Auxiliary power unit

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APIC APS3200 APU for Airbus 318/319/320/321

An auxiliary power unit (APU) is a relatively small, self-contained generator used in aircraft to start the jet engines, usually with compressed air, and to provide electricity, hydraulic pressure and air conditioning while the aircraft is on the ground. In many aircraft, the APU can also provide electrical power in the air.

APUs are also fitted to some tanks to provide electrical power when stationary, without the high fuel consumption caused by running the main engine.

A gasoline piston engine APU was first used on the Pemberton-Billing P.B.31 Night Hawk Scout aircraft in 1916. The Boeing 727 in 1963 was the first jetliner to feature a gas turbine APU, allowing it to operate at smaller, regional airports, independent from ground facilities.

Although APUs have been installed in many locations on various military and commercial aircraft, they are usually mounted at the rear of modern jet airliners. The APU exhaust can be seen on most modern airliners as a small pipe exiting at the aircraft tail.

In most cases the APU is powered by a small gas turbine engine that provides compressed air from within or drives an air compressor (load compressor). Recent designs have started to explore the use of the Wankel engine in this role. The Wankel offers power-to-weight ratios better than normal piston engines and better fuel economy than a turbine.

APUs fitted to ETOPS airplanes are more critical than others, as they supply backup electricity and compressed air in place of the dead engine during emergencies. While some APUs may not be startable while the aircraft is in flight, ETOPS compliant APUs must be flight-startable at all altitudes. Recent applications have specified starting up to 43,000 ft from a complete cold-soak condition. If the APU or its electrical generator is not available, the airplane cannot be released for ETOPS flight and is forced to take a longer route.

APUs are even more critical for space shuttle flight operations. Unlike aircraft APU's, they provide hydraulic pressure, not electrical power. The space shuttle has three redundant APUs, powered by hydrazine fuel. They only function during powered ascent and during re-entry and landing. During powered ascent, the APUs provides hydraulic power for gimballing of shuttle's engines and control surfaces. During landing, they power the control surfaces and brakes. Landing can be accomplished with only one APU working. On STS-9, two of Columbia's APUs caught fire, but the flight still landed successfully. [1]

A typical gas turbine APU for commercial transport aircraft comprises three main sections:

Power section
Load compressor
Gearbox

The power section is the gas generator portion of the engine and produces all the power for the APU. The load compressor is generally a shaft‐mounted compressor that provides all pneumatic power for the aircraft. There are two actuated devices, the inlet guide vanes that regulate airflow to the load compressor and the surge control valve that maintains stable or surge‐free operation of the turbo machine. The third section of the engine is the gearbox. The gearbox transfers power from the main shaft of the engine to an oil‐cooled generator for electrical power. Within the gearbox, power is also transferred to engine accessories such as the fuel control unit, the lube module, and cooling fan. In addition, there is also a starter motor connected through the gear train to perform the starting function of the APU.

With the Boeing 787 being an all electric airplane, the APU delivers only electricity to the aircraft. The absence of pneumatic system simplifies the design, but the demand for hundreds of kilowatts (kW) of electricity requires heavier generators and unique system requirements.

Two main corporations compete in the aircraft APU market: United Technologies Corporation, through its subsidiaries Hamilton Sundstrand and Pratt & Whitney Canada, and Honeywell International Inc.

In case of APU failure an air start unit (ASU) and ground power unit (GPU), respectively is needed.