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Fas1975
Original Poster
566 posts
33 months
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I'm a frequent flyer and have a couple of generic questions about aircraft. What's the hole in the back of the aircraft for?  Looks like an exhaust pipe, and I've seen "shimmer" from planes which have just arrived at the gate. But the engines are on the wing, why is the exhaust pipe where it is? Second, everytime the plane pulls up to the gate, and the engines are switched off, there's a drilling sound coming from underneath the aircraft, again, what on earth is that noise?
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RDM
1,292 posts
76 months
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Exhaust from the generator?
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Shar2
1,894 posts
82 months
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It's the exhaust for the Auxillary Power Unit, APU. This provides power when the main engines are shut down.
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Fluid
659 posts
54 months
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APU. Auxiliary Power Unit.
It powers the aircrafts electrical systems untill the engines are started.
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Fas1975
Original Poster
566 posts
33 months
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Very cool, thanks. And any idea what the noise is when the aircraft gets to gate and engines switched off?
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fozzy280472
48 posts
17 months
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As stated above exhaust for apu , the drilling sound you can hear depends on what aircraft type it is ,it can be brake cooling fans or could be hydraulic pumps or hyd power transfer units or even airco packs running , as I said earlier on very type dependant ,hope that helps .
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tank slapper
7,746 posts
152 months
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fozzy280472 said: As stated above exhaust for apu , the drilling sound you can hear depends on what aircraft type it is ,it can be brake cooling fans or could be hydraulic pumps or hyd power transfer units or even airco packs running , as I said earlier on very type dependant ,hope that helps . The power transfer units on Airbus aircraft are particularly noisy for some reason.
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Fas1975
Original Poster
566 posts
33 months
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I fly exclusively BA, not sure what they use on short haul fleet, will make a note next time for sure
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Simpo Two
54,231 posts
134 months
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Fas1975 said: What's the hole in the back of the aircraft for? Nobody's mentioned (d): tail gunner
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DuraAce
313 posts
29 months
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The 'drilling' noise is probably the engine fan blades clunking together as the engine slows to a stop, or windmills around in the breeze.
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Fat Albert
939 posts
50 months
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The APU is actually a miniature Jet engine (Gas Turbine) hence the high-pitched shrill tone and the heat-haze from the exhust
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HoHoHo
8,327 posts
119 months
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DuraAce said: The 'drilling' noise is probably the engine fan blades clunking together as the engine slows to a stop, or windmills around in the breeze. Are you suggesting the blades hit each other?
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bennyboydurham
1,207 posts
43 months
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TorqueVR
887 posts
68 months
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Simpo Two
54,231 posts
134 months
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Power transfer unit I deduce.
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Mr Sparkle
1,725 posts
39 months
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Where does the Auxillary Power Unit get its air from? Anyone know how much thrust they produce?
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tank slapper
7,746 posts
152 months
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HoHoHo said: DuraAce said: The 'drilling' noise is probably the engine fan blades clunking together as the engine slows to a stop, or windmills around in the breeze. Are you suggesting the blades hit each other? The blades are slightly loose in the hub, and have clappers to prevent them contacting each other at low speeds. It's those that you can hear touching when the fan windmills: http://www.youtube.com/watch?v=qv0sBTvpcOgAnd yes, that nutter does have an RB211 in his back garden: http://www.youtube.com/watch?v=qM7ksfRVF70
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tank slapper
7,746 posts
152 months
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Mr Sparkle said: Where does the Auxillary Power Unit get its air from? Anyone know how much thrust they produce? They have an intake to draw air through. It is in different places depending on the aircraft, but usually it's near to the APU. On some aircraft it is covered by a door when not in use, or is underneath the tail, so can be hard to spot. The APU doesn't produce a lot of thrust as they aren't optimised for that. They are turboshaft engines hooked up to a generator, but also supply high pressure air to start the main engines.
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motomk
1,239 posts
113 months
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Drilling sound might be the cargo bay doors being opened???
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Fas1975
Original Poster
566 posts
33 months
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bennyboydurham said: YES, that's the sound!
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