You have finally selected an airline service after hours of research having the best prices, comfort and most of all safety. You are very happy about the decision, but just as you are about the board the plane you see a huge hole at the tail of the plane! What in the name of science is that? Is that supposed to be there?
Before you have a panic attack and jump off the boarding bus, you need to read the article below!
The hole at the back of an aircraft, usually a large one, is the APU exhaust. An APU or Auxiliary Power Unit is usually found in large aircraft like the Airbus A320 or the Boeing 777 and is essentially a small turbine engine required to start the main engines. The turbine also needs to power aircraft systems when the plane is parked on the ground and power from a station is not available.
It can be thought of as an intake vent in a retractable scoop shape, located at the starboard (right) rear of the tail fin, also know as the vertical stabiliser.
The turbine is also used to suck in and provide high pressure compressed air (bleed air) that will be blown into the starter mechanism of the first engine. Once the engine starts running, bleed air from that engine is used to start others, meaning APU will not be required.
Once the plane takes off, the forward momentum of the aircraft is enough to spin the blades of the turbine and allow it to be restarted.