It definately is speed brakes. I fly the A320 over the OP's house anywhere from FL180-80 inbound to Bristol and its there where we often have to use speedbrakes to get down and slow down after being kept high on profile. Sure I said this a page ago
Its not the air rush noise, in the recording (which BTW is the lightsaber sound from star wars) its the low whoop... Have an Airbus driver here from JetBlue, fortunately, there is a pub along the west approach to KSEA, the altitude on the flightpath is around 5500-6000 when the whoop sounds, he says it is when you go to config 1.
We had the ADSB running on the laptop. He pointed out by sound when it was an A320, vs the 737 which just makes an air rush noise...
(not sure about the original FL15-FL12 poster)
Last edited by FlightPathOBN; 27th Aug 2012 at 18:27.
I am indebted to all you professionals who have taken the time to answer my query - I really didn't think it would generate that much interest !
Meant to say in my OP that I'm almost 'nose-on' to the aircraft concerned so maybe that makes a difference. In case Jontyisn't joking I'll sit under an umbrella ! and 1pudding1 - no need to apologise - I enjoy seeing you guys and sorely miss the Hercs, VC10s, C17s and other guests that no longer go over my house into Lyneham.
Interestingly enough, there is a fix for this! No really...
It appears that they even had the folks at DLR looked at this. The study was only on the A320, so I am going to assume it is specific to that ac. From the study and images provided, it appears the slat configuration causes the noise. As the slat extends, airflow is directed to the space behind the slat, rather than the bottom of the wing...creating what they call as 'cavitation resonance'. I find it similar to blowing across the top of a bottle.
The fix is to place a small tab on the bottom of the slat, or slots, to reduce the cavitation pressure and direct airflow back to the bottom of the wing, rather than up through the space...
(just so you know, they even tried messing with the AoA to solve this...but to no avail)
So, there you go....
Last edited by FlightPathOBN; 28th Aug 2012 at 20:52.
As the slat extends, airflow is directed to the space behind the slat, rather than the bottom of the wing...creating what they call as 'cavitation resonance'. I find it similar to blowing across the top of a bottle.
That's the noise! I opened a thread about two years ago, asking almost the same question as the OP, although from a rather different viewpoint. I live in south London, where you guys are descending to intercept the ILS on 28L at Heathrow. I used to hear the 'blowing over a bottle neck' noise - I thought of it as a 'sharp groan', or 'dropping out of warp' - quite frequently, but not as often of late.
Of course, you will be at or a bit below 5000ft over my house, so aeroplanes would be in quite a different configuration to the OP's 18000, which is a bit of a mystery. My nephew is a Boeing driver and hadn't got a clue what I was talking about, so maybe it is only buses?
From Denver - yes, "dropping out of warp," "blowing across bottle neck," are also nice descriptions for what I'm hearing. Slat deployment also seems to make sense in terms of altitude/distance from airport.
Side question - in that report .pdf, where slat noise mitigation was attempted by "filling in" the slot between the slat and wing with brushes or extensions - wouldn't that to some extent reduce slat effectiveness? I though slats delayed onset of stall in part by the venturi effect of the slot (similar to slotted flaps).
Ok, after hearing aircraft flying over my house the whole afternoon and paying close attention to them for a change I am now convinced that it is the speedbrake. I even had a chap who deployed, retracted and deployed them again (as I have seen some colleagues do when selecting CONF1 with the speedbrakes out), with all the howling that comes with it.
Also in the flightdeck you can hear this howling at certain speeds when moving the speedbrakes.
Thanks for that link RRR, although much of it went over my head. By mentioning folio 24, I take it you are thinking 'slat noise'? You may be right and I note that the noise propagation is to the rear of the aircraft, which might chime well with my, admitedly very unscientific, observations. I don't ever recall seeing the aeroplane I had just heard make the noise and surmise it had always just passed. With so many houses in close proximity round here, I don't get to see them very long.
We definitely must be talking about different sounds here. All the aircraft I have noticed making this brief groaning sound are flying the SID's and still below 6000ft.
It is definitely not the spoilers or a config change.. Not quite sure how some of you on the ground seem the know the flight crew are deploying the spoilers? X ray vision?
I'm still convinced it is some weird sound reflection of certain terrain, was hoping for a definitive answer from someone.
Brief groan is what I'm talking about Noland3 and only one. It is loud enough and sharp enough to be the only thing that calls my attention to that particular aeroplane. It is only unusual noises that make me look up much. Concorde always did, of course.
I always thought SIDs are 'Standard Instrument Departures', but I'm pretty certain the noise is only from aeroplanes descending and turning to intercept. I think my house is about 14 nautical miles from LHR and typically aircraft passing overhead come off the Biggin or Epsom stacks and are flying roughly north west. To give them 3000ft 10nm from touchdown, they need to be close to 4000ft overhead my house.
You guys will know exactly, but I can see from the ground that all sorts of config changes are going on above me, up to and including - rarely nowadays - wheels down or coming down. I can and have heard throttle up to arrest decent and throttle down to peg 240kts (is it 240 at that distance?)
My sense - as an engineer although not an aviation one - is the word resonance. Something about the noise says a resonant sound to me and since blowing across a bottle neck is exactly that, I can visualise slat or flap movements generating them. Just as they begin to move, new and temporary airsteams 'blow across' temporary cavities - bingo; a resonant oscillation that damps out almost immediately as the device reaches its normal or first operating position. It won't happen again because the cavity is no longer resonant.
I may be talking bo11ocks, but you have to admit it's good bo11ocks!