SkyDrill

What an APU supplies on the ground

Aviation Mechanic Powerplant (AMP) practice question 7 of 13 on Turbine Engines. ACS element AM.III.B.K9 · difficulty: recall. Want to answer it in a timed quiz? Use the Aviation Mechanic Powerplant (AMP) practice mode.

Question bank updated 2026-09-22

QUESTIONOn a turbine-powered aircraft, what does an auxiliary power unit provide while the aircraft is on the ground?

  1. AElectrical power and hydraulic pressure
  2. BElectrical power and bleed air
  3. CBleed air and engine lubricating oil
Show the answer and explanation

Correct answer: B. Electrical power and bleed air

  • A — incorrect The APU's ground outputs are described as electrical power and bleed air, not hydraulic pressure.
  • B — correct An APU provides electrical power and bleed air on the ground, and serves as a backup generator in flight.
  • C — incorrect An APU does not supply lubricating oil to the main engines; its other ground output is electrical power.

BACKGROUND

A turboshaft engine is a gas turbine engine built to transfer horsepower to a shaft, either to turn a helicopter transmission or to serve as an onboard auxiliary power unit. The APU is used on turbine-powered aircraft to provide electrical power and bleed air on the ground and to act as a backup generator in flight. On multi-engine aircraft, the engines are usually started by an onboard APU that supplies air pressure for a pneumatic starter on each engine. Air bled from the APU is therefore the normal source of power for starting the engines. An engine can also be started from an external air power source or from an already operating engine.

STUDY TIP

Think of the APU as a small turboshaft with two ground products, electrical power and bleed air, plus a backup electrical role in flight. Connect the bleed air output directly to pneumatic engine starting.

KEY POINTS

  • An APU is a turboshaft engine installed onboard the aircraft.
  • On the ground it supplies electrical power and bleed air; in flight it is a backup generator.
  • APU bleed air commonly drives the pneumatic starter on each main engine.

SOURCES

  • FAA-H-8083-32B ch01 "Gas Turbine Engines, Types & Construction"
  • FAA-H-8083-32B ch10 "Turbine Engine Operation"
Practice the Aviation Mechanic Powerplant (AMP) test →Timed 100-question exam, by ACS area, or review your misses. Free.
MORE ON TURBINE ENGINES

SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-32B Aviation Maintenance Technician Handbook – Powerplant, and 14 CFR part 43. Regulations and handbooks are revised; confirm details against the current edition.