What Are Aircraft Auxiliary Power Units and How Do They Work?
To carry out the many complex operations that are required for a safe and efficient flight, there needs to be reliable energy sources in place on aircraft that systems can use to maintain power and functionality. The auxiliary power unit (APU) is an energy source that is commonly found on airliners and business jets, and they typically serve as an additional energy source to start the main engines, as well as provide power for the operation of various onboard systems, avionics, and electronics while at the gate of an airport.
The standard APU is a small jet engine, placed towards the back of the aircraft in the tail cone. They may also be installed within an engine nacelle or in the wheel well of an aircraft for certain types if needed. To begin their functions, an aircraft battery is used to initialize APU operation, and then the APU may begin to provide electrical loads for systems, turbine engine starting, air conditioning, and more on its own.
The auxiliary power unit is also very important for safety through redundancy as the APU may serve as an additional source of power if the main engine fails. These APUs are certified for use in flight, and they can be used to ensure power is maintained in the case of an emergency. APUs are also sometimes used as a source of bleed air for carrying out an inflight engine relighting, as well as to power air conditioning packs when a takeoff must be conducted with the engine bleed shut off.
APUs are useful as they negate the need for relying on a ground power unit and other ground support equipment, allowing the aircraft to operate autonomously. As the main engine does not need to be in operation as passengers board, an APU also provides cost-saving benefits for the airliner, saving fuel and maintenance. During flight operations, the APU is shut off before takeoff, and it is later reactivated when the aircraft has cleared the runway and is fully landed. A majority of the APU’s use is during ground operations, and inflight operations are mostly reserved for emergencies such as a loss of power.
In the early days of APUs, they were mostly found on aircraft such as the B-29 Superfortress and the 727. In more modern times, APUs may now be found on a plethora of medium to large-sized civil and military aircraft, including those by Boeing and Airbus. APUs are typically never found on smaller aircraft, due to the fact that their high weights can lead to significant performance and load capability losses for such aircraft. Because of this, the decision to implement an APU is based on the size of the cabin, the generator size needed for powering and starting, and other various factors. Despite these requirements, APUs can serve as extremely beneficial systems to further an airliner’s safety and efficiency during standard flight operations.
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