FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
During troubleshooting, a technician finds that an aircraft's attitude indicator has become sluggish and tumbles shortly after engine start. A test of the vacuum system shows suction below the minimum specification. This finding is most consistent with a fault in which type of instrument system?
- AMagnetic compass system
- BPitot-static instrument system
- CElectric turn coordinator system
- DVacuum-driven gyroscopic instrument system
Show answer & explanationAnswer & explanation
Correct answer: D. Vacuum-driven gyroscopic instrument system
The attitude indicator (and often the heading indicator) on many light aircraft is spun by airflow drawn through the instrument by an engine-driven vacuum pump. Low suction reduces gyro RPM, causing sluggish response and tumbling, unlike electrically driven instruments such as the turn coordinator.
Why the other options are wrong
- A. The magnetic compass has no gyro or vacuum requirement.
- B. Pitot-static instruments (airspeed, altimeter, VSI) use ram air and static pressure, not vacuum.
- C. The turn coordinator is typically electrically driven, not vacuum driven.
Vacuum-Driven Gyro Instruments
Instruments such as the attitude and heading indicators that use airflow from an engine-driven vacuum pump to spin an internal gyro rotor at operating speed.
- Vacuum pump draws air through the instrument to spin the gyro
- Low suction causes sluggish or tumbling gyro response
- Turn coordinator is typically electrically powered instead
Memory trick: Vacuum sucks air to spin the gyro wheel