What Is The Hole On The Back Of An Airplane For?

If you've ever stared at the back of a plane while boarding or taxiing and noticed a small hole near the tail, you're not alone. It's easy to miss, but once you see it, you'll start spotting it on nearly every commercial jet. And, no, it is not a secret camera, nor a drain pipe. It is actually an exhaust point for something called the Auxiliary Power Unit, or APU. So, what are APUs and why do some planes use them?

APUs are the unsung heroes of air travel. They quietly keep planes running when the engines are off. You read that right. The APU is a small gas turbine engine tucked inside the tail cone of most commercial aircraft. It provides the electrical power and air pressure needed to run certain systems that are vital before and between flights. Think of it as the plane's personal generator. It's not there to move the aircraft forward, but to power vital systems while the plane is idle. But it has far-reaching uses and consequences.

The APU powers systems like lighting, air conditioning, cockpit displays, and avionics. It stops the aircraft from being a dead weight on the runway when not plugged into ground power. One of its most important jobs is to start one of the main engines. Modern jet engines require a kickstart similar to your car's ignition. That's where the APU comes into action. It produces compressed air that spins the engine's compressors and gets the turbines going. Once the engines are running, they take over power and air duties, and the APU can be shut down.

The position of the APU being in the tail coincides with certain airplane engine placements. But though it lives in the tail, the APU isn't involved in flying the aircraft. It doesn't generate thrust. Its purpose is pretty much limited to ground use with certain in-flight emergency usages. This includes when the engine fails. Where that happens, it may be called upon for electrical power or to restart the engines.

Remember, the hole is the exhaust port for the APU cause it gets hot back there. Just like a car or a full-sized jet engine, the APU burns fuel. This combustion produces emissions, and the tailcone exhaust port is responsible for dispelling these emissions. If you ever watch closely as a plane prepares for takeoff, you might spot a shimmer of heat waves coming from that tail hole. That'll be proof that the APU is doing its job.