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What is an APU?

An APU indicates an auxiliary group of power installed aboard planes (generally at the rear end of the fuselage) and intended to function in various circumstances where electric , pneumatic or hydraulic output  is necessary.    This power is normally provided by generators installed on the mainl engines of the plane, but there are circumstances where it is necessary to have an additionnal   source of power  either because the main engines are normally stopped, or in the event of difficulties with the basic generators of the plane.

How is an APU built?

An APU is composed of an aircraft engine  functioning on the same principle as what was explained on the page devoted to the bleed air system . It is thus made :

- of a compressor which sucts the external air and compresses it,
- of a combustion chamber fed with fuel,
- of several stages turbines placed in the hot air flow  resulting from combustion and driving all the line of shaft  ( load compressing  (see hereafter ), engine compressor, turbines).

It is an engine of much lower power and with a basic difference: its operation is designed to produce a torque on an output shaft and not a powerful gas jet to produce a force of reaction.

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    (document Hamilton Sundstrand)

The output shaft ( which is also the main engine shaft ) is connected to a double functions gear box:

1. It is first a question of distributing the power provided by the engine to various accessories. These are power  generators required to feed the plane networks :

2. These accessories not being able to run at high speed it is necessary to reduce the rotation speed provided by the engine shaft. This gear device also called accessories gearbox is thus a reducer ( speed).
An APU can also provide pneumatic power, one installs for that an additional compressor (load compressor) on the axis of the APU engine.
The APU shaft thus includes 2compressors : one supplies the engine, the other feeds the aircraft circuits  with high pressure air.


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Where is the APU on an aircraft installed?

In general in the tail of the plane because that in particular makes it possible to easily eject gases produced by the APU  in the fuselage center  line.

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In which circumstances do we use an APU?

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