Have you ever noticed a small hole in the back of an airplane’s tail assembly? It’s a common feature of many commercial airliners. Being that the engines are typically located on the wings, you might be wondering what exactly this small hole is.
APU Exhaust
The small hole in the back of an airplane’s tail assembly is an Auxillary Power Unit (APU) exhaust. Most modern airplanes use a small gas-powered engine for electrical and mechanical power. Known as an APU, it eliminates the need for ground power. Airplanes can run their APU to generate electrical and mechanical power. Because APUs are essentially turbine engines, however, they require an exhaust.
Why Purpose of an APU Exhaust
The purpose of an APU exhaust is to safely release combustion gases. APUs generate combustion gases just like all other gas-powered engines. Air and fuel will mix together in a combustion chamber. Upon being ignited, the APU will produce combustion gases. The APU exhaust will release these combustion gases.
Airplanes, of course, have other types of exhaust. They typically have a single exhaust for each engine. If an airplane has four main engines, it will likely have four exhausts — one for each main engine. The engine exhausts provide propulsion. As the hot combustion gases shoot out of the exhaust, the airplane will be propelled forward. APU exhausts aren’t designed for propulsion. Instead, they allow airplanes to power themselves without relying on ground power.
Why the APU Exhaust Is Located in the Tail
The reason the APU exhaust is located in the tail is so that it doesn’t interfere with the airplane’s other components. Airplanes are meticulously designed. As previously mentioned, the main engines and their respective exhaust systems are typically located in the wings. The APU exhaust, on the other hand, is located in the tail.
By integrating the APU exhaust into the tail, airplanes are able to take full advantage of their available space. The APU exhaust won’t interfere with the main engines or other components.
A tail placement for the APU exhaust also minimizes noise. APUs produce noise when running just like all other gas-powered engines. Therefore, the closer an APU is to the passenger cabin, the more noise the passengers will hear. By keeping the APU exhaust in the tail, airplanes will provide a quieter experience for their passengers.
In Conclusion
The small hole in the back of an airplane’s tail is an APU exhaust. It allows the combustion gases produced by the airplane’s APU engine to safely escape.
