The aviation industry has a wide range of equipment, machinery, and systems that ensure aircraft are working optimally at all times. Found both inside and outside of aircraft, certain instruments and apparatuses are used for preflight, onboard flight, and postflight operations. Two examples of such appliances include auxiliary power units (APUs) and ground power units (GPUs). Often conflated with one another, APUs and GPUs are incredibly important pieces of service equipment that many aircraft take advantage of, and you should familiarize yourself with them.
Auxiliary Power Unit (APU)
APUs are installed inside aircraft to provide power to the electrical system in the place of or as a supplement to engine-driven generator power. Moreover, they allow aircraft to operate without relying on ground support equipment (GSE) like a GPU, an external air-conditioning unit, or a high pressure air start cart. They consist of a small jet engine for their base, which is normally located in the tail cone of the aircraft. Some variations may be situated in an engine nacelle or in one of the wheels.
Generally, an APU can be started with the aircraft battery, and once it is running, it supplies electrical power to various aircraft systems as well as bleed air for air conditioning and engine start operations. Before being used on any aircraft, the APU must be certified for flight. Once this approval has been granted, it serves as an additional source of power in the event of the loss of an engine generator.
Ground Power Unit (GPU)
GPUs usually remain on the outside and also provide power in the place of engine-driven generator power. In general, they provide either 120V AC or 28V DC power, and they consist of a generator powered by a diesel engine but are available in an array of other configurations as well. They can be fixed or mobile units that are connected to the electrical system of an aircraft while they are on the ground, providing a power supply for the means of passenger disembarkation, aircraft servicing, and more.
When an aircraft lands, pilots will keep one jet engine on so that the aircraft has enough thrust to taxi around the airport as necessary and keep all of the electrical systems energized. To fulfill such requirements, an APU can be used. However, once the aircraft reaches a full stop, a GPU can be used to continue supplying power.
APUs v. GPUs
While APUs and GPUs serve a similar purpose, they each have their own unique characteristics. The most obvious difference we covered is mobility. Whereas an APU is found inside an aircraft, a GPU can be moved around to be used on different aircraft. Furthermore, each is utilized according to aircraft design, meaning that certain APUs and GPUs are not used for all aircraft. Nonetheless, certain aircraft can be powered by both. In terms of efficiency, GPUs win because they run on diesel fuel, making it cheaper to operate during long periods of ground operation. More than that, as previously mentioned, they can be used on multiple aircraft.
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