what is apu in aircraft

An Auxiliary Power Unit (APU) is a small jet engine located typically in the tail section of an aircraft, which provides electrical and pneumatic power when the main engines are not running. The APU allows the aircraft to operate autonomously from any ground support systems such as ground power unit (GPU), an external air-conditioning unit or a high pressure air start cart. The APU is typically used to power the aircrafts electrical systems, and provide air conditioning and pressurization for the aircraft when it is on the ground. 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, it 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 requirements dictate.