Aircraft General Knowledge: Instrumentation
What the exam tests
Instrumentation questions cover pitot-static instruments, gyroscopic instruments, magnetic compass, air data computers, electronic flight displays, autopilot and flight director, and warning systems such as GPWS and TCAS. The exam tests interpretation, failure modes and limitations.
Core instruments
- ASI uses pitot pressure and static pressure; a blocked pitot tube can cause the ASI to behave like an altimeter, while a blocked static source affects ASI, altimeter and VSI.
- Altimeter measures static pressure; it indicates altitude above the set pressure datum. QNH gives elevation above mean sea level; QFE gives height above the reference datum.
- VSI is rate of change of static pressure and lags during abrupt manoeuvres.
- Attitude and heading indicators use gyroscopes; vacuum or electric drive failure causes unreliable attitude or heading.
- Magnetic compass has turning and acceleration errors; it is reliable only in steady, level flight.
- EFIS displays combine ADC and AHRS data; disagreement between standby and primary instruments requires cross-check.
Worked example
The altimeter is set to 1013 hPa and indicates 620 ft at an aerodrome. The QNH is 1009 hPa. The altimeter subscale is 4 hPa too high, so indicated altitude is about 4 x 30 = 120 ft too high. Setting 1009 hPa makes the altimeter read 500 ft, the aerodrome elevation. The aircraft is not 620 ft above the aerodrome.
Traps
Do not use QFE for en-route levels. Do not chase a failing attitude indicator; use standby instruments and basic flight instruments. Do not treat VSI lag as a trend. Remember that a blocked static port affects several instruments at once.