What’s that hole in the tail? A giant screw hole for towing the aircraft? An air conditioning intake? It’s part of what’s called an auxiliary power unit. An airliner’s APU is a hidden extra engine, typically a gas turbine engine like jet engines themselves. But the APU doesn’t help the plane move. Rather, when an airplane is parked at the gate, pilots will use the APU to power onboard electric systems—from cabin lights and coffee makers to various cockpit systems—as well as to provide airflow to the air conditioning systems. Additionally, the APU is typically used to start the engines as the plane is pushed back from the gate. Once the engines are running, the APU is normally switched off, to be restarted only when the plane is on the ground at its destination.
When it comes to cooling and lighting a parked plane, the APU is much more efficient than the main engines. But it still uses fuel—indeed, the APU on a very large airliner may be as powerful as the main engines installed on a smaller aircraft—so it makes sense to use it as little as possible.
At many airports, to reduce APU usage, a parked plane can use external power—essentially, the jet can be plugged into the airport’s own power supply. It’s easy to see the cables from the windows of the terminal—they generally attach to the plane somewhere near the nose wheel. When a parked plane receives electrical power from external sources this way, there’s no need for the APU to be running. Similarly, a plane can also receive external air conditioning. You might see one or more inflated tubes, often yellow, snaking into the belly of the jet (the external air conditioning units themselves may be located under the jet bridge).
