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Aviation Glossary :: Auxiliary Power Unit  Aviation Glossary :: Auxiliary Power Unit FAA Written Test Preparation
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At Dauntless, our editorial staff maintains the web's largest unified glossary of aviation terms. This glossary is built from a combination of official, quasi-official, and proprietary sources (including original material that we develop oursselves). Uniquely, we often provide multiple definitions of a given term so that you can find that which best applies to you. In order to maximize your learning efficiency, this glossary (and similar ones for our international users) is incresingly fully integrated into our aviation learning apps, including our FAA written test prep and FAA practical test prep software and apps. If you like this glossary, you'll love them with their polished learning environments and world's best and clearest content (please do give them a try.).

Auxiliary Power Unit
Auxiliary Power Unit
A small turbine or reciprocating engine that drives a generator, hydraulic pump, and air pump. The APU is installed in the aircraft and is used to supply electrical power, compressed air, and hydraulic pressure when the main engines are not running.
source: FAA Aviation Maintenance Technician Airframe Handbook (FAA-H-8083-31)
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, naval ships, as well as some large land vehicles. Aircraft APUs generally produce 115 V alternating current (AC) at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircraft; others can produce 28 V direct current (DC). APUs can provide power through single or three-phase systems.
source: Wikitionary / Wikipedia and Related Sources (Edited)

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