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Aviation glossary

APU

Auxiliary Power Unit · Also: auxiliary power unit

The aircraft

Small gas turbine, usually in the tail cone, that supplies electrical power and compressed air when an aircraft's main engines are shut down.

What it is

On the ground with its engines off, an aircraft still needs power for lighting, the flight deck and air conditioning. The APU provides it: a gas turbine that drives an electrical generator and, on most types, supplies compressed air for air conditioning and for starting the main engines. It usually sits in the tail cone, where its intake and exhaust are visible.

In flight it serves as an electrical back-up, and for twins operating under ETOPS its reliability and ability to start at altitude are part of the requirements. On the Boeing 787, which largely does away with engine bleed air, the APU produces electricity only. To reduce noise and emissions, many airports restrict its use at the gate and provide ground power and pre-conditioned air instead.

On Vuelopedia

Boeing 787 Dreamliner

Related terms

ETOPSRules that let twin-engine aircraft fly routes far from suitable diversion airports, up to a maximum one-engine flying time approved for each aircraft type and airline. PressurisationSystem that keeps the air pressure inside an aircraft higher than outside, so that passengers and crew can breathe normally at high altitude. Twin-engine aircraftAircraft powered by two engines, now the dominant layout in airline service thanks to the reliability of modern engines. GatePoint in the terminal from which passengers on a flight reach the aircraft, whether by airbridge, on foot across the apron or by bus. TurbineSet of bladed wheels that extracts energy from the hot combustion gases to drive the engine's compressor, fan or propeller.
Whole glossary

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This page was last updated on October 9, 2026

APA

Vuelopedia. (2026, October 9). APU. Retrieved October 17, 2026, from https://vuelopedia.org/en/glossary/apu

MLA

“APU.” Vuelopedia, Fundación Poderia, 9 Oct. 2026, vuelopedia.org/en/glossary/apu. Accessed 17 Oct. 2026.

Chicago

Vuelopedia. “APU.” Fundación Poderia. Last modified October 9, 2026. https://vuelopedia.org/en/glossary/apu.

Harvard

Vuelopedia (2026) APU. Available at: https://vuelopedia.org/en/glossary/apu (Accessed: 17 October 2026).

BibTeX

@misc{vuelopedia_en_glossary_apu,
  author       = {{Vuelopedia}},
  title        = {{APU}},
  publisher    = {Fundación Poderia},
  year         = {2026},
  date         = {2026-10-09},
  url          = {https://vuelopedia.org/en/glossary/apu},
  howpublished = {\url{https://vuelopedia.org/en/glossary/apu}},
  urldate      = {2026-10-17},
  note         = {Accessed 2026-10-17}
}

Wikipedia

{{cite web |url=https://vuelopedia.org/en/glossary/apu |title=APU |website=Vuelopedia |publisher=Fundación Poderia |date=2026-10-09 |access-date=2026-10-17 |language=en}}

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