Aircraft General Knowledge – Instrumentation
Instrumentation tests pitot-static and gyroscopic instruments, magnetic compass, air data computers and EFIS/warning systems. Questions often give a blockage or setting error and ask what the pilot sees.
Pitot-static
The ASI compares pitot (total) pressure with static pressure. The altimeter senses static pressure against a set subscale; the VSI measures rate of change of static pressure. A blocked pitot leaves the ASI behaving like an altimeter in climb and descent. A blocked static port freezes the VSI, makes the ASI over-read in climb, and causes altimeter error.
Gyros and compass
Attitude and heading gyros use rigidity and precession; an erect gyro needs toppling and erection. The magnetic compass suffers acceleration and turning errors, and deviation is aircraft-specific. EFIS displays combine ADC and AHRS data; compare standby instruments after a failure.
Worked example
QNH at the airport is 1013 hPa; you set 1003 hPa by mistake. The 10 hPa error is about 10 × 30 ft = 300 ft. On the ground the altimeter reads 300 ft too high, so you would fly 300 ft low on approach if uncorrected.
Traps
Mixing static and pitot blockage symptoms; forgetting that VSI lags; using QFE instead of QNH; ignoring compass turning errors in the northern hemisphere.