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?
Exhaust for the auxiliary power unit.
The plane needs power for its electronics, air conditioning, etc, even when it is not moving. As long as the plane is waiting at the terminal, it can run off the airport’s grid. After the plug is pulled, (for example, taxiing while waiting for takeoff) the engines need to run in order to provide electricity. Operating the main engines just for electricity is extremely inefficient, so it has an auxiliary generator for this purpose. However, on such a large airplane this auxiliary engine is so large that it requires a gigantic tailpipe thing to vent waste gases.
So, yeah, plane farts.
Well, that was fast! Thanks, Chihuahua.
Heh, my father used to tell me that’s where everything exits the plane when they flush the toilets on board.
…and why do they always sit next to me?
I realize Wikipedia shouldn’t be assumed to be the final authority, but according to its article on auxiliary power units, the purpose and function of these units is a little different from what Chihuahua said. It says the primary purpose of an aircraft APU is to provide power to start the main engines. It also says it’s “used to run accessories while the engines are shut down. This allows the cabin to be comfortable while the passengers are boarding before the aircraft’s engines are started,” which implies the aircraft is not using airport grid power to do so while parked at the gate. Also, taxiing while waiting for takeoff requires at least one of the main engines to be started to provide thrust.
Like anything it depends. **Chihuahua and Arcite **have the basics. The APU provides electrical power and air-conditioning. The details of how it is actually used varies.
Many airport authorities don’t like noise and restrict the use of APUs (maybe they should be running a library instead). There may be a time limit for APU use, with ground power from the aerobridge or a power cart being used at other times. Airlines are also keen to save fuel and minimising APU use can help with this. Ground power only supplies electrics though, so you need the APU for air-conditioning (the B787 may be an exception as it does not need engine/APU “bleed air” for its air-conditioning.)
On hot days and/or short turn arounds the APU will most likely be running at the gate. If it is a comfortable temperature or the aircraft is parked for some time the APU will be shut down and the aircraft run off ground power.
During pushback the APU supplies the necessary power or air supply to start the main engines. Once the main engines are started and everything is switched on, the main engines are then providing electrics and can provide air supply for the air-conditioning but the air-conditioning may be left to the APU until after take-off.
The APU may be left running during take-off or it may be shut down after start. It depends on the aircraft type and company procedures. Using the main engines for air-conditioning reduces the thrust available by a small amount. This affects take-off performance and in some aircraft take-off with the engines providing air-conditioning is prohibited. In others it is allowed but there is a performance penalty in the form of a reduction in the maximum allowable take-off weight for that particular take-off. So you can leave the APU running after start and use it for air-conditioning until after take-off. You can shut down the APU, save a (tiny) bit of fuel, and use the engines for air-conditioning, or you can just not have any air-conditioning at all during take-off.
Another consideration for take-off is that if you have an engine failure and still have the APU running you have more electrical redundancy than if it was shut down.
Otherwise if they go too fast the tail fins puff up like zeppelin vanes.
You will note that the “buttholes” are somewhat bigger on the Airbus aircraft, since they were outsourced to Greece. . . LOL. . .
c’mon, it’s a little funny. . . :rolleyes:
Is it also true that some aircraft have a device that generates power by airflow?
In case all other power sources are dead? This would need an intake (mouth) and butthole.
That’s the Ram Air Turbine, which swings out of the fuselage to provide some minimal amount of power for the controls. No fuel required, it’s just a small windmill.
Well I guess it wasn’t always so, but it explains why I don’t see the big battery cart anymore that they used to use to start the jet engines.
I was on a flight recently where we were delayed at the gate after boarding. The captain came over the intercom to say that the reason for the delay was that the APU wasn’t working, so they were having to bring a power cart out to start the engines. So evidently, commercial airlines are totally unconcerned about proceeding with a passenger flight with no functioning APU.
It’s the poop deck.
Yes, an APU is a convenience item. Provided you have adequate ground support you can do without it.
I didn’t know (not that I would know) that you could start other engines from the APU. Having said that, when an airplane starts, they start the first engine from the start cart (very, very different from a cart start). The other engines are interconnected and start use bleed air to start. 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 engines can produce their own compressed air (in a sense) called bleed air, and that can be plumbed to the other engines to get them going.
While I’m sure it’s possible have an electric start engine, it’s my understanding that using a start cart/compressed air is how it’s typically done.
In fact, that winding up/spinning up noise you typically associate with a jet engine starting is actually the sound of the start cart getting ready to blast pressurized air into the engine starter.
Here’s a video if you’re interested.
Oh, and BTW, a start cart is a cart (because it’s like a cart, ya know, on wheels, that starts the engine). A cart start (sometimes called a shotgun start), OTOH, is when you start an engine with a cartridge. It’s basically a giant cartridge filled with an explosive, similar to a big giant bullet. It’s loaded in, set off and the pressure from it is used to start the engine. It’s useful for (military) jets that are taking off in the middle of nowhere.
This is the best video I could find, looking quickly, but I think AgentJayZ might have one. You’ll notice not only the smoke, but how quickly the engines start. No hesitation, no winding up, just hit the button and they’re running…in the best of circumstances. If something goes wrong, you deal with it like a dud bullet or if it (hopefully) at least fires, you have to reload, if you have another.
I was once stranded on the tarmac, amongst other issues there was a problem with the APU and a part needed to be swapped out. They said that if we were in the air and it broke, it would be no big deal but they wouldn’t let us take off with it not in proper working order.
Makes sense, I’m sure “APU works properly” is part of the pilot’s checklist.
Smaller engines may use an electric start while larger ones will use air. The BAe146 uses electrics for the engine start. B737 and bigger use air start (electric start does seem quite rare, the F100 uses air for start and it is a similar size to the 146). B737 can use air from a ground cart, the APU, or the other engine. Many airports discourage starts on the bay so using the APU for the engine start during pushback is normal.
Whether to depart with the APU u/s or not might be a complicated decision. Generally if something can be fixed with minimal delay then it will be fixed. If it can’t be fixed then it’s generally better to go with no APU than to cancel the flight or delay it significantly. There may be other factors at play. An engine driven generator may be unserviceable provided the APU is running for example.
I’ve never been on a flight where they used a start cart. The main engines have always either been started while the plane was being pushed back or after pushback. But the APU is just a smaller gas turbine, so why wouldn’t it be able to start the main engines? After all, a start cart is… A smaller gas turbine on a cart.
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?