How APUs Work ?

If you’ve ever seen the back end of an airplane, you’ve probably noticed what looks like a large black dot or hole at the end of the fuselage. It’s the exhaust pipe for the small “extra” jet engine known as the APU. But what is an APU?

An APU, or Auxiliary Power Unit, is the small turbine engine used to provide additional electrical energy and normally used to start one of the main engines on larger airliners. APUs start the engines by generating auxiliary “bleed air” when there is no ground pneumatic source available. First, an electric motor starts the APU, once up and running, the APU generates bleed air which is routed to the pneumatic starters on the airplane’s main engines to spin the engine compressors for starting.

In addition to starting the engines, APUs are used to run aircraft systems on the ground when the main engines aren’t running and no ground electrical power is available. The APU can power things like onboard lighting, galley electrics, cockpit avionics, and environmental systems for pre-cooling and pre-heating. By negating the need to start the main engines while waiting for passengers to board, using the APU saves on fuel and money.

While most of an APU’s purpose is on the ground, in some instances the APU can be used an emergency electrical power source while the aircraft is airborne. But, in most cases, the APU is shut down before takeoff and reignited when the aircraft clears the runway after landing. It’s also not very accurate to say that the APU is an extra jet engine since the turbine exhaust from the APU is vented overboard, which doesn’t really help propel the aircraft forward.

Today, APUs are commonly found in medium-sized and larger civil and military jets, some turboprop aircraft, and a handful of military fighter jets. Smaller civilian jets like those used for private charters don’t have APUs because the extra weight can have much a more significant impact than it would on a larger jet.


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