What Is an Auxiliary Power Unit (APU)?

For a typical aircraft to lift off from the ground and achieve flight, it must expend a massive amount of energy. While aircraft engines are advanced systems that harness the combustion of fuel to create thrust, the engines are not the only things aircraft have that utilize power. There are also many electronic systems and other devices, ranging from the lights that illuminate the cabin to the instruments that provide pilots with readings on flight conditions. To ensure that everything on and across the aircraft has the power needed to function as intended, most aircraft feature a special component known as an auxiliary power unit (APU). As a common fixture of many modern aircraft, it can be very useful to have a general understanding of their functionality and use.

While small aircraft may be limited in their electronics and are fairly self-sufficient in terms of power, APUs are commonly found on medium to large airplanes where power is required for various electrical devices and systems. These APUs provide an average of 115 volts of alternating current (AC), enough to sustain everything besides the engine. While APUs generate electricity, they should not be confused with conventional generators. Instead, they are actually miniature engines that are situated toward the tail or wheel well of the aircraft. Furthermore, many aircraft will have a separate generator that powers all systems during standard operations, while the APU simply serves as a backup system. One way to think of APUs is as wind generators, featuring turbines that spin during flight to create power. As long as the aircraft is in the air and moving forward, airflow will drive the APU turbine.

With a generator already on board, one may wonder why an aircraft needs an APU. As with many other choices made in the aviation realm, the inclusion of an APU, in addition to the onboard generator, is a result of the drive for redundancy. Aircraft often require electrical power to safely and efficiently operate, as a loss of power may spell a loss of lighting, air conditioning, cockpit control functionality, and more. As any safety hazard should be avoided at all costs, APUs are implemented for redundancy so that there is always a backup source of power that is reliable.

Aside from acting as emergency systems, there are other reasons that APUs are used as well. For example, many aircraft rely on their APU during the startup process to make kickstarting operations much easier and safer. As engines consume a great amount of power, operators will not always want them on during ground operations. Additionally, operating engines can present a safety hazard on the ground as flight preparations are carried out, making it more beneficial to only have them on when the flight is about to begin. As such, APUs can be used to create the necessary power to light the cabin, provide air conditioning, and energize all systems so that preflight inspections can be carried out with ease. Lastly, the APU may even be used for an additional boost of power for aircraft to better take off.

With these various roles, one can better understand APU uses and their grand importance. If you are searching for APU parts and components for an installation or repair, we at ASAP Aerospace Hub can help you procure everything you require for your operations with competitive pricing and rapid lead times for your benefit. As you explore our massive inventory, feel free to utilize our online RFQ forms to request quotes for your comparisons on items of interest, and we promise responses within 15 minutes or less of receiving your submission. If you have any further questions or concerns, please reach out to us by phone or email at any time, and we would be more than happy to assist you however we can!