What is an Auxiliary Power Unit (APU)?

The auxiliary power unit or APU as it is commonly known, is a small gas turbine engine, fitted to aircraft and can provide

Illustration of an Auxiliary Power Unit or APU

An Auxiliary Power Unit (APU) is an automatic engine, which normally runs at a governed speed of 100%. Some APUs have an idle facility that allows the engine to run at 85% when no loads are applied. As it is an automatic engine the fuel system must control the engine throughout the start and running phases of operation. The engine will be shut down if a critical control function is lost or a serious malfunction such as low oil pressure occurs.

APU’s are mainly used on the ground when their main engines are not running and ground carts (electrical and pneumatic) are not available. On most modern aircraft the APU will also be used in the air to provide air-conditioning during take off and landing phases, or to back up the main engines in case of a generator or air system failure.

Although the APU is usually rated to run at the max cruise altitude of the aircraft it is fitted to, its ability to take load diminishes with altitude. As the major load on any APU is the air load it can be usual that the APU’s ability to provide sufficient air for the aircraft is limited to 15-20,000 ft.

Above this height the APU will only provide electrical power, this may also be limited to less than the max cruise height. Most APU’s give shaft priority which means that if air and electric generators are on the generators are given priority. Most Aircraft use constant frequency generators, and their APU’s which run at a constant 100% do not therefore require a constant speed drive unit to maintain a constant output. If the air loads become to high the APU will reach its max EGT and the control system will back off the fuel to prevent damage, this would bring the APU generator off frequency and take the generator ‘off line’. Instead the air load is reduced to maintain a constant APU speed.