On larger, complex aircraft, powering all the equipment, lighting, and amenities can present a challenge to designers and manufacturers. Powering all that equipment and electronics can be difficult using batteries, and the added weight of extra batteries could reduce payload capabilities. Many aircraft designs include an auxiliary power unit, or APU.
The APU is a small gas turbine engine that provides electrical and pneumatic power for aircraft systems either independently or with aircraft engines and engine-driven generators. Often, aircraft designers place the APU in or near the tail cone of the aircraft.
An APU provides an easy way to transition from ground power to independent aircraft power. While the aircraft is parked at the gate a ground power unit (GPU) is often used to supplement or replace the use of aircraft batteries for powering the avionics, cabin electrical lighting, and other essential items. This allows the aircraft to be boarded with lights on; the lavatory operational; and the avionics programmable, set up, and ready for flight.
Once ready for gate departure, or immediately prior to, the APU is started to transition away from the GPU. APU starting, operation, and shutdown is controlled with the FADEC (full-authority digital engine control). Once up and running at a stabilized turbine rotation speed, the APU is capable of providing electrical power to aircraft systems as well as pneumatic power for engine starting and air conditioning.
Now that the aircraft can be powered independently, gate pushback occurs and the engines can be started for taxi. Bleed air is supplied by the APU compressor and is used to power the pneumatic starter on either aircraft engine. Once at least one engine is up and running the APU can be shut down, or left running to start other engines. By design, most aircraft bleed air systems are designed to allow cross-bleed engine starts, either to facilitate an inoperative APU or fuel savings by shutting down the APU immediately following the first engine start.
The APU is a versatile piece of equipment that not only enhances operational flexibility, but also reliability, systems redundancy, passenger comfort, and convenience. Welcoming passengers aboard an aircraft that is well lit, air-conditioned, and ready for flight is a competitive advantage as well as an operational one.
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