Aircraft General Knowledge - Instruments
This domain tests how cockpit instruments derive and display speed, altitude, attitude and heading, and whether you can recognise degraded data before it affects the flight.
Pitot-static instruments
The ASI depends on pitot ram pressure and static pressure. With the pitot blocked and static clear, the ASI under-reads in a climb and over-reads in a descent. With the static blocked and the pitot clear, the ASI behaves like an altimeter (over-reading in a climb), the altimeter freezes and the VSI reads zero. The alternate static source usually causes a small over-read of altitude and airspeed.
Gyroscopic instruments
Attitude and heading indicators are gyros, driven by vacuum or electricity; glass cockpits use AHRS. The turn coordinator shows rate of turn and roll and should indicate a standard rate turn of 3° per second. Magnetic compass errors include variation, deviation, dip and acceleration/deceleration errors.
Glass cockpit and redundancy
An air data computer and AHRS feed the EFIS displays. Standby ASI, altimeter and attitude indicator are required, and reversion modes allow comparison. Cross-check sources and decide which one is unreliable.
Worked example
Climbing in IMC, the static port blocks. The VSI stops at zero and the altimeter is frozen, while the ASI over-reads as altitude increases. You level off, select alternate static, cross-check against GPS and standby instruments, declare the problem and divert.
Marks-losing traps
Confusing pitot blockage with static blockage; forgetting pitot heat in icing; using the turn coordinator as an attitude indicator; trusting a single AHRS after a comparator disagreement; ignoring position error and lag.