AIRCRAFT INSTRUMENTS AND THEIR USAGES.ppt

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

1. AIRCRAFT INSTRUMENTS

2. LAY OUT OF AIRCRAFT
INSTRUMENT IN THE COCKPIT

3. 4 PRINCIPALS OF AIRCRAFT
INSTRUMENT ELEMENTS
1.DETECTING – which detects changes in value of the
physical quantity.
2. MEASURING – which measures the value of the
physical quantity.
3. COUPLING – by which displacements are magnified
and transmitted.
4. INDICATING – which exhibits the value of the
measured quantity transmitted by the
coupling element and to the pointer.

4. DETECTING
ELEMENT
MEASURING
ELEMENT
COUPLING
ELEMENT
INDICATING
ELEMENT
FIGURE I – Elements of an instrument
AIR SPEED INDICATOR

5. MECHANISMS ELEMENTS
•The detecting element always separated from the
three other elements.
•The detecting element is located outside of
casing.
•The measuring, coupling, and indicating are
located in the instrument casing.
It refers to all four elements as a composite unit
and contained within the case of an instrument

7. Class of Mechanisms
•Lever – simple link, lever, quadrant and
pinion forms the coupling element.

8. Figure 3 Simple lever mechanism
a) Effect of lever length
b) Effect of lever angle
on magnification.

9. • Rod – the used of pin or screw jointed
linkages for the interconnection of
component parts. The hairspring is used
to maintain the application tensions to the
connecting rods.
Note: These type of mechanism is widely
used in the flight instruments.

10. THREE TYPE OF ROD MECHANISMS
THE SINE MECHANISM
THE TANGENT MECHANISM THE DOUBLE-TANGENT MECHANISM

11. GEARS
Gear is used to
•Direct conversion of straight line or arc line
motion in full rotary motion.
•Increasing or decreasing the motion.

12. GEARS
Figure 5 Gear assembly for a multi-pointer indicating element
SECTOR GEAR ACTUATED
BY MEASURING AND
COUPLING ELEMENT
MESHING PINION

13. Hairsprings
•It is made up of phosphor bronze or
beryllium-copper.
Purposes
•It used to prevent backlash.
•As a controlling devices.
•To restoration of coupling and indicating
element to the original position when
deflecting forces are removed.

14. Forces acting of Hairspring
•Damping Force –
•Controlling Force –
•Deflecting Force –

15. Method of attachment
•The inner end fixed to a pointer or gear
shaft.
•The outer end being anchored to an
adjacent part of the mechanism
framework.
•The anchoring method permit the certain
degree of spring torque adjustment or
initial setting can be done.

16. THE INNER END
IS FITTED ON POINTER
OR GEAR SHAFT
TAPER PIN
ANCHOR POST ON
MECHANISM FRAME
Figure 6 (a) Method of attachment of hairspring
THE OUTER END
IS ANCHORED ON
FRAME

17. Method of adjustment
•When the screw is rotated it deflects the
fork and rotate the spring, shaft and
pointer to a new position without altering
the torque loading of the spring.

18. POINTER
HAIRSPRING
FRICTION LOADING
JEWELED
PIVOT SCREW
ZERO ADJUSTING SCREW
LOCATED ON BAZEL OR FRONT COVER
Figure 6 (b) Method of zero adjustment of hairspring
FORK
PIN

19. Temperature Compensation
•Temperature can cause the change in
physical characteristic of a metal.
•It will cause an error on indication.
•To neutralize it, several method of
compensation is applied.

20. Bimetal-strip Method
•Consists of two metals joined together to
form a single strip.
•Invar and brass (steel) are used.
•Invar is a form of steel with a 36% nickel
content and have a negligible coefficient of
linear expansion.
•Brass (steel) has high linear expansion
coefficient.
•Not complete yet

21. Theory of operation of
Bimetal-strip
•Increase in temperature the brass or
steel will expand.
•Will contract when the strip is subjected to
a decrease in temperature.
•Therefore the invar will try to maintain the
same length and being firmly joined to
other metal.
•Will cause the whole strip to bend.

22. ROCKING SHAFT
PIN
SECTOR GEAR
POINTER
CAPSULE
Figure 7 Application of bimetal-strip
INVAR STRIP

23. Thermo-resistance Method
•This method of compensation used a thermo-
resistor or thermistor connected in the
indicator circuit.
•The thermistor consisted of mixture of
metallic oxide which has negative
temperature coefficient.
•Increase in temperature decrease in
resistance. It allowed more current to flow.

24. Thermo-magnetic Shunt
Method
•Some temperature measuring instrument utilize
a device known as a thermo-magnetic shunt.
•This is a strip of nickel-iron alloy sensitive to
temperature changes.
•The strip is clamped across the poles of the
permanent magnet so it divert the air gap
magnetic flux through itself.

25. Sealing Of Instrument
Against Atmospheric Effects
•Some indicator using external atmospheric pressure as a
datum.
•To prevent inaccuracies of reading of indication after air
cabin entered their case.
•The cases therefore sealed to withstand external
pressure.
•Many instrument filled in the case by nitrogen or helium
gas to protected against the adverse effects of
atmospheric temperature, pressure and humidity and
case is sealed.

26. Questions
Instrument Elements and Mechanism
• State the meaning of the term back lash.
• State three function of hair spring.
• State the material used to make the hair spring.
• Name three methods for temperature compensation of instrument mechanism.
• State the different between invar and brass.
• What is thermistor.
• State the important of sealing of an instrument from atmosphere effect.
Instrument Display
• What is the advantage and disadvantage of straight scale and circular scale.
• What is platform scale.
• What is number alpha numeric display.
Altimeter
• State the purpose of Altimeter.
• State type of diaphragm used in Altimeter.
• State the working principle of Altimeter.
• Name three type of Altimeter.
• State the purpose barometric setting knob.
• State the error subject to an Altimeter.
Air Speed Indicator
• State different form of Air Speed reading.
• What types of error that are corrected to give indication of True Air Speed
indicator.