Stupid question about aircraft

[Fas1975]

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?

[RDM]

Exhaust from the generator?

[Shar2]

It's the exhaust for the Auxillary Power Unit, APU. This provides power when the main engines are shut down.

[Fluid]

APU. Auxiliary Power Unit.

It powers the aircrafts electrical systems untill the engines are started.

[Fas1975]

Very cool, thanks. And any idea what the noise is when the aircraft gets to gate and engines switched off?

[fozzy280472]

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 .

[tank slapper]

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.

[Fas1975]

I fly exclusively BA, not sure what they use on short haul fleet, will make a note next time for sure

[Simpo Two]

Fas1975 said:

What's the hole in the back of the aircraft for?

Nobody's mentioned (d): tail gunner

[DuraAce]

The 'drilling' noise is probably the engine fan blades clunking together as the engine slows to a stop, or windmills around in the breeze.

[Fat Albert]

The APU is actually a miniature Jet engine (Gas Turbine) hence the high-pitched shrill tone and the heat-haze from the exhust

[HoHoHo]

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?

[bennyboydurham]

If it's an Airbus you might be hearing the PTU, which sounds a bit like a barking dog.

http://www.youtube.com/watch?v=-nU3T4_cZZM&fea...

Someone cleverer than me will explain what it does and why you hear it.

[TorqueVR]

What's a PTU?

[Simpo Two]

Power transfer unit I deduce.

[Mr Sparkle]

Where does the Auxillary Power Unit get its air from? Anyone know how much thrust they produce?

[tank slapper]

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=qv0sBTvpcOg

And yes, that nutter does have an RB211 in his back garden: http://www.youtube.com/watch?v=qM7ksfRVF70

[tank slapper]

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.

[motomk]

Drilling sound might be the cargo bay doors being opened???

[Fas1975]

bennyboydurham said:

If it's an Airbus you might be hearing the PTU, which sounds a bit like a barking dog.

http://www.youtube.com/watch?v=-nU3T4_cZZM&fea...

Someone cleverer than me will explain what it does and why you hear it.

YES, that's the sound!