APU — Auxiliary Power Unit

What the APU Actually Does

An Auxiliary Power Unit is a small, self-contained gas turbine engine mounted in an aircraft — typically in the tail cone — that exists to provide electrical power, compressed air, and in most cases the pneumatic power needed to start the main engines, all without needing those main engines running. In practice, this makes the APU the thing that lets an aircraft be fully self-sufficient on the ground: air conditioning, cabin lighting, avionics and other electrical systems can all run entirely off the APU while parked at a gate, and the compressed air it generates is what's typically used to spin up the main engines for engine start, whether or not any ground equipment is available.

Independence From Ground Infrastructure

Without an APU, an aircraft parked at a gate without ground power and ground air-conditioning connected would have no electrical power and no climate control at all — which is exactly the situation the APU is designed to make unnecessary. This matters most at airports or gates that lack ground power units and preconditioned air carts (common at smaller or older airports, or when those systems are simply unavailable or broken), and for aircraft that need to start engines somewhere without ground-based pneumatic start carts, such as a remote parking stand.

The Trade-Off: Convenience vs Cost and Emissions

Running the APU burns fuel and produces emissions and noise just like running any small jet engine, because that's exactly what it is — which is why airports increasingly push airlines to plug into ground power and preconditioned air wherever it's available rather than running the APU during extended gate turnarounds, both for local air quality and noise reasons and because ground power is typically cheaper than APU fuel burn. Many airports now specifically restrict or fee-penalise extended APU use at the gate for exactly this reason, and modern aircraft and airline operating procedures increasingly favour minimising APU run time whenever ground infrastructure can substitute for it.

Beyond the Gate

The APU's usefulness isn't limited to the ground — it also serves as a backup power source in flight on many aircraft types, available to be started if needed to support electrical or pneumatic systems in the event of an issue with the main engines' own generators or bleed air systems, giving the aircraft an independent backup source of power that doesn't depend on either main engine.