An Auxiliary Power Unit or APU allows an aircraft to operate autonomously without reliance on ground support equipment such as a ground power unit, an external air-conditioning unit or a high pressure air start cart.
The APU is a small jet engine which is normally located in the tail cone of the aircraft but, in some cases, is located in an engine nacelle or in the wheel well. The APU can be started utilizing only the aircraft battery(s) and, once running, will provide electrical power to aircraft systems as well as bleed air for air conditioning and for engine start.
When the APU is certified for use in flight, the APU can be used, as required, to provide an additional source of electrical power in the event of the loss of an engine generator. It can also be used as a source of bleed air for starter assist for an inflight engine relight or to power the air-conditioning packs in the event conditions or company policy dictate that the takeoff be conducted with the engine bleed turned off (see Hot and High Operations).