Airplane exhaust systems are part of the airframe. Small piston-engine aircraft may carry a muffler to reduce engine noise. The small hole in the tail is an APU exhaust, allowing combustion gases produced by the Auxiliary Power Unit to escape safely.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
Does a plane have an exhaust?
Yes, airplanes have exhaust. Each engine has its own exhaust. A four-engine jet will usually have four separate exhaust cones at the rear of the nacelles. Turboprop planes are different as they send their gases out through small rectangular outlets arranged along the side of the fuselage. Even when the main engines are shut down, a thin stream of warm gas still leaves the tail, because every passenger plane carries an auxiliary power unit (APU) that needs its own exhaust. On models like the MD-80, Boeing 717 or Boeing 777 this APU exhaust is not a pipe but a flush grid or a ring of simple holes in the aft fuselage skin.
Does a plane have exhaust pipes? Not in the everyday sense. The metal tail cone that you see behind a jet engine is part of the engine itself, not an add-on pipe. The APU does not use a long external pipe as the grids or holes in the skin act as the final passage for the gases. Because these manufacturer-designed exhaust paths are short and aerodynamically designed, no separate muffler is added: a muffler adds weight, creates unwanted back-pressure and does not survive the intense heat.
Commercial jets rely on their exhaust to produce thrust. The hot, high-speed gases leave the engine core and mix with bypass air, forming the white plume. Heat exchangers buried in the pylon pick up some of that waste heat to warm cabin air, but the gases themselves are not silenced. Regulatory bodies like the FAA and EASA therefore set strict safety standards for the whole aircraft exhaust system, assuring that the emitted gases - whether at cruise above 3,000 ft (914.4 m) or on approach - meet environmental and airworthiness limits.
What is the purpose of the plane tail exhaust?
The plane tail exhaust hides a small turbine called the auxiliary power unit. Having the APU in the tail places it as far away from the cabin and fuel tanks as possible, and being outside the pressure vessel makes isolation easier. The tail has otherwise mostly unused space, so the exhaust pipe exits without hot gases hitting anything, preventing interference with other parts of the airplane. Tail placement for the APU exhaust minimizes noise and keeps the unit up and away from ground staff who are working on the aircraft.
Where is the airplane exhaust?
The airplane exhaust is usually located in the tail. The thin stream you notice curling from the tail-top of a parked jet is the auxiliary power unit exhaust. The APU is a small turbine engine that sits in the tail, and the location in the tail, away from ground staff, is helpful. The white streaks drawn high across the sky are contrails; they occur at aircraft cruise altitudes where water vapor condenses because it is very cold and the atmosphere at high altitude is of much lower vapor pressure than the exhaust gas.
What is the exhaust at the back of a plane?
The small hole in the tail is the exhaust point for the Auxiliary Power Unit. The APU is a small gas turbine engine tucked inside the tail cone of most commercial aircraft. It draws in air, compresses it, mixes it with fuel, and ignites the mixture to produce hot compressed exhaust gas. This gas shoots out through a tailcone exhaust port, a simple tailpipe that needs heat shielding to protect the airframe. The APU does not eject gas out of a nozzle to create thrust but provides electrical power and compressed air on the ground and occasionally in flight. It starts the main engines, powers on-board electronics at the gate, and runs the cabin air-conditioning. While a conventional jet engine will eject hot gas out of a nozzle to provide propulsion, the APU provides no thrust.
