An Auxiliary Power Unit (APU) is a component found in fixed-wing aircraft, near the rear of the fuselage. It functions as an additional energy source, used to start up one of the main engines. It can be found in many medium to large jets – and some turboprops – while smaller civilian jets do not have them (due to weight limitations).
A Brief History of APUs
APUs date back to aircraft produced in the 40s, during World War II, such as the Boeing B-29 Superfortress. Flying Magazine describes it “like a motorcycle engine installed inside the fuselage”. In 1958, the B727 became the first Boeing jetliner to have an APU. Today, APUs can be found on almost all common jetliners.
The Basic Mechanics
An APU is a gas turbine with three sections: the power section, load compressor and gearbox. The power section is a turbine engine that provides power to the APU, the load compressor supplies pneumatic pressure to the aircraft and the gearbox transmits power from the APU to an electric generator. The electric generator then supplies electrical power to the aircraft.
This power is enough to operate cockpit avionics, on-board lighting and galley electrics when the aircraft is parked at the gate. It also draws bleed air from its own compressor to drive the environmental packs used for cabin heating and cooling.
APUs are sometimes seen as jet engines, but they aren’t considered so. This is because, while an APU provides electrical and pneumatic energy, it doesn’t provide propulsion, as jet engines do.
Advantages of the APU
APUs help airlines reduce fuel consumption by avoiding the need to start the aircraft’s main engines while waiting for passengers to arrive. This also saves on maintenance costs on the main engines.
It also can be used as an emergency electrical power source should a power failure occur when the aircraft is airborne. While it is highly unlikely that all engines will shut down mid-flight, if needed, the APU can provide the power needed to start up the engines in-flight to execute a landing.
APUs in the Future
As with all components, APUs undergo continuous observation and improvement. We expect to see lighter APUs in the future; currently, an A320 Auxiliary Power System (APS) weighs up to 170kg. We also anticipate further reductions in fuel consumption through the application of e-Taxi, an application that allows the aircraft to taxi with partial or complete energy supply from the APU without having to use the main engines (AERTEC Solutions).
We’re looking forward to exciting APU developments in the near future! Are you?
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