# Why do airplanes have buttholes? **URL:** \ **Category:** Factual Questions\ **Created:** [June 22, 2016, 10:49pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096 "2016-06-22T22:49:57Z")\ **Posts on this page:** 17\ **Page:** 2
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [June 23, 2016, 1:35pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/21 "2016-06-23T13:35:41Z")
> [@Leo\_Bloom](#): > > Wait, so there’s three energy outputs of the APU, or two? (And _all_ can do electricity and air, or only the “larger” options [ref **Richard** above])? > > 1: Electric power, self generated (after ignition) > 2: “Ground power,” which comes off the airport main circuit > 3: Compressed air > > or am I mixed up about #1 and #2? APUs are generally small-ish turboshaft engines. the turbine powers an output shaft which then spins pneumatic compressors and electrical generators. The pneumatic compressor(s) can supply cabin air _or_ air to start the main engines. On most flights I’ve been on, the cabin air circulation runs while the plane is at the gate and during pushback until the main engines are to start. then the cabin air circulation cuts off. Once the main engines are started, they take over supplying cabin air circulation (via bleed air\*) and electrical power from engine-driven generators. (\* the Boeing 787 doesn’t use bleed air in favor of using electric air compressors) ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [June 23, 2016, 1:44pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/22 "2016-06-23T13:44:18Z")
here’s an example of a DC-9: at first, the whine you hear is the APU turbine. Then at about 42 seconds you hear the cabin air stop, then a couple later one engine starts. ---
**Author:** ![Richard\_Pearse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/richard_pearse/32/13144_2.png) [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)\ **Post date:** [June 23, 2016, 1:54pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/23 "2016-06-23T13:54:07Z")
> [@Leo\_Bloom](#): > > Wait, so there’s three energy outputs of the APU, or two? (And _all_ can do electricity and air, or only the “larger” options [ref **Richard** above])? > > 1: Electric power, self generated (after ignition) > 2: “Ground power,” which comes off the airport main circuit > 3: Compressed air > > or am I mixed up about #1 and #2? Two. APU provides electrical power and an air supply. Ground power is power supplied to the aircraft by either a ground power cart or a plug direct from the aerobridge. Ground power is an alternative electrical power source to the APU. You use one or the other, not both. All APUs I’m aware of can supply air and electrical but not all jet engines (propulsive) need air to start, some small jets such as the ALF502 are started with an electric starter motor. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [June 23, 2016, 1:59pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/24 "2016-06-23T13:59:32Z")
> [@Richard\_Pearse](#): > > Two. APU provides electrical power and an air supply. > > Ground power is power supplied to the aircraft by either a ground power cart or a plug direct from the aerobridge. Ground power is an alternative electrical power source to the APU. You use one or the other, not both. > > All APUs I’m aware of can supply air and electrical but not all jet engines (propulsive) need air to start, some small jets such as the ALF502 are started with an electric starter motor. That’s because the ALF502 was a Lycoming T55 helicopter turboshaft converted into a turbofan. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [June 23, 2016, 2:25pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/25 "2016-06-23T14:25:28Z")
> [@Richard\_Pearse](#): > > Two. APU provides electrical power and an air supply. > > Ground power is power supplied to the aircraft by either a ground power cart or a plug direct from the aerobridge. Ground power is an alternative electrical power source to the APU. You use one or the other, not both. > > All APUs I’m aware of can supply air and electrical but not all jet engines (propulsive) need air to start, some small jets such as the ALF502 are started with an electric starter motor. Thanks - Off to find out what an aerobridge looks like (a giant three-holed socket?). Give you guys an inch … \* ---
**Author:** ![swampspruce](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/swampspruce/32/261_2.png) [@swampspruce](https://boards.straightdope.com/u/swampspruce)\ **Post date:** [June 23, 2016, 2:28pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/26 "2016-06-23T14:28:10Z")
Not really relevant to airliners but early jets also used explosive cartridges to fire up ( A nice video clip of a [Canberra](https://www.youtube.com/watch?v=g9fkmqPHTDEhttp://) starting). A 747 cart starting might be up there on the List Of Things I’d Like To See, or maybe not… 🙂 ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [June 23, 2016, 2:34pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/27 "2016-06-23T14:34:16Z")
> [@Chihuahua](#): > > However, on such a large airplane this auxiliary engine is so large that it requires a gigantic tailpipe thing to vent waste gases. It’s not a gas-powered generator like you get at Home Depot, it’s another jet engine. Jet engines work by blowing air through them, so the butthole is that engine’s output airflow. I assume they use a jet engine because they already have jet fuel on board. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 23, 2016, 2:54pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/28 "2016-06-23T14:54:55Z")
> [@CurtC](#): > > I assume they use a jet engine because they already have jet fuel on board. Tangent: The B-29 _Kee Bird_ (‘Kee-_rist_, it’s cold!’ – It made an emergency landing in Greenland) was destroyed by fire when the jury-rigged fuel tank for its reciprocating (gasoline-powered) APU began to leak. ☹ ---
**Author:** ![Arcite](https://avatars.discourse-cdn.com/v4/letter/a/67e7ee/32.png) [@Arcite](https://boards.straightdope.com/u/Arcite)\ **Post date:** [June 23, 2016, 4:11pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/29 "2016-06-23T16:11:04Z")
> [@CurtC](#): > > It’s not a gas-powered generator like you get at Home Depot, it’s another jet engine. Jet engines work by blowing air through them, so the butthole is that engine’s output airflow. I assume they use a jet engine because they already have jet fuel on board. Also, jet engines have higher power-to-weight ratios and greater efficiencies than piston engines, which is one reason they’re good for aircraft. ---
**Author:** ![Just\_Asking\_Questions](https://avatars.discourse-cdn.com/v4/letter/j/ba9def/32.png) [@Just\_Asking\_Questions](https://boards.straightdope.com/u/Just_Asking_Questions)\ **Post date:** [June 23, 2016, 4:53pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/30 "2016-06-23T16:53:02Z")
> [@Johnny\_L.A](#): > > Tangent: The B-29 _Kee Bird_ (‘Kee-_rist_, it’s cold!’ – It made an emergency landing in Greenland) was destroyed by fire when the jury-rigged fuel tank for its reciprocating (gasoline-powered) APU began to leak. ☹ Oh that pissed me off! All that work, all that money - four new engines, and stupid carelessness sent the whole thing to the bottom of the lake. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [June 23, 2016, 4:55pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/31 "2016-06-23T16:55:46Z")
> [@Just\_Asking\_Questions](#): > > Oh that pissed me off! All that work, all that money - four new engines, and stupid carelessness sent the whole thing to the bottom of the lake. Me too. I saw the _Nova_ documentary once. Once. I knew beforehand how it would end, because I’d been following the story. It was hard to watch. I can’t bear to watch it again. ---
**Author:** ![Count\_Blucher](https://avatars.discourse-cdn.com/v4/letter/c/c5a1d2/32.png) [@Count\_Blucher](https://boards.straightdope.com/u/Count_Blucher)\ **Post date:** [June 23, 2016, 4:57pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/32 "2016-06-23T16:57:04Z")
> [@bizzwire](#): > > Whenever I’m taxiing into or out of the gate, and I’m looking out the window, every airplane has an…orifice on the tail thingy. What’s it for? V.I.P.s…? ---
**Author:** ![Joey\_P](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@Joey\_P](https://boards.straightdope.com/u/Joey_P)\ **Post date:** [June 23, 2016, 6:08pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/33 "2016-06-23T18:08:38Z")
> [@swampspruce](#): > > Not really relevant to airliners but early jets also used explosive cartridges to fire up ( A nice video clip of a [Canberra](https://www.youtube.com/watch?v=g9fkmqPHTDEhttp://) starting). A 747 cart starting might be up there on the List Of Things I’d Like To See, or maybe not… 🙂 The difference between a cart start and a start cart was mentioned up thread. 🆒 ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [June 23, 2016, 6:27pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/34 "2016-06-23T18:27:09Z")
> [@Joey\_P](#): > > The start cart is essentially just a big, gas powered, air compressor and pumps very high velocity heated air into a starter that spins up the engine. The start cart for the SR-71 actually had a driveshaft that provided for direct transfer of mechanical power from two V8 engines to the J58 engines on the plane. ---
**Author:** ![jz78817](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jz78817/32/1028_2.png) [@jz78817](https://boards.straightdope.com/u/jz78817)\ **Post date:** [June 23, 2016, 7:19pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/35 "2016-06-23T19:19:13Z")
> [@Arcite](#): > > Also, jet engines have higher power-to-weight ratios and greater efficiencies than piston engines, which is one reason they’re good for aircraft. they’re not jet engines; they’re turboshafts. a turboshaft uses a second turbine in the exhaust which turns an output shaft to do mechanical work (spin a generator, or prop, etc.) A jet engine produces thrust which moves the vehicle. and gas turbines do have higher power-to-weight ratios, they are not more efficient than piston engines. especially not compared to diesel engines. Turbines are a good fit for aircraft because of that power-to-weight ratio and their relative mechanical simplicity. Plus, you try designing a piston engine which can be used to push a 777 or A380 around which has both long service life and minimal down time. ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [June 23, 2016, 8:32pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/36 "2016-06-23T20:32:08Z")
> [@Machine\_Elf](#): > > The start cart for the SR-71 actually had a driveshaft that provided for direct transfer of mechanical power from two V8 engines to the J58 engines on the plane. Cool. ---
**Author:** ![swampspruce](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/swampspruce/32/261_2.png) [@swampspruce](https://boards.straightdope.com/u/swampspruce)\ **Post date:** [June 23, 2016, 9:51pm UTC](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096/37 "2016-06-23T21:51:41Z")
> [@Joey\_P](#): > > The difference between a cart start and a start cart was mentioned up thread. 🆒 Cartridge start, not a start cart. Sorry for the obfuscation…😉 I used to use a CSU (Combined Services Unit, air and electrical) to start the F-5 when I was in the military. I know the ins and outs of starting airpplanes generally… [Previous page](https://boards.straightdope.com/t/why-do-airplanes-have-buttholes/758096.md?page=1)