FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesEasy
An aircraft is parked at the gate with both engines shut down. Ground crew reports that cabin air conditioning and electrical power are being supplied without a ground cart connected. What onboard component is most likely providing this power?
- AThe ram air turbine (RAT)
- BThe auxiliary power unit (APU)
- CThe battery bus alone
- DThe main engine starter-generator
Show answer & explanationAnswer & explanation
Correct answer: B. The auxiliary power unit (APU)
The APU is a small self-contained gas turbine engine mounted in the aircraft (typically in the tail) that supplies electrical power and bleed air for air conditioning and engine starting when the main engines are not running.
Why the other options are wrong
- A. The RAT is an emergency device deployed only in flight during power loss.
- C. A battery alone cannot sustain air conditioning loads for extended periods.
- D. Main engine starter-generators only function when the main engines are running.
Auxiliary Power Unit (APU)
A small gas turbine engine installed on the aircraft that provides electrical power and bleed air for air conditioning and engine starting independent of ground support equipment.
- Self-contained small turbine engine
- Supplies electrical and pneumatic power
- Used for main engine starting via bleed air
- Typically mounted in the tail cone
Memory trick: APU = the airplane's own little engine, always ready