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The APU (auxiliary power unit) does not need to supply bleed air when the main engines are not operating. Aerodynamics are improved due to the lack of bleed air vent holes on the wings. By driving cabin air supply compressors at the minimum required speed, no energy wasting modulating valves are required.
The first German jet engines built during the Second World War used a mechanical APU starting system designed by the German engineer Norbert Riedel.It consisted of a 10 horsepower (7.5 kW) two-stroke flat engine, which for the Junkers Jumo 004 design was hidden in the engine nose cone, essentially functioning as a pioneering example of an auxiliary power unit for starting a jet engine.
The compressed air can be supplied from an on-board auxiliary power unit (APU), a portable gas generator used by ground crew or by cross feeding bleed air from a running engine in the case of multi-engined aircraft. [19] The Turbomeca Palouste gas generator was used to start the Spey engines of the Blackburn Buccaneer.
This bleed air is directed into a mechanism to start the engine turning and begin pulling in air. The starter is usually an air turbine type, similar to the cartridge starter, but uses the APU's bleed air instead of the burning gases of the propellant cartridge. Most cart starters can also use APU air to turn them.
The quantity of bleed air flowing to the AC pack is regulated by the flow control valve (FCV). One FCV is installed for each PACK. A normally closed isolation valve prevents air from the left bleed system from reaching the right PACK (and vice versa), although this valve may be opened in the event of loss of one bleed system.
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Compressed air is sent to the ATS turbine from the aircraft auxiliary power unit ( bleed air from the gas generator or from a free-turbine load compressor, eg PW901 APU), from an already-running engine (bleed air) on a multi-engined aircraft or, for early jet aircraft, from an air compressor mounted on ground support equipment.
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