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Landroger
1st Apr 2009, 00:29
Here's a real geeky question, but a genuine one I have pondered for many years. I live under one of the LHR approach routes from, I think, the Biggin and Epsom stacks. They pass over my house on their way to acquire 27L - again I speculate - I'm sure you know roughly where I mean.

Not only do I often stop and watch as they pass over me at about 5000', but I listen too and apart from the usual low power jet scream, and the occasional sound of engines spooling up, some of the aircraft make the most peculiar noise, just once. I have never been able to correlate the noise with any particular type - they all seem to do it, sometimes.

It seems to happen when the aircraft is slowing down and takes the form of a loud groan, like a wooden door that is tight in its frame. It only happens once and it doesn't always happen, but its always from aeroplanes maneouvering to acquire the ILS. When we camp at Broadstone Warren Scout camp, you can hear aeroplanes on their way in to LGW.

I've asked my nephew - a B757/767 F/O - and he doesn't know. Any ideas? (I told you it was geeky)

Roger.

dany4kin
1st Apr 2009, 03:05
Think I've seen this question (and answer) somewhere here before.

I believe that sound is caused by the extension of the speedbrakes, I guess the airflow being disrupted and all that.

I stand to be corrected however.

Flight Detent
1st Apr 2009, 03:25
Hi Landroger...

I don't go along with 'dany4skin' at all...the use of speedbrakes anywhere near that sector of the flight would be very unusual indeed!

My best guess would be that you are near the spot where, as the airplane is slowing on the localizer (electronic signal that is an extension of the runway centerline), probably with 5 flap set. Then the intercept with the glideslope is made (glideslope is another electronic signal that tells the autopilot to start/maintain a 3 degree slope towards the runway threshhold, arriving there at about 50 feet above), followed right there with selection of 15 flap and gear down.

This large amount of increased drag (15 flap and gear down) then forces the auotothrottle system to increase thrust to maintain the desired speed...what you are hearing is that initial 'growling' windup of the engines from their low(ish) setting to something quite a bit higher.

Sorry if I have over-simplified this, but I don't know how familiar you are with these basic maneuvers during a normal ILS approach.

Cheers...FD...:)

mvsb1863
1st Apr 2009, 04:33
Hi Landroger

I don't know if you are a pilot or not but I have the answer for your question, and if you are a pilot you will understand the reason for this noise.

"AIRPLANE NOISE, THE SOUND OF FREEDOM"

:ok:

tartare
1st Apr 2009, 08:38
I suspect they're lowering flaps.
The Bae146 made a quite spectacular sound due to increased drag when flaps were lowered, clearly audible inside cabin, to the point that many airlines would incorporate mentioning it in the cabin crew PA announcement on approach... basically, "don't worry, the sound is normal, the plane is not falling apart."
I live under the approach to AKL, and the sound of increased drag as flaps are lowered is often the first thing to wake me at 5am, as NZ1 arrives from LHR.

Rainboe
1st Apr 2009, 09:53
Living on the south coast, lots of planes pass over Portsmouth heading up to London Airports from Southwestern Europe and Atlantic flights. They would be usually about 10,000' plus. I very frequently hear this peculiar noise. The suggestion it could be a spool up is good, but they can't all be spooling up at the same place. The noise pattern on the ground of a jet engine is very complex, and i think it may possibly be caused by the movement of the noise 'carpet' over the ground, as you pas from one region to another. It is a very strange noise, like a spooling-up growl. It sounds engine related to me.

Yobbo
1st Apr 2009, 12:30
The BAC-111 flaps made a distinct noise when going from 0 to 3*.

Landroger
1st Apr 2009, 15:37
Hi Landroger...

I don't go along with 'dany4skin' at all...the use of speedbrakes anywhere near that sector of the flight would be very unusual indeed!

My best guess would be that you are near the spot where, as the airplane is slowing on the localizer (electronic signal that is an extension of the runway centerline), probably with 5 flap set. .................................

............................................................ ...................................
Cheers...FD...http://static.pprune.org/images/smilies/smile.gif


Thanks for the detailed answer FD, but I think I'm further out than that. The centerlines for both 27L and 27R extend back to just over the Thames at Battersea and more or less Buckingham Palace. You want acquisition by 10 miles out and 3000' as I understand it? I live about 9 miles south of that point and aircraft that peeled off the bottom of either Biggin or Epsom stacks (at 5000' is it?) are descending and slowing, but have not yet acquired the ILS.

Tartare

You may be right as I think aircraft have some level of flap selected as they pass and a few even seem to extend their undercarriage, but this is less common. My nephew thought it may have been some change in bleed pressures within one or more of the engines, but he admitted he was guessing. I don't think the noise is audible from inside the aeroplane and, unless you live close to the pattern of a large airport - as I do - you may never have heard it.

Its like the slightly desperate sounding sigh some cattle make - not 'aerodynamic' at all, but its quite load and distinctive.

Roger.

goudie
1st Apr 2009, 15:51
I live North East of Luton and recently, have heard an engine sound I cannot recall hearing before, on the approach to Luton from the East. It's a wailing sound with decreasing pitch and is in two stages. The latter stage being lower in pitch than the first. The timing between the two stages is brief and always the same. Could it be the same noise, I wonder?

Capt Pit Bull
1st Apr 2009, 15:59
Sounds like something hydraulic related to me, hydraulics often groan when they are deploying or retracting things, especially if an electrical pump or PTU triggers to top the pressure off after some demands been on the system and dropped the pressure a bit. E.g. configuring.

pb

Mark 1
1st Apr 2009, 16:05
It's difficult to know without hearing the noise, but some possibilities:

Compressor blade interaction tones - the fan will be subsonic probably at approach power and internal turbomachinery noise more easily propagates against a slower axial flow. It will be fairly directional, so may only last a few seconds, but will sound like discrete tones at mid to highish frequencies (e.g. 80 blades at 4000 rpm shaft speed gives ~5KHz tone)

Combustion instability - a low frequency 'growl' or 'hoot' at around 500hz, more commonly heard at very low powers when starting up or taxying.

Airframe generated noise - this is almost the most dominant noise source on some big iron at approach powers. Flaps, slats, gear bogeys etc all generate vortices. Usually quite broadband and lowish frequency, but smaller items can generate more high pitched disturbances.

Handling bleeds - often open during power changes to offload the compressor by leaking air into the bypass duct. This can create a bit of a whistle on some powerplants.

I can't give you a definite, but maybe some food for thought.

dany4kin
1st Apr 2009, 22:55
"I don't go along with 'dany4skin' at all..."

Oh get my username right please!

One letter makes an awfully big difference....

Capt Claret
2nd Apr 2009, 00:04
Flaps and/or slats being extended.

As mentioned above, the 146 flaps positively howl when travelling from 0 to 18 or vice versa. So much so that at the co I work for we mention it in the safety brief pa.

The Douglas/Boeing 717 that I now fly, also has a mini-howl as the slats extend or retract, but NO WHERE near as loud as the 146. :}

muduckace
2nd Apr 2009, 01:43
All aircraft have different environmental noise levels. The MD-11 is a Cat 4 noise abatment aircraft while the 747-400 is not. Extra engine? Or environmental air displacement? Or increased thrust as A result of flap settings?. The aircraft share the same engine options.

This is just an example,

Flight Detent
2nd Apr 2009, 03:17
So "dany4kin"....you still think that was unintentional....

I chuckled for 20 minutes after I wrote that,

and wondered how long it would be until somebody noticed!!!

I hope I didn't offend you (too much!), it's just one of those simply random things that just seemed right at the time!

Cheers...FD...:E

FLCH
2nd Apr 2009, 04:01
Could it be the sound of the slats extending ? At least on the 757's on initial extension there is a brief "groan" that is always heard. I'm guessing as the air gap increases between the leading edge and slat forming a brief harmonic vibration. I also hear it on retraction, kind of reminds me of when the tank in the toilet is approaching full the plumbing gives that slight and brief groaning sound !
That's my opinion though I may not necessarily agree with it.

dany4kin
2nd Apr 2009, 04:13
Flight Detent

No offence taken, it's so strange I didn't even think of that conotation when I came up with that username... I probably should have done!

The dany bit is an anagram of my name (cryptic eh?), can't remember the 4kin logic however... type it in to youtube if you like guitar though :ok:

Sorry chaps, back on subject... That 146 noise flap is very noticable from inside the cabin.

Diesel Fitter
2nd Apr 2009, 07:22
I hope I didn't offend you (too much!)

Forget about him - think of poor Danny! :eek:

Lightning5
2nd Apr 2009, 08:07
Could be engine bleed valves opening/closing with throttle movement. Just a thought.

HighHeeled-FA
2nd Apr 2009, 09:27
Judging by your position, I'd go with the flaps down in order to maintain 160 till 4DME.

Cremeegg
2nd Apr 2009, 21:42
Seem to recall a similar thread on here a while ago where the noises on the ground were discussed. I seem to recall that it was 146 & the smaller Airbuses A320 series that were the main offenders. I believe early flap extension noise not engine noise at all.

I often work outside not too far from your position and am able to enjoy the "entertainment" overhead on fine days. I thought that aircraft stacked at Ockham not Epsom - not too far apart - but that's just the pedant in me.

Time to try the search function.........

matblack
2nd Apr 2009, 22:42
The noise may be caused by an exhaust valve which opens when aircraft are descending and the pressure differential reaches a low level. You can also hear it in reverse when ascending. You can hear it clearly whilst on board an MD80 as it climbs. I think this exhaust port is open on the ground and it closes as the pressure differential increases with altitude.On the way down it opens as the pressure differential reduces to a low level. In flight at altitude it is normally closed. As far as I understand it's an exhaust that dissipates warm air from the electronics bay.

Slow Progress
2nd Apr 2009, 22:54
Could it be the apu starting up?

Capt Claret
3rd Apr 2009, 00:19
matblack, you could be pulling one's leg here but incase you're not; at 5000' most jets will have a cabin altitude above aerodrome level, will be at low power and it's unlikely that the exhaust (outflow) valve will be open and the idea is to pump air into the cabin to cause its level to decrease to be at aerodrome level by landing.

Landroger
3rd Apr 2009, 00:31
:ok: I must admit I'm feeling a bit smug. I really thought someone would straight away come on and say; 'Stupid boy Rog', its the Internal Change Over Low Pressure Flange operating - duh!' :rolleyes: But they haven't. :)

I like the '757 slats opening' momentary resonance - that could be it and there are enough 757's, Lord knows. Is that about right for distance from touch down?

Mark 1's 'Compressor blade interaction tones' are interesting, because it seems to me the noise is preceded by a short, quite quick 'down doppler' engine effect, very suggestive of slowing rapidly. I always think of it as 'dropping out of warp', but there is an element of 'hoot' about it.

Keep the ideas coming guys, lets see if we can get a definitive answer, because I really want to know. :)

Roger.

dany4kin
3rd Apr 2009, 02:35
Is anybody else thinking of standing under a flight path this weekend to find out just what this noise is? I hear the weather in the South East is supposed to be lovely...

Wjh308
3rd Apr 2009, 03:51
I think it is the engine spooling up briefly. The RB211's having 3 stages tend to make that growl. Watch video's of L1011's taking off, and tell me if that growl is the same sound you are hearing.