Aircraft General Knowledge: Instruments and Electronics
Instruments and Electronics tests pitot-static and gyroscopic instruments, magnetic sensing, air data computers, EFIS, flight management, autopilot and warning systems. Questions focus on indications, failure modes and interpretation under IMC, including reversionary display modes.
Core rules
- Pitot pressure feeds the ASI; static pressure feeds ASI, altimeter and VSI. A blocked pitot affects airspeed; a blocked static affects all three.
- Altimeter indicates altitude above the set pressure datum: QNH gives elevation, QFE gives height above aerodrome, standard gives pressure altitude.
- Gyros: attitude and heading references depend on rigidity and precession; erection systems can be unreliable in prolonged turns or acceleration.
- The air data computer converts pressure and temperature into air data; EFIS displays depend on valid ADC and inertial reference inputs.
- Autopilot modes have armed, captured and active states; disconnect and warning behaviour must be understood.
Worked example
Actual QNH is 1003 hPa but the altimeter is set to 1013 hPa. Near sea level 1 hPa is about 27 ft, so the altimeter reads about (1013 minus 1003) times 27 = 270 ft too high. The aircraft is lower than indicated; correct QNH must be set before instrument approach.
Traps
- Reversing the direction of the altimeter error.
- Reading VSI lag as an immediate trend.
- Trusting an attitude indicator after a long acceleration or turn without cross-check.
- Confusing heading indicator drift with magnetic variation.