Auxiliary Power Units and Their Importance For Aircraft

For airlines that require high amounts of power for their standard set of operations, having a device that provides energy for functions outside of propulsion can be highly beneficial. Common to large aircraft such as commercial airliners, devices such as Auxiliary Power Units (APU) are regularly used for the vehicle to operate autonomously without the aid of a ground power unit, high pressure air start cart, external air conditioning unit, or other various ground support equipment. In this blog, we will discuss the design and operation of auxiliary power units, allowing you to better understand how they augment the operations of various aircraft.

When implemented on an aircraft, the APU will come in the form of a small airplane engine which is situated near the tail cone structure. In some instances, the APU may also be installed in an engine nacelle or within the wheel well. While the APU may be considered a type of jet engine, they differ from standard engines in the sense that they vent all exhaust overboard rather than using it for increased thrust. For a business jet or commercial airliner, the APU may serve to provide additional energy for starting the engine, and the inclusion of an extra electrical generator ensures that there is enough power for the operations of onboard lighting, cockpit avionics, galley electronics, and other electrical systems while parked at an airport terminal.

In order to optimally produce power, auxiliary power units utilize bleed air that is produced with their own compressor, and included environmental packs allow for the aircraft to be heated and cooled as needed. With these general applications and functions, the APU can reduce operational costs as compared to the operations of the main engine, also decreasing the amount of fuel burned and maintenance required for such devices. Generally, the APU is not needed during standard flight operations, instead being shut off prior to takeoff and turned on once a touchdown is complete. Nevertheless, the APU can serve as an emergency source of energy during flight in the case of an electrical source failure.

Some of the earliest APUs were found on the B-29 Superfortress and other World War II aircraft, typically being used as a starting system. These early variations were often small and were placed within the fuselage of such aircraft. Nowadays, however, APUs are regularly found on civil airliners and military jets that are medium to large sized. Smaller aircraft often forgo the APU due to weight limitations which can affect their performance and aerodynamics. APU requirements will often be established based on the size of the cabin, the piston engine and its required starting power, and the generator size. Then, the APU will be specifically designed to accommodate all requirements of the particular aircraft.

While auxiliary power units are very useful for a number of standard operations, it is important to note that they can produce high amounts of noise when turned on. Because of this, many airports may enact restrictions on APUs during night hours, ensuring that noise pollution is minimized for the benefit of surrounding communities. Additionally, as such engines face the risk of fires and explosions, such equipment should have an included fire extinguisher present for safety. For many modern APUs, the extinguisher may be automatically operated.

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