Definition
Auxiliary power units (APUs) are small onboard engines, typically small gas turbines, that provide electrical power, compressed air, and sometimes hydraulic power to an aircraft independently of its main engines. They allow the aircraft to operate cabin systems, start the main engines, and run avionics while on the ground without relying on external ground equipment.
Plain English
An APU is a small extra engine on the aircraft whose job is to make electricity and compressed air for the rest of the aircraft. It lets you run the lights, air conditioning, and instruments — and start the main engines — without plugging into a ground cart.
Context Anchor
Seen in turbine-engine and turboshaft discussions when describing how an aircraft can power systems or start engines without using the main engine first.
Derivation
Auxiliary comes from the Latin auxilium meaning 'help' or 'aid.' An auxiliary power unit is literally a 'helping power unit' — it does not move the aircraft; it supports the systems that do.
Why Pilots Care
It enables independent engine starts and ground operations without external power carts and supplies emergency electrical power if a main generator fails.
Intuition Check
Do not read auxiliary as meaning unimportant. Here it means a separate helper power source, not the main engine and not a spare engine used for normal flight power.
Example Sentence 1
Before requesting engine start, the crew ran the APU to power the avionics and supply bleed air to spin up the main engines.
Example Sentence 2
If a generator fails in flight, the auxiliary power unit can be started to restore essential electrical power.