Auxiliary Power Unit
Flying|March 2018

Auxiliary Power Unit

Rob Mark

An aircraft auxiliary power unit serves as an additional energy source normally used to start one of the main engines on an airliner or business jet. The APU is equipped with an extra electrical generator to create enough power to operate onboard lighting, galley electrics and cockpit avionics, usually while the aircraft is parked at the gate. Drawing bleed air from its own compressor, an APU also drives the environmental packs used to heat and cool the aircraft.

And most important,operating an APU negates the need to start one of the aircraft’s main engines while waiting for passengers to arrive, thereby saving on fuel and maintenance for a more expensive power plant.

In most cases, the APU is shut down before takeoff and reignited when the aircraft clears the runway after landing. While most of an APU’s active service life occurs as the aircraft sits on the ground, in some instances the APU is used as an emergency electrical power source while the aircraft is airborne.

The APU is a small turbine engine installed near the rear of the fuselage. But calling the APU an extra jet engine is not accurate because the turbine exhaust from the APU is vented overboard. A jet engine would be used to propel the aircraft forward.

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