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TheCommodore
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Identify This: Strange Approach Noise B767

Fri Dec 28, 2007 12:54 am

I live at Wahroonga under the northern approach for Sydney Kingsford Smith's main runway so I'm a little familiar with the variations of engine sounds of different aircraft types. One sound that has me perplexed though is a strange noise that seems to be associated with B767's on approach.

I would estimate that aircraft passing directly overhead are probably within 5 minutes of touchdown, and almost never have extended their landing gear.

I often hear a short, loud sound lasting no more than about a second that sounds like a low pitched 'BWaaaaRP'. It's definitely not the 'ripping' sound I've read about in other discussions. It's a bit more like the type of sound you might expect a rubber footed chair leg to make as it is dragged across the floor.

I'm wondering if it could be an aerodynamic sound that occurs as slats, flaps, or spoilers are beginning to be deployed or whether it's something else all together? Perhaps it's a sound made as the doors begin to open so the undercarriage can be extended, but Wahroonga's a bit too far from the airport for that isn't it?

I've searched the forum for similar discussion and can't seem to find anything. Apologies in advance if this subject has already been discussed.  confused 
 
rendezvous
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RE: Identify This: Strange Approach Noise B767

Fri Dec 28, 2007 1:21 am

From what I've seen of planes, there's quite often a highish pitch short noise when the slats at the front of the wing move. If you're on the early part of the approach, then this would seem about the right place for this to happen.
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Sun Dec 30, 2007 9:59 pm

Thanks Rendezvous,

Slats it is then!
 
cptspeaking
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RE: Identify This: Strange Approach Noise B767

Sun Dec 30, 2007 10:39 pm

I sincerely doubt what you're hearing is any of the aerodynamic devices extending...not over the engine noise from that sort of distance anyway. I also live under the approach path to an airport and from this as well as other spotting, I've noticed there are times where as the plane is just about to go over, you end up in just the right spot where there is some sort of resonance off seemingly the engine cowling that makes a fairly loud noise, somewhat like you're describing. I notice it almost every time with heavy jets (A300, 777, 757 - may not be a heavy..see other discussions for that one  Wink ), and sometimes on the smaller ERJs and CRJs, but not often.

Ever heard a do-jet start? Those make a noise similar to what I'm describing every time they start...

Your CptSpeaking
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Sun Dec 30, 2007 11:53 pm

Hi CptSpeaking,

You're right. There's no way you'd hear the noise of the motors that power the slats as they extend, over the sound of the jets, which is why I was fairly certain it must be some sort of aerodynamic noise.

It's just like a sudden 'skud' sort of sound, if that makes any sense.

Interesting that you seem to be in a similar location to me. Not on final leg but only a few kms away from that. I'll find out what height the aircraft are at if possible. That might help someone to offer other clues.

Your idea that it might be related to the position of the observer and be a resonance from the engine cowling is interesting. Aircraft overhead of me are sometimes banking steeply, which could be a relevant fact I failed to mention initially. Most though, are just flying straight over.

Unlike Lynchburg, Sydney sees few A300's or B757's passing overhead regularly but 777's are dead common. I'll keep a closer watch over the next few days and see if I can confirm whether the noise is only generated by 767's, or 777's, or both.
 
cptspeaking
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RE: Identify This: Strange Approach Noise B767

Sun Dec 30, 2007 11:59 pm



Quoting TheCommodore (Reply 4):
Unlike Lynchburg, Sydney sees few A300's or B757's passing overhead regularly but 777's are dead common. I'll keep a closer watch over the next few days and see if I can confirm whether the noise is only generated by 767's, or 777's, or both.

I only wish we saw those in Lynchburg!! Biggest thing I've seen in there is a BBJ  Wink I live (when I'm not in school) on about a 5 mi. final to RDU...we see 4-5 A300s each day, about 7 757-200s, and one daily AA 777-200, substituted seasonally by a 767-300.

Your CptSpeaking
 
boeing767mech
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 12:58 am



Quoting TheCommodore (Thread starter):
One sound that has me perplexed though is a strange noise that seems to be associated with B767's on approach.

I would estimate that aircraft passing directly overhead are probably within 5 minutes of touchdown, and almost never have extended their landing gear.

I often hear a short, loud sound lasting no more than about a second that sounds like a low pitched 'BWaaaaRP'. It's definitely not the 'ripping' sound I've read about in other discussions. It's a bit more like the type of sound you might expect a rubber footed chair leg to make as it is dragged across the floor.

If you where sitting in front of the wing you where heaing the Slat power drive unit PDU extending the slats, and since these like the flaps are driven by jack screws you can hear them over the sound of the engines. If you where sitting aft of the wing you hear the Flap PDU or if you where sitting aft LH side of the aircraft you hear the ADHP of Air Driven Hydraulic Pump which aslo can be heard over the engines.

David
 
cptspeaking
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 2:26 am



Quoting Boeing767mech (Reply 6):

I believe the OP was sitting on the ground  Wink I would definitely agree with a mechanical sound if he were in the airplane though...
 
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jetmech
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 2:42 am



Quoting TheCommodore (Thread starter):
I live at Wahroonga under the northern approach for Sydney Kingsford Smith's main runway

Lucky you, cossetted away in the leafy tree lined avenues of the upper North shore!

Quoting TheCommodore (Thread starter):
I often hear a short, loud sound lasting no more than about a second that sounds like a low pitched 'BWaaaaRP'.

It could be something to do with the engine air bleeds. Air is bled from the engine for two purposes, supplying the pneumatic system, and controlling the air flow through the compressor. What you may have possibly heard was the pneumatic system switching over from the low stage to high stage supply due to the engines throttling back, or the opening or closing of some of the compressor flow control bleed valves. The noise sounds like it was too brief to have been flaps or slats.

Regards, JetMech
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 4:09 am

Ah huh!

JetMech... you may have the winning answer!

When not on the ground, 'cossetted away in the leafy, tree lined avenues of the Upper North Shore', but actually up in the air in QF767's, on approach to SYD, I have certainly noticed the air conditioning surging as the jets spool up and down. I've read about this in other Tech/Ops discussions and those threads definitely note the pneumatic system switching over from the low stage to high stage. That wouldn't be unique to a 767 though would it? I suppose most jets employ a similar system.

Then there's another factor. If you're talking QF 767's, there's the GE powered ones and then the RR powered ones, which, correct me if I'm wrong, are the one's on lease from BA. Still, they probably have the same two stage pneumatics I guess.

It's true that a one second 'bwaaarp' sound couldn't be the slats extending, alone, as they take a bit longer than that, but I wondered if it might just be a sound the occurs as the airflow over the leading edge is disturbed, as the slat begins to move.

However, opening or closing of some of the compressor flow control bleed valves sounds like a plausible explanation as well!

Regards from the leafy bosom of the Nth Shore!  cheeky 
 
dl757md
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 4:26 am



Quoting TheCommodore (Thread starter):
I often hear a short, loud sound lasting no more than about a second that sounds like a low pitched 'BWaaaaRP'.



Quoting TheCommodore (Reply 9):
I have certainly noticed the air conditioning surging as the jets spool up and down.

If you could hear that it would be a very high pitch sound coming from the ACMs. Sort of like a turbo charger.

Pneumatic bleeds on an engine can't really be heard cycling over the engine noise unless you are talking about an overboard dump like from the 2.5 and 14th stage bleed valves on the PW2037. That is the unmistakable ripping or cascading sound which you've said isn't it. The sound the pneumatic bleed valves make is really no more than a momentary clunk and a sound of air going through the ducts. This isn't readily audible when you are standing next to a running engine let alone some distance away. At least not in my experience. Is there any way you could record this sound and get it online so we can hear exactly what you are hearing? That would be the best way to get an accurate answer.

DL757Md
 
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jetmech
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 4:55 am

Quoting TheCommodore (Reply 9):
That wouldn't be unique to a 767 though would it? I

Such a system is found on a lot of commercial airliners, not just the 767.

Quoting TheCommodore (Reply 9):
QF 767's, there's the GE powered ones and then the RR powered ones

I was actually thinking of a GE powered QF 767 when I wrote my previous reply. I'm pretty sure the RR / 767 engine combination has a similar two stage pneumatic supply system. I'm think the low stage to high stage changeover is a reasonable candidate for the noise you heard, as you are dealing with large diameter ducts supplying a large volume flow rate of air. The changeover from pneumatic supply air at one pressure to another would be sure to make some noise. The pneumatic supply ducting of a RR Trent 700 is shown below.


View Large View Medium
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Photo © Pier Picone



Regards, JetMech ( Lower North Shore! )

[Edited 2007-12-30 20:57:48]
 
SFOMB67
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 5:52 am

The ADP in the ctr hyd system is quite noisy. Might be that, if the flaps/slats or gear are operating. Don't know if you'd hear it from the ground though.
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 8:37 pm

DL757MD,

I guess I could try and grab a sound bite with my mobile and then upload but SYD has a noise sharing policy and since I started this thread it has been strangely silent overhead. Some opportunities might present over the next few days. Thanks for your ideas.
 
FlyASAGuy2005
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RE: Identify This: Strange Approach Noise B767

Mon Dec 31, 2007 8:46 pm

The CRJ does it too. When on the ground, you'd hear this loud shreek noise for about 2 seconds. People new to the plane usually jump and think something's wrong with the engine. It's quite funny.
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Wed Jan 09, 2008 6:09 am

OK... More clues.

I've been watching and listening closely for the past few days and can confirm that on four occasions I've heard the sound described originally, and when I've rushed to the window to look, it's been a Qantas B767 every single time. Twice, the sighted aircraft were banking steeply. I'll also confirm that all aircraft passing overhead, as best I can see, do not seem to have begun deploying flaps, slats and definitely not undercarriages although obviously a small amount of flaps would not be visible from the ground.

I've been trying to grab a sound bite but have only managed to grab sound bites for A320's, A330's and a variety of other aircraft but haven't caught a 767 'Bwaarping' yet. I'll be lucky to catch this sound bite over the damn leaf blowers and Kookaburras!  Wink

Interestingly though, I think I might have narrowed down the circumstances when this sound happens. For the past few days there has been plenty of broken cloud around and the 'BWaaaRP' sound only seems to happen when the aircraft are passing through cloud. Not when conditions are completely clear.

A couple of thoughts. QF767's don't have winglets and I'm wondering whether a 767 banking through cloud can generate this sort of noise as a result of some sort of vortices around the wing tip. A320's and 30's don't seem to be affected, nor are 747's, but then neither are 777's and they don't have winglets (but then they're a much more modern design). And before someone flames me, yes, I know not every other aircraft has winglets.

Who knows... maybe it's just some aerodynamic phenomenon! I'm determined to catch a recording of this weird sound.

 cheerful 
 
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BreninTW
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RE: Identify This: Strange Approach Noise B767

Wed Jan 09, 2008 6:17 am

Does it sound like a huge flag flapping in the wind? I've heard it a few times when MD-82s and -90s are landing in humid weather in Taipei.

Quoting TheCommodore (Reply 15):
Who knows... maybe it's just some aerodynamic phenomenon! I'm determined to catch a recording of this weird sound.

I think you're right on that.

Bren
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Wed Jan 09, 2008 9:05 pm

Hmmm... nope, nothing like that I'm afraid but thanks anyway.
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Mon Apr 21, 2008 9:52 pm

Thanks everyone who shared their theories. I think Boeing767Mech came closest to having the right answer.

The other day I flew into SYD from AKL on an Air New Zealand B767 ZK-NCG, seated in 19B - the exit row just forward of the wing.

Just as slats began to extend the short "Bwaaarp' sound we've been discussing could be clearly heard. It's not the sound of the PDU though as the sound made by the drive unit seems quite distinct from the more aerodynamic sound I've been talking about. The sound only occurs for a second, just as the slats begin to move forward - not for the duration of their travel.

I'm back to guessing that is must be related to the change in air flow over the wing as the configuration is changed.
 
dl757md
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RE: Identify This: Strange Approach Noise B767

Tue Apr 22, 2008 1:59 am



Quoting TheCommodore (Reply 18):
I'm back to guessing that is must be related to the change in air flow over the wing as the configuration is changed.

I'm glad you heard the sound you're curious about while in flight. I must say though that in several hundred 767 flights I've never heard a sound that you described that I might correlate to that part of flight. I'm not saying you're wrong...you heard what you heard. Still if you can ever capture the sound so that we can listen to it, maybe we can finally and definitively identify it.  Smile

DL757Md
 
TheCommodore
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RE: Identify This: Strange Approach Noise B767

Tue Apr 22, 2008 9:36 pm

Couldn't agree more...

I guess I'm back to carrying my mobile, hoping to capture a sound-bite from the ground. Sad huh?  blush 

It's a pity it's now my company's policy to fly VirginBlue rather than Qantas domestically otherwise I'd have regular opportunities to capture the sound in-flight - that is, of course, if I had a mobile phone with flight mode.  Wink

I guess it'll only be another year or two that we even see 767s regularly in the sky here. Once the QF787s begin arriving the Qantas fleet will be retired, leaving just the international carriers with 767s.

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