FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsHard
During a checkride debrief, a CFI explains to an applicant that a failed vacuum pump in a conventionally equipped light airplane would most directly affect which flight instruments?
- AThe turn coordinator and magnetic compass
- BThe airspeed indicator and altimeter
- CThe vertical speed indicator and turn coordinator
- DThe attitude indicator and heading indicator
Show answer & explanationAnswer & explanation
Correct answer: D. The attitude indicator and heading indicator
In most conventionally equipped light airplanes, the attitude indicator and heading indicator are driven by vacuum-powered gyros, while the turn coordinator is typically electrically powered and the airspeed indicator, altimeter, and VSI are pitot-static instruments unaffected by vacuum failure. A vacuum pump failure therefore causes the attitude and heading indicators to slowly spin down and become unreliable.
Why the other options are wrong
- A. The turn coordinator is electric, and the compass is magnetic; neither depends on vacuum.
- B. These are pitot-static instruments, independent of the vacuum system.
- C. The VSI is pitot-static, and the turn coordinator is electric, not vacuum-powered.
Vacuum System Instruments
In most light airplanes, the attitude indicator and heading indicator are powered by engine-driven vacuum pumps.
- Turn coordinator is usually electric as a backup gyro source
- Vacuum pump failure causes gradual, insidious instrument errors
- Pitot-static instruments are unaffected by vacuum failure
Memory trick: 'Vacuum powers the Attitude and Heading pair—everything else has its own supply'