# Why do airplanes have a rear exhaust port? **URL:** \ **Category:** Factual Questions\ **Created:** [April 3, 2008, 8:48pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845 "2008-04-03T20:48:19Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Sophistry\_and\_Illusion](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@Sophistry\_and\_Illusion](https://boards.straightdope.com/u/Sophistry_and_Illusion)\ **Post date:** [April 3, 2008, 8:48pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845/1 "2008-04-03T20:48:19Z")
I’ve noticed that many passenger jets, even those without an engine mounted in the tail, seem to have an exhaust port in the tail of the plane. You can tell that hot gases are being exhausted from the port, so it’s like a car tailpipe. But what is its purpose? Where are the hot gases coming from? ---
**Author:** ![brewha](https://avatars.discourse-cdn.com/v4/letter/b/91b2a8/32.png) [@brewha](https://boards.straightdope.com/u/brewha)\ **Post date:** [April 3, 2008, 8:53pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845/2 "2008-04-03T20:53:01Z")
I’m guessing you mean something like [THIS](http://www.aerospace-technology.com/projects/dassault900ex/images/Falcon900EX_2.jpg-.jpg) That is an engine on top of the plane in front of the vertical fin. And that is the exhaust for that engine. If you mean something else, could you post a pic? I can think of no reason to have a rear exhaust other than for a third, center mounted, engine. Other than for an APU of course (after reading susbsequent posts :smack: ) ---
**Author:** ![Soul](https://avatars.discourse-cdn.com/v4/letter/s/e9bcb4/32.png) [@Soul](https://boards.straightdope.com/u/Soul)\ **Post date:** [April 3, 2008, 8:53pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845/3 "2008-04-03T20:53:55Z")
> **[Auxiliary power unit](https://en.wikipedia.org/wiki/Auxiliary_power_unit)** > > An auxiliary power unit (APU) is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft and naval ships as well as some large land vehicles. Aircraft APUs generally produce 115 V AC voltage at 400 Hz (rather than 50/60 Hz in mains supply), to run the electrical systems of the aircraft; others can produce 28 V DC voltage. APUs can provide power through single or three-phase systems. > During World War I, the British Coastal class bl... That rear exhaust port is used to vent the Auxiliary Power Unit, which provides power for the plane for purposes other than propulsion. ---
**Author:** ![puppygod](https://avatars.discourse-cdn.com/v4/letter/p/ce7236/32.png) [@puppygod](https://boards.straightdope.com/u/puppygod)\ **Post date:** [April 3, 2008, 8:54pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845/4 "2008-04-03T20:54:10Z")
Probably [APU](http://en.wikipedia.org/wiki/Auxiliary_power_unit). ---
**Author:** ![Sophistry\_and\_Illusion](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@Sophistry\_and\_Illusion](https://boards.straightdope.com/u/Sophistry_and_Illusion)\ **Post date:** [April 3, 2008, 9:03pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845/5 "2008-04-03T21:03:53Z")
Thanks. I googled every combination of ‘airplane rear exhaust port’ that I could think of, no joy. Now my ignorance done bin fitted. ---
**Author:** ![Sophistry\_and\_Illusion](https://avatars.discourse-cdn.com/v4/letter/s/b3f665/32.png) [@Sophistry\_and\_Illusion](https://boards.straightdope.com/u/Sophistry_and_Illusion)\ **Post date:** [April 3, 2008, 9:08pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845/6 "2008-04-03T21:08:17Z")
Where’s the air intake for that sucker? The wikipedia article talks like the APU is run off of compressed air, implying the plane actually carries the air supply for the APU, but the wording in the article is not clear. Do you guys know where the air for combustion comes from? ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [April 3, 2008, 9:42pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-a-rear-exhaust-port/443845/7 "2008-04-03T21:42:07Z")
No, the compressed air is used to spin up the main engines. The APU provides power to drive the air compressor. Since an APU doesn’t generate thrust, it doesn’t need a lot of air. You can see the air intake on a 747 here: [Boeing 747-4H6 - Qantas | Aviation Photo #0573755 | Airliners.net](http://www.airliners.net/open.file/573755/M/) I think the intake is covered up when the APU isn’t running.