Aircraft General Knowledge: Instrumentation
Pitot-static instruments
Understand the altimeter, airspeed indicator, and vertical speed indicator. The altimeter measures static pressure against a set subscale; know the difference between indicated, calibrated, true, and pressure altitude. The ASI measures dynamic pressure (total minus static); know IAS, CAS, EAS, and TAS and their corrections. The VSI measures rate of change of static pressure with a lag.
Gyroscopic instruments
Know the attitude indicator and heading indicator as rigidity-in-space instruments, and the turn coordinator/turn-and-slip as rate instruments. Understand the difference between a real and apparent drift, and between a turn indicator and a turn coordinator. Know the sources: vacuum or electric, and the consequences of a failed source.
Compass and magnetic errors
Know the magnetic compass, deviation, variation, and the turning and acceleration errors. The compass is the only independent heading source in many aircraft; know when to trust it and when not.
Modern displays and warnings
Know the EFIS, EICAS/ECAM, PFD, ND, and the meaning of colour coding (red warning, amber caution, green normal). Understand the master warning and master caution philosophy and the difference between a warning and a caution.
Traps
Questions confuse pressure altitude with density altitude, indicated with true airspeed, and gyroscopic drift with compass error. Another favourite: which instrument uses which source (pitot, static, or both).
Worked example
At an airfield elevation of 500 ft with QNH 1013 hPa, the altimeter reads 500 ft. If QNH falls to 1003 hPa (10 hPa lower), the altimeter over-reads by about 10 × 30 = 300 ft, so it would indicate 800 ft while the aircraft is still on the ground. Rule of thumb: 1 hPa ≈ 30 ft near sea level.