APU

APU (Auxiliary Power Unit) is a small gas turbine engine, typically located in the tail cone of an aircraft, that provides electrical power and bleed air (for air conditioning and main engine starting) independently of the main engines.

The APU allows the aircraft to operate its systems at the gate without running the main engines or relying on external ground power. It is also used as a backup power source in flight and can restart engines at altitude if needed.