Auxiliary Power Unit (APU)

An APU is a small gas turbine that supplies electrical power and air conditioning (and engine-start air) without running the main engines or relying on ground equipment — so the cabin stays comfortable while boarding. APU hours and condition are tracked like the engines and factor into maintenance planning.

Why it matters: an APU frees the aircraft from ground infrastructure — cabin climate, electrical power and engine-start capability without a ground cart. On hot ramps and at remote airfields the difference is passengers boarding a cool cabin rather than sitting in a hot one while support is found.

An APU is also a maintained component with its own hours, cycles and overhaul economics, so its condition belongs in any evaluation of a candidate aircraft. Where APU use is restricted or expensive on the ground, crews manage its running time deliberately — comfort, cost and noise rules all trade against each other.