It is most common to use counter in combination with the circular type of display.

• It is used in the application to the altimeters and,

• there are two types of counters, static counter – baro settingdynamic counter – altitude



Digital Displays



Dual indicator display• Designed principally as a means of conserving panel space.

• There are two basic forms:Two separate indicators and scales are embodied in one

case.Two indicators in a case but with the pointers registering

against a common scale.



Dual indicator display



Colored Display

• Provides a means of indicating specific operational ranges of the systems with which they are associated,

• so as to make more rapid assessment of conditions prevailing with the scanning of instruments.

• Color may be applied to scales; in the form of sectors and arcs which embrace the number

of scale marks appropriate to the required part of the range, and

in the form radial lines coinciding with appropriate individual scale marks.



Colored displays …cont’d

• It is usual to find that colored sectors are applied to those parts of a range in which it is sufficient to know that a certain condition has been reached rather than knowing actual quantitative values.

• Depending on the condition to be monitored, the colors may beRedYellowGreen



Range markings• Arcs and radial lines are usually called range

markings.• Definitions of range markings are

RED radial line – maximum and minimum limitsYELLOW arc – take off and precautionary rangesGREEN arc – normal operating rangeRED arc – range in which operation is prohibited.WHITE arc (airspeed only) – indicates the airspeed range

over which the aircraft landing flaps may be extended in the take-off, approach and landing configurations of the aircraft.



Range markings



2. Qualitative Displays.• These are of special type in which the information is presented

in , a symbolic or pictorial form, to show the condition of a system whether the value of an output is

increasing or decreasing, by the movement of a component and so on.

• Example – applications,Engine synchronizing displaysTo show the movement of flight controls



Qualitative displays


Engine synchronizing

Position of flight control systems


Director Displays• Are those which are associated principally with flight attitudes

and, navigational data and presenting it in a manner which indicates the

movements of Gyro Horizon ILS Indicator ,

• Are integrated instrument systems of present day aircrafts. • There are three elements making up the integrated display

A pointer registering a bank angle against a bank-angle scale; An element symbolizing the aircraft An element symbolizing the natural horizon.

• Both the bank pointer and horizon symbol are stabilized by gyroscope. 132aircraft instrumnets


Director displays



Cont….

ILS + RMI = HORIZONTAL SITUATION INDICATOR (HSI)


Head-up Displays:

• To present vital flight data at the same level as the pilot’s line-of-sight when viewing external references, i.e., when he/she maintains a “head up” position.

• The principle of the method is to display data on the face of a special Cathode Ray Tube and, to project them optically as a composite symbolic image on to a transparent reflector plate, or directly on the windscreen.




Head-up display system



Visual Approach Monitor

• It is a head-up display designed to aid pilots when approaching airfields not equipped with ILS or other approach aids.

• The display unit is mounted on a sliding tray located at a glare shield panel in front of the pilot, when required, the tray is pulled out to automatically raise the lens through which the display is projected.



Cont.

• The display provides the following cues, based on selection on a control module:The vertical approach anglehorizontal attitude speed errors



Visual approach monitor



Light Emitting Displays

• There are two types of light emitting displays used in aircraft instruments.

• These are – Liquid Crystal Display (LCD), and Light Emitting Diode (LED)



Liquid Crystal Display (LCD)• Are found in many state-of-the-art aircraft

instruments.• The display can be configured to form letter and

number patterns, or it can form a full picture.• Commonly, liquid crystals are gray, but they can be

full color.• Liquid crystals are fluid materials that contain

molecules arranged in crystal forms.• The molecules are typically twisted and therefore

“bend” the light that passes through the crystal.142aircraft instrumnets


LCD – construction• Consists of two glass plates, coated on their inner

surface with a thin transparent conductor such asindium oxide.

• The conductor on the front plate is etched into a standard display format of seven bars or segments, each segment forming an electrode.

• Each bar is electrically separate and is selected by a logic/driver circuit which causes the bars to illuminate in patterns forming the digit to be displayed.



Cont.

• A mirror image of the digits with its associated electrical contacts is also etched into the oxide layer of the back glass plate,

• but this is not segmented since it constitutes a common return for all segments.

• The space between the plates is filled with a liquid crystal material, referred to as a nematic (Greek – nemator means thread) material.



LCD



L.C.D.



LCD – operation

• If a voltage is applied to the liquid crystal, the molecules align and the light passes “straight” through the material.

• LCD’s use this phenomenon to align light waves with polarized filters.

• The polarized filters block, or pass the light to form specific patterns for the display.



LCD



7-Segment LCD display



Cont.

• The magnitude of the optical changes, contrast, is basically a measure of the light reflected from or transmitted through the segment area to the light reflected from the background area typical ratio 15: 1



Cont.

• The current applied to the segment is of alternating type ,to avoid undesirable electrolytic effects

• Energizing of the segments is accomplished by simultaneous application of a symmetrical out of phase signal to the front and back electrodes of a segment ,and thereby producing a net voltage difference.



Cont.

• When two in phase signals are applied the display segments spontaneously relax to the de-energized state.



Types of LCD

• There are two types:Dynamic-scattering typeField-effect type



Dynamic Scattering • Operates on the principle of forward light scattering,

which is caused by turbulence of the ions of the liquid crystal material when current is applied to the segment electrodes.



Cont.

• For Transmissive read out a back light source is provided ,the light being directed down by a light -control film similar in its action - a Venetian Blind.

• In the area defined by the energized segment ,the light is then scattered up towards the observer to produce a light digit or character on a dark background.



Field-effect Type

• Incorporates additional plates called polarizers on the front and back plates of the assembly.



L.C.D

• This molecular configuration causes the plane of polarization to be reflected 900 as it passes through the LCD.



Light Emitting Diode (LED)

• The light emitting diode is widely used on aircraft instruments and test equipment.

• Single LEDs are used as indicator lights.• An arrangement of LEDs is used to display letters

and numbers.• LEDs require 1.5 to 2.5 V and 10 to 20 mA to

produce adequate light for most applications.• For an LED to conduct, the applied voltage must be

connected in the forward-biased condition.158aircraft instrumnets


LED



LED cont…• The light of an LED comes from the energy given off when

the diode is forward-biased.

• At this time free electrons travel from a high to a low energy level and produce light and heat.

• Those diodes which do not emit light will expend all their “extra” energy in heat.

• LEDs expend most of their extra energy in light.

• The various colors available from LEDs are determined by their active elements, such as Gallium, Phosphorus, and arsenic 160aircraft instrumnets


LED – Operation

• When current flows through the chip it produces light which is directly transmitted in proportion to the current flow.

• In the normal 7 segment display format, it is usual to employ LED per segment.

• But the number depends on the overall size of the digits required for the display and its appearance.



LED – 7 segment display



LED Construction



Cont.• LED ‘s can be adopted for the

circular or vertical scale display

• The light intensity of LED column is graded, the highest intensity being provided at the leading edge of the column

• To vary the color of the light By varying the proportion of GaAs

&Gap, or By doping with other elements like

oxygen and nitrogen.



Types of LED



II. INSTRUMENT PANELS

AND LAYOUTS

B. INSTRUMENT PANELS AND LAYOUTS


Instrument Panels

• All instruments essential to the operation of an aircraft are accommodated on special panels.

• Panels – Main instrument panel – positioned in front of pilots.

• Common to all types of aircraft

• Known as P1 and P3 panels

• Center instrument panel – P2

Flight engineer’s panel (P4) Overhead - known as P5 On the glare shield – P7 On a control pedestal located centrally between the pilots (P8 &

P9) At the side as well as the back of the pilots’ seats. 167aircraft instrumnets


Panels – material• Alloy of sufficient strength and rigidity to

accommodate the required number of instruments;• Attachment Requirement – a panel or an individual

instrument should be easily installed and removed;• Instrument panels may be of

the single unit type or made up of two or three sub-panels assemblies, and are supported on shockproof mountings since they

accommodate the flight instruments and their sensitive mechanisms.



Panel mountings

• All panels are mounted in the vertical;• In some aircraft types the practice of sloping main

instrument panels forward at about 150 from the vertical is adopted to minimize parallax errors.



Color of panels

• Instrument and control panels were painted black.• Then changed into matt grey finish, which apart from

its “softer” effects, provides a better contrasting background for instrument dials.

• Currently an additional panel color is the use of light brownish color.



Properties of Instrument Panels

• They have to be light in weight and must have sufficient strength to accommodate flight instruments.

• They are non- magnetic. • They are attached in such a way that they can be

installed and removed easily.• The vibration characteristics of instrument panels

shouldn’t be in position to impair seriously of the instruments or to damage them.



Boeing 767



Boeing 747 Cockpit



A380 Cockpit



B787 Cockpit




AND LAYOUTS

C. INSTRUMENT GROUPINGS


Groupings of instruments

• There are two ways of categorizing aircraft instruments:Categories according to Application.Categories according to means of Operation.



Categories according to application

• Powerplant InstrumentsProvide information related with the power plant (engine)

of the aircraft.

• Flight InstrumentsProvide information related with altitude, speed, etc.

• Navigation InstrumentsProvide information required for navigating the aircraft in

space and while landing and ascending.

• Systems InstrumentsThese concern aircraft systems such as electrical,

hydraulic, pressurization, bleed air systems, etc.178aircraft instrumnets


Categories according to operation

• Mechanical InstrumentsUse mechanical system to obtain and/or transmit

information.

• Gyro InstrumentsUse principles of gyroscope and are primarily used for IFR

flight.

• Electrical and electronic instrumentsAdvances in electronics and digital technology



Flight and Navigational

Instruments Group• Flight and Navigational instruments are composed of:

Pitot - static instrumentsGyroscopic instruments Magnetic Compass



Flight and Navigational Instruments

• Altimeter• Airspeed Indicator• Vertical Speed Indicator• Attitude Indicator (Gyro Horizon)• Turn and Bank (Rate of Turn) Indicator• Directional gyro• Magnetic Compass• Outside Air Temperature Indicator• Clock

Pitot Static Instruments

Gyroscopic Instruments



Flight Instruments• There are six flight instruments:

Airspeed indicator,Altimeter,Gyro horizon,Direction indicator,Vertical speed indicator, andTurn and bank indicator

• Proper grouping is forMaintaining co-ordination to create a ‘picture’ of an

aircraft’s flight condition,Assisting the pilot to observe the instruments with

minimum effort. 182aircraft instrumnets


Types of grouping

• Basic six layoutGyro horizon at top centre position, and considered as the

master instrument.The airspeed indicator, altimeter and vertical speed

indicators flank the gyro horizonThe direction indicator is position directly below the gyro

horizon.The turn and bank indicator then will be mounted near to

the direction indicator as a means for secondary reference instrument for heading changes.






Basic Six Grouping



Basic ‘T’

• There are four key instruments:Airspeed, gyro horizon and altimeter form the horizontal

bar, andThe direction indicator form the vertical bar.At the side of the direction indicator less specifically

essential flight instruments may be mounted.



Basic ‘T’



Power plant Instruments

• The specific grouping of instruments required for the operation of power plants is governed by:Type of power plant,Size of aircraft, andSpace available



Grouping

• For single engine aircraft, facing the pilot is a good enough arrangement.

• For twin engine aircrafts, the instruments are grouped at the centre instrument panel, between the two groups of flight instruments.

• The position of power plant instruments are arranged so that those relating to each power plant correspond to the power plant positions as seen in plan view.



Grouping of PP Instruments



Grouping cont…



Grouping cont…

• On large multi-engine aircrafts, a flight engineer’s station is provided and all the power plant instruments are grouped at the control panels at this station.

• Those instruments measuring parameters required to be used by a pilot during takeoff, cruising and landing are duplicated on the main centre instrument panel,RPM,Turbine temperature, etc.



Flight engineer’s station



B747 Power plant instruments




AND LAYOUTS

D. METHODS OF MOUNTING INSTRUMENTS


Types

• There are two common types used for panel mounting of instruments:Flanged case method, andClamp method



Flanged case method

• Requires the use of screws inserted into locking nuts which, in some instruments, are fitted integral with the flange.

• Could be either front mounted or rear mounted on the panel.



Flange mounting



Clamp mounting

• The cases of some instruments are flangeless permitting them to be mounted from the front of the instruments panels.

• Special clamps are provided at cutout locations to secure each instrument, circular or square.

• Clamping of the instruments is effected by rotating adjusting screws which draw the clamp bands tightly around the case.



Clamp mounting




AND LAYOUTS

E. ILLUMINATION OF INSTRUMENTS AND INSTRUMENT PANELS


Lighting

• Adequate illumination of instruments and the panels to which they are fitted is important during night flight or under adverse conditions of visibility.

• The color chosen for lighting was red, and currently has been replaced with white lighting.

• There are three types of lighting methods:Pillar lightingBridge lightingWedge type lighting



Pillar lighting

• Provides lighting for individual instruments and controls on the various cockpit panels.

• Consists of miniature center contact filament lamp inside a housing which is a push fit into the body of the assembly.

• Light is distributed through a red filter and an aperture in the lamp housing.

• The shape of the aperture distributes the sector of light which extends downwards over an arc of approximately 900 to a depth slightly less than 2 in from the mounting point. 205aircraft instrumnets


Pillar Light



The Bridge Lighting

• Is a multi-lamp development of the pillar lighting.• Two or four lamps are fitted to a bridge structure

designed to fit over a variety of the standardized instrument cases.

• The bridge fitting is composed of two light alloy pressings secured together by rivets and spacers carrying the required number of center contact assemblies above which the lamp housing is mounted.



The Bridge Lighting



Wedge Type Lighting• This method of instrument

lighting derives its name from the shape of the two portions which together make up the instrument cover glass.

• It relies for its operation upon the physical law that the angle at which light leaves a reflecting surface equals the angle at which it strikes that surface.

• The two wedges are mounted opposite to each other and with a narrow airspace separating them.



Cont.

• Light is introduced into Wedge A from two 6V lamps set into the wedge by its polished surfaces ,

• The angle at which the light rays strike the wedge surface governs the amount of light reflected – the lower angle the more light reflected.



Cont.

• The double wedge mechanically changes the angle at which the light rays strike out one of the reflecting surfaces of each wedge,

• thus distributing the light evenly across the dial and also limiting the amount of light given off by the instrument.



Cont.

• Light escaping into Wedge B is confronted with constantly decreasing angles and this has the effect of trapping the light within the wedge and directing it to its wide end.

• Absorption of light reflected into the wide end of wedge B is ensured by painting its outer part BLACK.



INSTRUMENT ELEMENTS

AND MECHANISMS

A. INSTRUMENT ELEMENTS


Instrument Elements• From the operating point of view, an instrument is

made up of the following four main elements.1. Detecting element – detects changes in value of the

physical quantity or condition presented to it;

2. Measuring elements – measures the value of the physical quantity or condition in terms of small translational or angular displacement;

3. Coupling elements – by which displacements are magnified and transmitted; and

4. Indicating element – exhibits the value of the measured quantity transmitted by the coupling element, by the relative positions of a pointer, or index, and scale.



Elements of an instrument



INSTRUMENT ELEMENTS

AND MECHANISMS

B. INSTRUMENT MECHANISMS


Mechanism in pitot-static tube


Mechanisms

• Refers to all four elements as a composite unit and contained within the case of an instrument.

• However, because of the operating principles and the construction, the above definition of the term mechanism applies to only very few instruments.



Composite Unit of Mechanical Elements



Separated Mechanical Elements



Coupling Elements• There are classes of mechanisms that are based on the principles of:

Levers, and Rods Gears

• In aircraft instrument applications, such lever and rod mechanisms are confined principally to Direct reading pressure gauges, and Pitot static flight instruments.

• In the majority of applications to aircraft, a separation of some of the elements is necessary so that three or maybe only two, elements form the mechanism within the instrument case.



Simple lever mechanism – bourdon tube

• The bourdon tube forms Detecting element, and Measuring element

• A simple link, lever, quadrant and pinion form Coupling element

• Pointer and scale Indicating element

• The lever mechanism is the complete coupling element.



Cont…

• The lever type mechanism uses two terms that are related to the movement and calibration of the indicating element:Lever length:- the distance between the point of operation

of measuring element and pivoting point of the lever.Lever angle:- is the angle between the lever and the link

connecting it to the measuring element.



Lever length



Lever angle



Rod Mechanisms

• Unlike pure lever mechanisms, rod mechanisms dispense with pin or screw-jointed linkages for the interconnection of component parts, and

• rely on rods in contact with, and sliding relative to, each other for the generation of the input/output relationship.

• Contact between the rods under all operating conditions is maintained by the use of a hairspring which tensions the whole mechanism.



mechanism


Cont…

• Rod mechanisms are widely applied in flight instruments.

• They are classified into three classes, named after the trigonometrical relationships governing their operation:The sine mechanism,The tangent mechanism, andThe double-tangent mechanism.



A. Sine Mechanism

• h2- h1= r( sinθ2- sinθ1)

• It is employed in certain types of ASI, as the first stage of the coupling element.



B The Tangent mechanism

• h2 - h1 = d( tanθ2 -tan θ1)

• The point of contact between the two rods remains at a constant perpendicular distance d from the center of the rocking shaft.



Double tangent mechanism.

• It is employed where rotary motion of a shaft is to be transferred through a right angle .

• Typical application is as the 2nd stage of an airspeed indicator coupling element and for the gearing of the indicating element.



C)The double tangent mechanism

• d(tanθd1 - tanθd2) =f (tanθc1-tanθc2



THE GEARS

• PURPOSE: Are used for the conversion of straight line or arc like motion into full

rotary motion. Are used for increasing and decreasing motion.

• TYPES OF AIRCRAFT GEARS

• 1 Sector Gears needs only few degrees to rotate the pinion through a complete

revolution..

• 2 Pinion Gears Provide the coupling of the sector gear with other mechanisms.



Gears



Backlash• It is the loss of motion in the gear connection,• because a gear can turn a small amount before it will drive the

one in mesh with it.• It is commonly referred to as the “play”, as it is the

dimensioning of the gear teeth which causes play between gear meshes to avoid jamming of gears.

• This undesirable effect of backlash can be controlled by two methods: Using coiled hairsprings – the spring due to tensioning always has a

tendency to unwind so that the inherent play between gear teeth is taken up and they are maintained in contact, or

Using “anti backlash gear” - Two gears mounted freely face to face on a common hub and interconnected with each other.



Hairspring

• Hairsprings are precision-made devices which serves as, Anti-backlash device, Controlling devices against which deflecting forces are balanced to

establish required calibration laws, and For the restoration of coupling and indicating elements to their original

positions as and when deflecting forces are removed.

• It is the flat coil type of wire with the inner end attached to the collet on the pointer shaft or gear shaft, and the other end of the spring is anchored to the frame.



Coiled hairspring



Zero adjustment

• A fork is an integral part of the pole and engages with the pin eccentrically.

• When a zero adjusting screw externally is rotated, • it deflects the plate thus rotating the spring, shaft and

pointer to a new position without altering the torque loading of the spring.

• Hairsprings are made from:Phosphor bronze, andBeryllium-copper



hairspring



INSTRUMENT ELEMENTS

AND MECHANISMS

C. TEMPERATURE COMPENSATION OF INSTRUMENT MECHANISMS


Temperature Compensation Of

Instrument Mechanisms

• In the construction of instrument mechanisms, various metals and alloys are used and,

• changes in the physical characteristics can occur with the changes in the surrounding temperature.

• As a result this change would introduce indication errors on the instruments.



Temperature compensation• It becomes necessary to neutralize this change in

temperature, through a method known as temperature compensation.

• The methods adopted for temperature compensation are varied depending on the type of instrument to which they are applied:

i. A bi-metal strip method

ii. Thermo-resistance method, and

iii. Thermo-magnetic shunt method



i. Bi-metal Strip Method

• It is the oldest method of compensation.• It is applied to instruments such as, air speed

indicators, altimeters, VSI & EGT.



CONSTRUCTION• It is made of two strips of

metals:

INVAR – (It is an alloy of metal containing 36% nickel. (It has negligible coefficient of expansion).

BRASS OR STEEL:It has high linear coefficient of expansion



OPERATION

• When the strip is subjected to an increase or a decrease in temperature the brass will expand or contract.

• The principal effect of temperature changes on pitot static instruments is the expansion and contraction of the capsule, thus tending to make over read or under read.

• Therefore, to compensate for temperature variations, as the ranging bar is in contact with the sector gear arm the increase in temperature indicating element has the tendency to over read



Cont…

• The increase in temperature has a simultaneous effect on the ranging bar which being a bi- metal and,

• on account of the position of the INVAR portion it will sag or deflect in the direction indicated by the dotted arrow,

• thus counteracting the capsule expansion and keeping the indicating element at a constant reading.



Bi-metal strip method


Thermo-resistance Method• A thermistor , is composed of a mixture of metallic

oxides and has very large temperature coefficients of resistance which is usually negative .

248

temp

resistance


Thermo Resistance Method• The rise in temperature of the indicator increases the

resistance of the components,

• resulting in the reduction of the current flowing through the indicator and hence the indicator under-reads.

• Thermister resistance, on the other hand, decreases with the increase in temperature,

• so that for the same temperature change the resistance will balance out to maintain a constant current and ,

• therefore a constant indication of the quantity being measured is indicated.



Thermo-resistance method

• Its resistance decreases with the increase in temperature.



Thermo-magnetic shunt method • It is an alternative to thermistor method and,

• it is commonly employed on temperature gages.

• The vital part of this method is the strip made from nickel-iron alloy is sensitive to temperature changes.

• The strip is clamped across the poles of the permanent magnet.



Thermo magnetic shunt Cont.• Let us assume that the indicator temperature increases.

• the moving coil resistance increases as well as the strip’s magnetic reluctance.

• Since the deflecting torque exerted on the moving coil is proportional to the product of the current and flux , the increased air gap flux counterbalances the reduction in current to

maintain a constant torque and indicated reading.

• Depending on the size of the permanent magnet, the number of strips that may be fitted to effect the required compensation are determined.



Thermo-magnetic shunt

method



End of Week 1

Ethiopian Aviation Academy

Documents

text book aircraft instruments

illumination of instruments

instrument mountings

instrument classification

mechanisms chapter

types of mechanisms

layouts chapter

standards chapter