Definition
A small, self-contained engine -- usually a small gas turbine -- installed on an aircraft to provide electrical power, compressed air, and sometimes hydraulic power independently of the main engines. It allows the aircraft to operate its systems on the ground with the main engines shut down, and to start the main engines without external ground equipment.
Plain English
A small extra engine on the aircraft that runs the electrics, air conditioning, and engine-starting air when the main engines are off. It lets the aircraft power itself on the ground without needing a ground cart, and it can also start the main engines.
Context Anchor
Pilots encounter this term during ground operations, engine start procedures, electrical system discussions, and aircraft systems checks.
Derivation
Auxiliary comes from the Latin auxilium, meaning help or aid. The APU is literally a 'helper power unit' -- it isn't what flies the aircraft, but it supports the systems that do.
Why Pilots Care
It allows systems to operate and engines to start on the ground without running the main engines, saving fuel, reducing wear, and providing emergency electrical power in flight if needed.
Analogy
Think of it like a small generator in an RV. The main engine moves the vehicle, but the small generator keeps the lights, AC, and outlets running when you're parked.
Intuition Check
Do not think of an Auxiliary Power Unit as an extra engine for pushing the aircraft through the air. It supplies power to aircraft systems; it is not used as the aircraft’s main source of thrust.
Example Sentence 1
Before requesting pushback, the crew started the APU so the cabin would stay cool after the ground power cart was disconnected.
Example Sentence 2
With the auxiliary power unit running, the cabin stayed cool during the long ground delay.