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The auxiliary power unit (APU) is a compact self contained unit that provides electrical power and compressed air during airplane ground operations. The APU consists of a gas turbine engine and AC generator together with controls, enclosures and mountings necessary for safe and continuous operation. The gas turbine engine incorporates a bleed air system that provides bleed air for the engine starters and air conditioning. The generator supplies electrical power to energize the airplane electrical systems. The APU, which is completely enclosed in a shroud, is attached to the airplane keel beam aft of the main landing gear wheel well forward bulkhead.
The APU gas turbine engine is composed of a two-stage centrifugal compressor coupled to a single-stage inward flow turbine. The accessory drive section coupled to the turbine shaft provides power for driving engine accessories and the generator. Ambient air is compressed by the primary and secondary compressors and then directed to the diffuser and plenum where bleed air is extracted for pneumatic equipment. Air that is not bled off is directed to the combustion chamber, mixed with the fuel, and burned to drive the turbine.