Auxiliary Power Unit (APU)

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.
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.