An Auxiliary Power Unit is a small turbine engine, usually mounted in the tail cone, that generates electrical power and — on most installations — bleed air for air conditioning while the main engines are shut down. It is the reason a well-run charter cabin is cool, lit, and quiet-humming when you step aboard on a 95-degree ramp: the crew started the APU twenty minutes before your arrival, ran the packs, and chilled the cabin without ever spinning the main engines.
Mechanically, the APU is a miniature jet engine burning the same fuel from the same tanks. Consumption is modest but real — roughly 15–30 gallons per hour on midsize and larger aircraft, so an hour of ground comfort costs the operator $100–250 in fuel plus cycles on a component with its own maintenance schedule. The alternative is ground power: a GPU cart plugged into the aircraft, supplied by the FBO for roughly $50–250 per use. A GPU runs lights, avionics, and charging, but on many aircraft it cannot drive the air conditioning packs the way APU bleed air can — which is why crews on hot ramps run the APU anyway, and why in extreme climates FBOs also offer preconditioned-air carts.
For the charter customer, the APU is mostly invisible and already priced in — extended ground comfort is part of what the hourly rate and incidental fees cover. Where it becomes worth understanding is at the edges. Long waits with passengers aboard (a "hold at the FBO, we may leave within the hour" afternoon) mean hours of APU burn that can appear on a final invoice as a modest ground-power or wait-time charge. Some airports — notably noise-sensitive European fields and a few US neighbors-first airports — restrict APU running to 10–30 minutes before departure, so the cabin may be warmer or cooler at boarding than usual, through no fault of the crew.
One distinction saves confusion on the ramp: hearing a turbine whine does not mean the engines are running or that departure is imminent — it is almost always the APU. And not every aircraft has one: many turboprops and some very light jets skip the APU entirely to save weight, relying on GPU carts and quick engine starts instead. On those aircraft, boarding a few minutes closer to departure time, into a cabin that cools down in flight rather than before it, is normal — a small comfort difference between the very light jet and midsize classes that rarely makes it into the brochure.
