An Auxiliary Power Unit (APU) is a small jet engine usually found in the tail section of an aircraft. It supplies electrical and pneumatic power when the main engines are not in use. The APU enables the aircraft to operate independently without relying on ground support systems like ground power units, external air-conditioning units, or high-pressure air start carts.
The APU in an aircraft powers electrical systems, air conditioning, and pressurization when the aircraft is on the ground. Using Ground Power Units (GPUs) allows turning off the APU while parking, reducing airport noise, emissions, fuel consumption, and wear on the APU. The APU is started again only when the aircraft is about to leave the parking spot to provide compressed air to start the main engines.
The Auxiliary Power Unit in an aircraft runs independently from the main engines and is typically fueled by the same fuel as the engines. It is activated by a battery or GPU.
When certified for in-flight use, the Auxiliary Power Unit can supply extra electrical power if an engine generator fails. It can also provide bleed air for engine restarts or to operate air-conditioning packs if takeoff needs to be done without engine bleed. However, using the APU during flight is limited to avoid higher fuel consumption and costs.
The aircraft manufacturer determines the necessary APU specifications by taking into account factors such as the cabin size, the amount of bleed air needed for the environmental packs, and the generator size for cockpit and cabin power and engine starting.
The responsibility of meeting these specifications lies with the Auxiliary Power Unit manufacturer, typically a subcontractor to the aircraft manufacturer.While ensuring the readiness of avionics is crucial for departure, creating a comfortable cabin environment is often even more important for most operators before passengers arrive.
