FAA Sport Pilot Airplane (SPA)Aircraft Performance and SystemsEasy
A light sport airplane's attitude indicator and heading indicator are driven by a vacuum system powered by an engine-driven pump. If the vacuum pump fails in flight, which instruments would most likely become unreliable?
- AAirspeed indicator and altimeter
- BVertical speed indicator and magnetic compass
- CAttitude indicator and heading indicator
- DTurn coordinator and airspeed indicator
Show answer & explanationAnswer & explanation
Correct answer: C. Attitude indicator and heading indicator
The attitude indicator (artificial horizon) and heading indicator (directional gyro) are typically driven by the vacuum system in many light aircraft. The pitot-static instruments (airspeed, altimeter, VSI) and the electrically-powered turn coordinator are unaffected by vacuum pump failure.
Why the other options are wrong
- A. These are pitot-static instruments, unaffected by vacuum failure.
- B. The VSI is pitot-static and the compass is magnetic; neither depends on vacuum.
- D. The turn coordinator is usually electrically powered as a backup, and airspeed is pitot-static.
Vacuum System
An engine-driven pump creates suction to spin gyros in the attitude and heading indicators in many light aircraft.
- Powers attitude indicator and heading indicator
- Turn coordinator is usually electric for redundancy
- Pump failure causes gradual precession/errors in vacuum instruments
- Pilot should cross-check with electric turn coordinator and compass if vacuum fails
Memory trick: Vacuum runs the 'AH' and 'HI' — Attitude & Heading