Definition
Small onboard engines, typically gas turbines, fitted to many aircraft to supply electrical power and compressed air independently of the main engines. APUs allow systems such as avionics, lighting, air conditioning, and engine starting to operate while the aircraft is on the ground with the main engines shut down, and can also serve as a backup power source in flight on some aircraft.
Plain English
An APU is a small extra engine inside the aircraft whose only job is to make electricity and air pressure for the plane's systems, so the main engines don't have to be running to keep the lights, instruments, and air conditioning on.
Context Anchor
Seen in aircraft systems discussions, engine-start procedures, and operations at the gate or on the ramp before the main engines are started.
Derivation
Auxiliary comes from the Latin auxilium, meaning 'help' or 'support.' The name fits the role: the APU is a helper engine, not the one that flies the airplane.
Why Pilots Care
Enables independent operations at airports without ground power units and supports safe engine starts and cabin conditioning.
Analogy
An APU is like a built-in generator for the aircraft. It does not fly the airplane forward, but it can keep important systems powered when the main engines are not doing that job.
Intuition Check
Do not assume an APU is another engine used for thrust. It is mainly a helper power source for aircraft systems, not a propulsion engine.
Example Sentence 1
Before starting the main engines, the crew started the APU to power the avionics and provide air for engine start.
Example Sentence 2
With the APU running, the aircraft maintained cabin air conditioning during a long ground delay.