Auxiliary power unit

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An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft, as well as some large land vehicles.


Transport aircraft

Functions of APU

The primary purpose of an aircraft APU is to provide power to start the main engines. Turbine engines must be accelerated to a high rotational speed in order to provide sufficient air compression for self-sustaining operation. Smaller jet engines are usually started by an electric motor, while larger engines are usually started by an air turbine motor. Before engines are to be turned, the APU is started, generally by a battery or hydraulic accumulator. Once the APU is running, it provides power (electric, pneumatic, or hydraulic, depending on the design) to start the aircraft's main engines.

APUs are also used to run accessories while the engines are shut down. This allows the cabin to be comfortable while the passengers are boarding before the aircraft's engines are started. Electrical power is used to run systems for preflight checks. Some APUs are also connected to a hydraulic pump, allowing crews to operate hydraulic equipment (such as flight controls or flaps) prior to engine start. This function can also be used, on some aircraft, as a backup in flight in case of engine or hydraulic failure.

Aircraft with APUs can also accept electrical and pneumatic power from ground equipment when an APU has failed or is not to be used.

APUs fitted to ETOPS (Extended-range Twin-engine Operations) aircraft are a critical safety device, as they supply backup electricity and compressed air in place of the dead engine or failed main engine generator. While some APUs may not be startable in flight, ETOPS-compliant APUs must be flight-startable at altitudes up to the aircraft service ceiling. Recent applications have specified starting up to 43,000 ft. (≈ 13 000 m) from a complete cold-soak condition such as the Hamilton Sundstrand APS5000 for the Boeing 787 Dreamliner. If the APU or its electrical generator is not available, the airplane cannot be released for ETOPS flight and is forced to take a longer non-ETOPS route.

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