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Strange Sound From 757

Posted: Mon May 24, 2004 9:15 am
by GBOAA1PAX
I've heard a unique sound coming from Boeing 757 aircraft. Sometimes while flying at altitude, or when using reverse thrust on the ground, a sudden high pressure rushing air noise can be heard around the engines. It begins very suddenly, as if the termination popped off a pressurized hose, and sounds like a sharp and fast puncture to a stationary automotive tire (tyre). It does not sound like regular engine blast or reverse thrust, but rather like high pressure air escaping from a relatively small opening at very high speed.

It often seems to occur when throttles are being reduced, rather than increased. The sound stops just as suddenly as it starts. When it occurs while flying, reversers are not being deployed.

If we have any 757 mechanics on the forum today, I would certainly appreciate your clearing up this mystery. I’ve wondered about this for twenty years!

Many thanks.

RE: Strange Sound From 757

Posted: Mon May 24, 2004 9:22 am
by MD11LuxuryLinr
There are a bunch of people in the Tech/Ops forum that could answer this question, I'm sure. Either they'll see in this forum and reply, or you'll have to wait for a moderator to move this thread to Tech/Ops..

Edit: BTW, Welcome to A.net!  Smile

[Edited 2004-05-24 02:23:13]

RE: Strange Sound From 757

Posted: Mon May 24, 2004 9:26 am
by NWFltAttendant
GBOAA....

Ive been looking for the answer to that question for 10 years. Ive asked flight crew members, tech ops people at NWA, and mechanics at NWA. Nobody has been able to give the answer to that question. If you sit in row 38 or 39, on the starboard side of the aircraft, you'll intermitently hear it in flight in 10-15 second blasts. Im still interested in the answer to this question and would like to really know what 'it' is.

RE: Strange Sound From 757

Posted: Mon May 24, 2004 9:27 am
by Dazed767
Are you only hearing this on P&W powered engines?? I notice it too, on the ground and when flying on one. I did a search and the answer seems to be 'that the noise is coming from the airbleed valves that open to dump air to maintain compressor stability'.

Justin

https://www.airliners.net/discussions/general_aviation/read.main/253067/

RE: Strange Sound From 757

Posted: Mon May 24, 2004 9:55 am
by dl757md
What you are hearing is an overpressure inside the engine being relieved through the 14th stage bleed valve. This is for PW as I am unfamiliar with RR. You are very astute in your observation that it seems to happen when throttles are being reduced. The fan on the front of the engine is a very large rotating mass with a lot of inertial energy. The PW engines on a 57 are a dual spool design. The fan is on one shaft N1 and the rest of the compressor sections are on another concentric shaft N2 surrounding the N1 shaft.
They are not mechanically linked. That is to say that rotate independently of each other - to a point. As power is reduced N2 which has far less inertial energy slows more quickly than N1. The quicker the power reduction the greater the speed differential between N1 and N2. As the speed differential increases the fan pushes too much air into the compressor and the resulting overpressure can cause damage. The 14th stage bleed valve senses this and relieves this pressure overboard through a vent on the left core cowl of the engine. This is very high pressure air and quite hot, the exact figures I'm not sure of off the top of my head, resulting in the very loud hissing noise.

RE: Strange Sound From 757

Posted: Mon May 24, 2004 9:58 am
by AA777MIA
I have never heard that sound, but then again, I was always on the RR aircraft..

RE: Strange Sound From 757

Posted: Mon May 24, 2004 10:00 am
by MD11LuxuryLinr
Dl757md..

Super description. Thanks for the info. Big grin

Edit:.. BTW, welcome to A.net to you too!  Laugh out loud

[Edited 2004-05-24 03:02:21]

RE: Strange Sound From 757

Posted: Mon May 24, 2004 10:02 am
by GBOAA1PAX
That's the symptom! They described it perfectly. It's great to finally know what that sound is, at long last.

Many thanks!

This is a terrific forum. Sorry for not posting to the Tech Ops section.


RE: Strange Sound From 757

Posted: Mon May 24, 2004 10:30 am
by Womack17
Dl757md - Thanks so much for that description - I have flown the 752 for many years back and forth between RDU and ATL - I have heard that noise many times and always wondered if it was just me.

Also welcome to the forum - You earned a respected user vote from me for that fantastic description - Thanks for breaking it down in to layman's terms.

RE: Strange Sound From 757

Posted: Mon May 24, 2004 12:14 pm
by dairbus
Is the same rushing sound that you hear when the engines are taken out of reverse thrust on landing? I have heard it many times watching Delta's 757's turning off the runway. They are PW powered by the way.

RE: Strange Sound From 757

Posted: Mon May 24, 2004 12:19 pm
by fspilot747
Sounds like water gushing through the engines. I posted a thread on this in tech ops a couple years ago and got a good answer, I'll find it.

RE: Strange Sound From 757

Posted: Mon May 24, 2004 12:39 pm
by dl757md
DAirbus

Yes, this is the sound that PW powered 757s make when they come out of reverse. Any time the engine is decelerated quickly such as from full power reverse to idle. If a pilot gets clearance to cross an active runway and expedite he'll run the engines up to a high power setting to get moving quickly and clear the runway and then chop the throttles quickly to avoid going too fast and having to use a lot of brakes. This will cause the noise you have described. It also happens during flight, usually during descent, and can be heard on the ground from aircraft at relatively high altitude.

Question for you Air Force guys do the engines on the C-17 have 14th stage bleed? Do they make the same noise?

Thanks you guys for the positive feedback on my earlier reply. Finally decided to join so I could participate in this great forum rather than just read it.

Dl757md

RE: Strange Sound From 757

Posted: Mon May 24, 2004 2:36 pm
by kbuf737
Is this the same bleed air valve noise that the 737-300/400/500 models have on deceleration?

RE: Strange Sound From 757

Posted: Mon May 24, 2004 3:34 pm
by dl757md
KBUF737

The 737-300/400/500 as well as the 737NG have a system that has the same function as the 14th stage bleed valve on the 757. It is called the Variable Bleed Valve or VBV. There are differences between the VBV on the 300/400/500 and the VBV on the NG, however they work basically the same way. The noise created by the BVB system is much less than that of the 14th stage bleed on the 57 due to the fact that the air that is being dumped overboard is going into the fan air inside the fan cowl rather than an uncowled exit as on the 57. Additionally the exhaust area for this dumped air is much larger resulting in a lower velocity stream and less noise. One last reason for less noise is that the 57 14th stage bleed works as a kind of pop off valve. It's either opened or closed. The VBV on the 37 as it's name implies has variable positions and modulates the pressure relief more precisely, not requiring as much pressure for activation. The noise from the 57 is clearly evident no matter where you sit on the plane even in the cockpit.
On the 37 I have never heard it in the cockpit while doing a maint. engine run nor have I heard it as a pax on dozens of flights. Not to say that this isn't the noise you're hearing. I'll listen for it next time I fly a 37 with fans.

Regards,
Dl757md