Odd aircraft noises...

[FullWings]

I've just been sitting at home with the window open and heard a noise like the front fans of jet engines winding down but much more quickly than you'd expect to happen in the air. Sort of "WHOOooo" in rapidly descending pitch, followed shortly afterwards by the reverse. A little later, the normal sound of an airliner descending overhead at 10-15,000' (this is in Oxfordshire) became audible.

This is not the first time I've heard these kind of noises but they are very distinct tonight. We do have a significant inversion and light winds aloft, so there may be some odd refraction/tunnelling effects going on...

Has anyone else heard anything like this?

[flyboyike]

Absolutely.

[AtomKraft]

Flap hoot?

BAe 146

[compressor stall]

Speed brakes. Very noticeable on a32F aircraft.

[FullWings]

Votes seem to be coming in for speedbrakes. Can you hear this from inside the aircraft as it seems to be louder than any of the other noises, observed from the ground?

All the types I've operated have produced a fairly wide-spectrum rumbling rather than an identifiable note - peculiarity of the A32X...?

[MCDU2]

Lots of military bases around Oxfordshire and of course the Bovingdon hold for LHR is only down the road so could be anything.

[BARKINGMAD]

Doubt if a 146/RJ would be cycling the flaps at that altitude.

[Uplinker]

It will almost certainly be speedbrake deployment.

Aircraft that have been held up over certain airspace then need to descend more quickly after they have cleared that zone, in order to get back onto their descent profile.

To do this we extend the speedbrakes. The aircraft 'sees' the extra drag and with the engines already at idle, it will have to put the nose down to keep the speed correct - which results in an increased rate of descent. As the spoilers/speed brakes raise and lower, they cause an audible howl whose tone changes depending on the changing angle of the spoiler surfaces. You can't really hear this from Airbus cockpits, don't know about Boeing.

Alternatively, it could be flap howl. The BAe146 for example is very well known for this, but of course flaps are only extended to land, not while in transit or descent.

[dubbleyew eight]

do you hear it in the aeroplane? YEP. especially in the BAE146.