Definition
A small onboard engine, usually a gas turbine, that provides electrical power and compressed air to an aircraft independently of its main engines. It allows systems such as lighting, avionics, air conditioning, and engine starting to operate while the main engines are shut down or before they are started.
Plain English
A second, smaller engine on the aircraft whose job is to power the airplane's systems when the main engines aren't running.
Context Anchor
Seen in airplane system descriptions, preflight and dispatch discussions, and Configuration Deviation List entries for APU doors, inlets, exhausts, or related equipment.
Derivation
Auxiliary comes from the Latin auxilium, meaning help or support. The APU is literally a helper power unit — it supports the airplane when the main engines can't or shouldn't.
Why Pilots Care
Enables independent ground operations and reliable engine starts, reducing delays and dependence on airport ground equipment.
Analogy
An APU is like a built-in generator for the airplane. It helps power the airplane’s systems without needing the main engines to be running.
Intuition Check
Do not think of the APU as another engine used to pull or push the airplane through the air. It is a helper power source for airplane systems, not a source of flight thrust.
Example Sentence 1
Before starting the main engines, the crew started the APU to power the cockpit displays and air conditioning.
Example Sentence 2
With the APU listed as inoperative on the CDL, the flight required a ground power cart for departure.