FAA Instrument Flight Instructor (FII)Instruments and SystemsMedium
During preflight, a pilot notes that the aircraft's vacuum pump has failed. In an aircraft with a conventional vacuum-driven gyro system and electric turn coordinator, which flight instruments will become unreliable?
- Amagnetic compass and airspeed indicator
- Battitude indicator and heading indicator
- Cairspeed indicator and altimeter
- Dturn coordinator and vertical speed indicator
Show answer & explanationAnswer & explanation
Correct answer: B. attitude indicator and heading indicator
In most light aircraft, the engine-driven vacuum pump powers the attitude indicator and heading indicator gyros, while the turn coordinator is electrically powered and the airspeed indicator, altimeter, and VSI rely on the pitot-static system. A vacuum failure therefore renders the attitude and heading indicators unreliable while the other instruments remain functional.
Why the other options are wrong
- A. The magnetic compass is independent of any powered system, and the airspeed indicator is pitot-static driven.
- C. These instruments are pitot-static driven, not vacuum-driven, so they are unaffected.
- D. The turn coordinator is electric and the VSI is pitot-static; neither depends on vacuum.
Vacuum System Instruments
In conventional light aircraft, the vacuum system typically powers the attitude indicator and heading indicator, while the turn coordinator is electric and the pitot-static instruments are independent.
- Vacuum pump is usually engine-driven
- Powers attitude indicator and heading indicator gyros
- Turn coordinator is electrically powered as a backup source
- Vacuum failure is often first detected via a suction gauge or annunciator light
Memory trick: Vacuum feeds the 'A' and 'H' — Attitude and Heading, nothing else.