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. In some cases, the APU is located in an engine nacelle or in the wheel well.
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 aircraft's electrical systems, and provide air conditioning and pressurization for the aircraft when it is on the ground.
However, the utilization of GPUs allows the auxiliary power unit (APU) to be deactivated when the aircraft is parking. Switching of the APU results in a decrease in airport noise and emissions, in addition to the reduction of fuel consumption and auxiliary power unit wear. Only when about to leaving the parking position, the APU must be started to provide compressed air to start the main fan engines of the aircraft.
The APU operates independently of the aircraft's main engines, and is typically powered by the same fuel that the main engines use. The APU is started by a battery or gGPU.
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. However, using the APU in flight is generally avoided due to the additional fuel consumption and associated costs.