Aircraft Instruments
This domain tests pitot-static instruments, gyroscopic flight instruments, magnetic compass behaviour and the engine and system indications used in normal and abnormal operation. Questions often require you to predict an indication after a blockage, a setting change or a failure.
Pitot-static group
The airspeed indicator compares pitot pressure with static pressure. The altimeter is a sensitive barometer calibrated to the standard atmosphere; the subscale setting determines the reference pressure. The vertical speed indicator responds to the rate of change of static pressure and lags briefly after a climb or descent begins. A blocked pitot tube with an open drain can cause the ASI to behave like an altimeter; a blocked static source affects ASI, altimeter and VSI.
Gyroscopic instruments
Rigidity in space and precession explain attitude and heading indications. The attitude indicator and heading indicator are usually vacuum- or electrically driven; the turn coordinator uses a rate gyro and shows rate of turn and coordination. A vacuum failure commonly affects the attitude and heading indicators, not the electric turn coordinator.
Compass and engine instruments
The magnetic compass suffers variation, deviation, turning error and acceleration error. In the Northern Hemisphere, acceleration on an east or west heading produces an apparent turn toward north; deceleration toward south. Tachometers, EGT gauges, oil pressure and temperature gauges and fuel quantity indicators have their own sender and power requirements.
Worked example
An altimeter subscale is changed from 1013 hPa to 1000 hPa while the aircraft remains at the same true altitude. The change is 13 hPa. Using approximately 27 ft per hPa near sea level, the indication falls by 13 × 27 = 351 ft, so the altimeter reads about 350 ft lower.
Traps
Do not apply the altimeter-setting rule without noting whether pressure is rising or falling. VSI lag is normal and is not a failure. Compass turning and acceleration errors are not instrument defects. A blocked static source does not leave the ASI unaffected.