Auxiliary power unit

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Auxiliary Power International Corporation (APIC) APU APS3200 for the Airbus A320 Family
Auxiliary engine outlet and inlet on the tail of an Airbus A380

An auxiliary power unit ( English auxiliary power unit , APU ) is an auxiliary power unit, mainly in the aircraft is used.

Aviation technology

The auxiliary power unit is not designed to power the aircraft. It usually supplies electrical energy and compressed air for autonomous operation of the aircraft equipment on the ground without the main engines having to run. The auxiliary engine is also often required to start the main engines. Other systems on the aircraft that can be operated by the auxiliary power unit are the on-board electrics / electronics ( avionics ), the air conditioning system and the de-icing device . The auxiliary power unit itself is started either with batteries or, if the main engines are still running, by the on-board electrics / pneumatics. The auxiliary power unit is normally switched off after the main engines have started. There are exceptions, however. In some situations, for example, the auxiliary power unit can be used to operate the air-conditioning system during the start-up phase, if a particularly high engine power is required for start-up, since air-conditioning systems can demand a significant proportion of the engine power. In some types of aircraft, it also serves as an emergency unit. It must be fully available if the aircraft is too far from an emergency landing pad or alternate airport ( ETOPS ).

An auxiliary power unit consists of an internal combustion engine, today mostly a gas turbine , previously also an internal combustion engine , as well as a generator and various auxiliary units for regulation. The installation space for the auxiliary power unit is equipped with a fire extinguishing system.

The auxiliary power unit is usually installed in the rear , recognizable by a small exhaust pipe. When a plane has no functional APU on board or is not allowed to reduce emissions of the operation of the APU at an airport, then either ground power units ( English "ground power units" GPU), ground launchers ( English air starter unit , ASU) and air-conditioning car back .

Riedel starter with cable in the center of the air inlet of a Jumo 004 jet engine on a Messerschmitt Me 262
APU in a Curtiss C-46


The airship Akron , built in 1911, had three propeller pods with a total of 280 hp and a 17 hp auxiliary power unit that maintained the pressure in the ballonets and operated a generator for the radio system.

Starter motor for jet engines

During the Second World War, boxer engines were used as starter motors for the first German jet engines to bring the engine up to starting speed. The by Norbert Riedel developed two-cylinder two-stroke -Boxermotor "Riedel-starter" had a capacity of 270 cm³ and a power of 8 kW (10.5 HP) at 7150 min -1 . It was designed as an extremely short stroke ( bore / stroke : 70 mm / 35 mm = 2: 1) so that it could fit into the hub of the turbine compressor and was started electrically or with a pull starter. The engine was produced by the Victoria works in Nuremberg and served as a starter for the Junkers Jumo 004 and BMW 003 jet engines .

APUs instead of shore power

Some bombers and cargo planes in World War II, e.g. B. the B-24 , the B-29 and the C-47 had piston-driven auxiliary power units, which were also called "putt-putts". For a long time, civil aircraft, such as the Boeing 707 , managed without an APU because their supply was guaranteed at large airports with shore power or ground power units . The Boeing 727 , which was also designed for the independent operation of smaller airfields, was then equipped with an auxiliary turbine for the first time.

Vehicle technology

Auxiliary units that are built into modern battle tanks are also referred to as auxiliary engines . It can be small gas turbines, but also piston engines. The APU (e.g. with the M1A2 Abrams 6.5  kW strong) is used to operate the aggregates of the tank when the tank is stationary, without the main engine (in the example 1119 kW strong) having to be switched on. This saves operating materials, reduces wear and tear and significantly reduces the heat and sound signature of the tank. Without an APU, it is inconceivable to maintain overpressure in the combat area for days in an ABC contaminated environment, if necessary.

Space travel

The orbiters of the space shuttle were equipped with three independent APUs, which supplied the necessary hydraulics for aligning the main engines , operating the control surfaces and landing gear with energy during takeoff and landing . The APU, most comparable to that of the General Dynamics F-16 (usually called EPU - Emergency Power Unit there), was operated with hydrazine and made a characteristic hissing noise that was audible every second , similar to a steam locomotive . This sound could be heard well in several videos of the last minutes of the countdown . The APU tended to fail more frequently on flights, most seriously on flight STS-9 , in which two of the three units failed.

Web links


Individual evidence

  1. D'Orcy's airship manual; an international register of airships with a compendium of the airship's elementary mechanics; October 1917; The Century co. New York; Pages 177-179; online at ; last accessed on October 6, 2016