Instrument Failure, Degradation and Warnings
Domain 11 pulls the course together: what happens when a sensor, computer or display fails, what the crew sees, and how the aircraft degrades safely. The exam favours the classic pitot-static failures, display reversion, and the logic of redundancy, voting and warning prioritisation.
Blocked pitot and static sources
- Pitot blocked, static clear - in a climb the static pressure falls, so q appears to rise and the ASI over-reads like an altimeter; in a descent it under-reads. The altimeter and VSI remain correct.
- Static blocked - the ASI behaves like a rough altimeter relative to the trapped pressure; the altimeter freezes and the VSI reads zero. Compare with the standby instruments and GNSS altitude.
Unreliable airspeed is a memory item: pitch and thrust to a known attitude, then cross-check before trusting any single source.
Other failures
- Attitude and heading: compare with the standby attitude indicator and the magnetic compass; an IRS cannot be fully realigned in flight, so it may be placed in attitude mode.
- Display failure: automatic or manual reversion to the remaining display units; loss of a DMC forces a different display configuration.
- GNSS loss: RAIM alert, reversion to radio navigation or INS.
- Autopilot disconnect: aural warning plus red master warning; the aircraft reverts to manual flight.
Redundancy concepts
Fail-passive - a failure leaves the system usable but degraded (the autopilot drops out and the aircraft remains controllable). Fail-operational - the system continues after a failure, as required for autoland. Voting (two-out-of-three) lets a system identify and reject a faulty source. The MEL defines what may be inoperative for dispatch.
Worked example
Climbing through FL200, the ASI climbs steadily while altitude and vertical speed behave normally, and the standby ASI shows a normal speed. The pitot head is blocked but the static port is clear; pitot heat was probably left off in icing. The crew should suspect the primary ASI, not the altimeter.
Traps that cost marks
- Mixing up the pitot-blocked and static-blocked signatures.
- Forgetting that the standby instruments are the primary cross-check.
- Assuming a failed IRS can be realigned in flight.
- Treating a caution-level failure as dispatchable without checking the MEL.