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Explain Curious 737 Engine Noise Overhead.

Posted: Sun Dec 07, 2014 5:30 pm
by 26point2
Something I have wondered for years...perhaps someone here can help. This is a long-shot and perhaps my description won't come across very well in printed word but I am curious:

I live in Santa Cruz, California and directly below the BOLDR intersection of the Big Sur 2 arrival into KSFO. This is relevant because ATC requires traffic to cross BOLDR at 10,000' and 250 kts. Now, while I am outside I often hear an odd sound overhead similar to someone, with a sing-song voice, saying WHOOOOOO with a descending tone toward the end? Sound lasts about 2-3 seconds and then a normal, but faint, overhead jet sound is heard. Prior to this sound there is no engine noise apparent on the ground. Sound always seems to be coming from B-737 on the BSR2 arrival. Of course the plane is not EXACTLY overhead but slightly downrange due to doppler effect when the sound reaches the ground.

My question is about the acoustics of the WHOOOO sound that precedes the normal engine noise. The engines will be at flight idle to meet the crossing restriction most likely. Can anyone explain this noise signature 2 miles overhead and why it would produce this odd sound? And why the B-737's engines and not others?

26.2

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sun Dec 07, 2014 8:16 pm
by Okie
Quoting 26point2 (Thread starter):
My question is about the acoustics of the WHOOOO sound that precedes the normal engine noise. The engines will be at flight idle to meet the crossing restriction most likely. Can anyone explain this noise signature 2 miles overhead and why it would produce this odd sound? And why the B-737's engines and not others?

First of all @ 10,000ft agl the noise you hear was generated 8.89 seconds before you hear it not counting time delay for distance travelled at 250kts.

Quoting 26point2 (Thread starter):
Of course the plane is not EXACTLY overhead but slightly downrange due to doppler effect when the sound reaches the ground

I expect the Doppler effect of the noise being compressed and then contracted is part of what you hear.

I used to live close to an approach to KOKC and after a short period of time with a little observation you could determine the aircraft/engine type from the sound without looking.
Most have some distinct sounds.

I live close to KTIK now so a B1B is pretty easy to figure out.


Okie

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sun Dec 07, 2014 9:51 pm
by trnswrld
I don't recall hearing 737s making the strange noises, but always here the A319/320/321 aircraft do that at between 7-9k ft above my house southwest of ORD. I just always thought it was a change in power settings on that specific aircraft.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sun Dec 07, 2014 10:27 pm
by 113312
Most likely retracting of speed brakes.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sun Dec 07, 2014 11:18 pm
by Whiteguy
Quoting 113312 (Reply 3):

Most likely retracting of speed brakes.


Maybe but not likely. Not often the speed brakes get used, usually used more often to help with descent profiles. My guess is it's just normal air over the airframe noise.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Mon Dec 08, 2014 3:15 am
by ATA L1011
Quoting trnswrld (Reply 2):
I don't recall hearing 737s making the strange noises, but always here the A319/320/321 aircraft do that at between 7-9k ft above my house southwest of ORD. I just always thought it was a change in power settings on that specific aircraft.

I have not noticed it either on 737 Classics nor NG's, but that noise on all of the airbus A320 family is very prominent throughout the approach phase.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Mon Dec 08, 2014 5:07 am
by FlyHossD
That's probably the sound of the engines spooling up from idle RPM to now maintain speed at a level altitude. Generally, the VNAV PATH descents cross the fix simultaneous with reaching the specified altitude (so the engines would have been at idle until reaching the fix).

Quoting okie (Reply 1):
I expect the Doppler effect of the noise being compressed and then contracted is part of what you hear.

Probably some truth to that, too.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Tue Dec 09, 2014 3:43 pm
by LH707330
I hear this a lot too from other aircraft types, and I live right under where the Seattle traffic straightens out on final. As others have said, it could be engines spooling+doppler, which is why you hear little to nothing, and then that sound, followed by the sound the plane normally makes.

Quoting okie (Reply 1):
I used to live close to an approach to KOKC and after a short period of time with a little observation you could determine the aircraft/engine type from the sound without looking.
Most have some distinct sounds.

Too true. I've gotten to the stage where I can pick out engine types on certain planes. The sound I'm most curious about is this warble that A330s make as they fly away from you on approach.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Wed Dec 10, 2014 1:39 am
by Mender
Just an idea. Could this sound be created as the slats are extened creating a sort of reed like that of a wood wind instrument? It would explain why you only hear it for a few seconds as the reed disapears when the gap between the slat and the leading edge opens up.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Wed Dec 10, 2014 4:10 am
by AA737-823
One must keep in mind that the 737 is the ONLY airframe in the world with engines like it has. While the Airbus can also be ordered with CMF56 engines, they are NOT the wide-chord fan blade variant of the engines.
The 737's 24-blade fan does produce some interesting acoustical characteristics.
As others have said, the A320 cFM also produces distinct (and sometimes plain weird) noises, but they're different from the noises the 737NG makes.

Quoting Mender (Reply 8):
Just an idea. Could this sound be created as the slats are extened creating a sort of reed like that of a wood wind instrument?

While that's a plausible theory, the slats are not usually extended at or before 10000 feet. And the far noisier component during that process is the 737s kruger flaps extending at awkward angles to the airflow. You can hear it very distinctly, even from the back of the cabin. The slats don't disturb the airflow as much as the krugers.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Wed Dec 10, 2014 5:33 am
by FlyHossD
Quoting Mender (Reply 8):
Just an idea. Could this sound be created as the slats are extened creating a sort of reed like that of a wood wind instrument? It would explain why you only hear it for a few seconds as the reed disapears when the gap between the slat and the leading edge opens up.
Quoting AA737-823 (Reply 9):
While that's a plausible theory, the slats are not usually extended at or before 10000 feet. And the far noisier component during that process is the 737s kruger flaps extending at awkward angles to the airflow. You can hear it very distinctly, even from the back of the cabin. The slats don't disturb the airflow as much as the krugers.

Very, very unlikely to extend flaps at that altitude and speed. In fact, the maximum flap extension speed for the 737s is 250 knots, so it's prudent to get the speed below that so that a gust (or other fluctuation in indicated airspeed) doesn't put you about 250.

I do agree that extension of the Krueger flaps makes more noise than the other leading edge devices or the trailing edge flaps.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Thu Dec 11, 2014 12:47 am
by l39driver
Hi all, this is my first post (longtime lurker) so please be gentle.

While not necessarily the cause of the OP's situation, landing lights come on at 10k and below. A320s make a rumble when lights extend from the wing fairing (when seated inside near the wing). I know the 733 have lights that extend from the outboard flap canoe.

Given the 200-250kt speeds, extended landing lights may possibly be heard from the ground.

T

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Thu Dec 11, 2014 1:02 am
by FlyHossD
Quoting l39driver (Reply 11):

Hi all, this is my first post (longtime lurker) so please be gentle.

While not necessarily the cause of the OP's situation, landing lights come on at 10k and below. A320s make a rumble when lights extend from the wing fairing (when seated inside near the wing). I know the 733 have lights that extend from the outboard flap canoe.

Given the 200-250kt speeds, extended landing lights may possibly be heard from the ground.

One, welcome to the board.

Two, depending on the company and situation, the landing lights are generally turned on at higher altitudes; many companies use FL180 (18.000').

During my years flying the 737NG, the company suggested to us to limit the use of retractable landing lights as the drive motors were failure prone, expensive to replace and caused some objectionable vibration. It was my practice to turn on the landing lights at FL180/18,000 or the transition altitude, whichever was higher. The landing lights (all) were normally turned off for CAT II or III approaches.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Thu Dec 11, 2014 3:34 am
by Jetlagged
Quoting 26point2 (Thread starter):
Of course the plane is not EXACTLY overhead but slightly downrange due to doppler effect when the sound reaches the ground.

That's not the Doppler effect, just the speed of sound and the time it takes for sound to reach the ground.

CFM56s on 737s do tend to make that kind of whoooo noise spooling up from idle, at least inside the cabin. A320s much less so. If the aircraft is levelling off for some reason that might explain it. Also, in a VNAV Path descent the engines might spool up to maintain airspeed (pitch controlling the flight path).

The other thing to remember is that lower sound frequencies travel further. At 10,000 feet the aircraft is nearly 5 miles away at least, so the higher frequencies are less audible, making lower frequency components of engine whine more dominant.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Thu Dec 11, 2014 5:07 am
by nz2
Quoting trnswrld (Reply 2):
I don't recall hearing 737s making the strange noises, but always here the A319/320/321 aircraft do that at between 7-9k ft above my house southwest of ORD. I just always thought it was a change in power settings on that specific aircraft.

Agree, here in AKL , NZ it is always the a320's, not the boeings

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Thu Dec 11, 2014 6:39 pm
by l39driver
Quoting FlyHossD (Reply 12):
One, welcome to the board.

Two, depending on the company and situation, the landing lights are generally turned on at higher altitudes; many companies use FL180 (18.000').

During my years flying the 737NG, the company suggested to us to limit the use of retractable landing lights as the drive motors were failure prone, expensive to replace and caused some objectionable vibration. It was my practice to turn on the landing lights at FL180/18,000 or the transition altitude, whichever was higher. The landing lights (all) were normally turned off for CAT II or III approaches.

Thank you very much for the welcome! I have learned a ton from the forum over the years for sure!

As a non-commercial pilot my general practice is 10 and below but come to think of it I pass a lot of aircraft descending through 18ish with lights on.

Whenever I ride a320s the lights coming out makes a whoosh sound pattern kind of like the OP states hence my original thought.

Thanks again for the background.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Thu Dec 11, 2014 9:37 pm
by LU9092
I live in Brooklyn and LGA arrivals to rwy 4, 31 and sometimes 22 go over me at varying altitudes, ~1600 ft. if approaching 4, ~2000 ft. if flying the expressway visual, and 3500 to 4000 ft. for the 22 approach that goes out past Queens before a big turn to final. I'm pretty sure I've never heard the WHOOOOO from aircraft approaching rwy 4, when they already have flaps out and are lowering the gear as they go over. I've never been certain the plane that made the sound was a Boeing, but I've definitely heard it from A319/320s, followed by the engine noise and the "hum" or "whistle" that makes them easy to distinguish from any other types. As the OP stated, there is no other sound from the plane that is heard before the "WHOOO."

[Edited 2014-12-11 13:43:26]

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Fri Dec 12, 2014 6:39 am
by 737tdi
I used to live right under the flight deck/fantail on a few different aircraft carriers and I can tell you the most annoying aircraft engine noise is from the S3 Viking. The first few times I heard it I was surprised out how much they sounded like a giant vacuum cleaner. You do get used to it though.

I have never really noticed this effect on 737s except on engine start where you get that start up moan.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Fri Dec 12, 2014 1:08 pm
by bueb0g
Quoting 26point2 (Thread starter):
Sound always seems to be coming from B-737 on the BSR2 arrival. Of course the plane is not EXACTLY overhead but slightly downrange due to doppler effect when the sound reaches the ground.

Nothing to do with the doppler effect, that's just the limit of the speed of sound.

Quoting 26point2 (Thread starter):
Can anyone explain this noise signature 2 miles overhead and why it would produce this odd sound?

The sound itself, however, may well be caused (or made odd) by the doppler effect. I expect (as others have said) that it's the engines spooling up as they reach the waypoint, probably just having finished a descent.

Quoting FlyHossD (Reply 6):
That's probably the sound of the engines spooling up from idle RPM to now maintain speed at a level altitude. Generally, the VNAV PATH descents cross the fix simultaneous with reaching the specified altitude (so the engines would have been at idle until reaching the fix).

Agreed...

Quoting Mender (Reply 8):
Just an idea. Could this sound be created as the slats are extened creating a sort of reed like that of a wood wind instrument? It would explain why you only hear it for a few seconds as the reed disapears when the gap between the slat and the leading edge opens up.

Wouldn't normally happen at FL100

Quoting l39driver (Reply 11):
landing lights come on at 10k and below.

Depends on the airline. Over here it's common to put the landing lights on only when you've been cleared to land.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sun Dec 14, 2014 7:32 am
by 737tdi
Oh, so you have all heard that sound. An S3 landing on an aircraft carrier? Oh, you haven't heard that. But I would bet you argue with me later on a jet engine question???

You guys just kill me. You have no idea but can argue when it comes to commercial jets over prop engine noise.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sun Dec 14, 2014 6:18 pm
by PGNCS
Quoting AA737-823 (Reply 9):






One must keep in mind that the 737 is the ONLY airframe in the world with engines like it has. While the Airbus can also be ordered with CMF56 engines, they are NOT the wide-chord fan blade variant of the engines.
The 737's 24-blade fan does produce some interesting acoustical characteristics.

A very good post AA737...I am not an engine guy but have flown a lot of different types of aircraft including the 737; my first reaction from reading this given my background on the 757 in particular was that it could be a bleed valve deal; do you have info on that germane to this phase of flight?

Quoting 26point2 (Thread starter):
I live in Santa Cruz, California and directly below the BOLDR intersection of the Big Sur 2 arrival into KSFO. This is relevant because ATC requires traffic to cross BOLDR at 10,000' and 250 kts.

So if it's an "at 250K" restriction there is no reason in a 737 to be configuring with flaps/slats (given the max speed to start configuring for a 737 is 250K if I remember right.)

Quoting whiteguy (Reply 4):
Quoting 113312 (Reply 3):
Most likely retracting of speed brakes.


Maybe but not likely. Not often the speed brakes get used

VERY often the speedbrakes get used in the 737, though I don't think that has anything to do with this situation.

Quoting Mender (Reply 8):
Could this sound be created as the slats are extened creating a sort of reed like that of a wood wind instrument?

Not at 250K.

Quoting FlyHossD (Reply 10):
Very, very unlikely to extend flaps at that altitude and speed. In fact, the maximum flap extension speed for the 737s is 250 knots, so it's prudent to get the speed below that so that a gust (or other fluctuation in indicated airspeed) doesn't put you about 250.

Thanks for the refresher, Hoss. Yeah, there is no reason to believe that repetitive 737's at 250K would be all be configuring at the max extension speed. I completely agree with you that that makes no sense.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Tue Dec 16, 2014 12:39 am
by Whiteguy
Quoting PGNCS (Reply 20):
VERY often the speedbrakes get used in the 737, though I don't think that has anything to do with this situation.

As I said in the rest of my comment.....

Speed brakes are more often used for descent profiles rather than slowing the aircraft. They help a little to slow but not a lot.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Tue Dec 16, 2014 3:00 am
by AJ
Regarding the odd tones made by the Airbus narrow bodies; it has been traced to the equalisation vents under the wings:
http://www.lufthansagroup.com/en/pre.../2013/october/29/article/2651.html

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Tue Dec 16, 2014 7:26 pm
by Mender
Quoting AJ (Reply 22):
Regarding the odd tones made by the Airbus narrow bodies; it has been traced to the equalisation vents under the wings:
http://www.lufthansagroup.com/en/pre.../2013/october/29/article/2651.html

That makes a lot of sense.

I was nearly right when I thought the sound was created by a sort of reed. IE. The noise was not made by the engine or the undercarriage.

However, I was wrong about what part of the aircraft was creating that reed.

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sun Feb 08, 2015 12:27 am
by OMP777X
Quoting AJ (Reply 22):
Regarding the odd tones made by the Airbus narrow bodies; it has been traced to the equalisation vents under the wings:
http://www.lufthansagroup.com/en/pre.../2013/october/29/article/2651.html

This must be it. I hear the same thing over here several miles out from ORD coming from narrow body Airbii on a daily basis. Thanks for finding and sharing this info!

Best,

OMP777X

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Sat Feb 14, 2015 11:13 pm
by Ruscoe
I live in Brisbane Australia near a flight path.
I have never heard the Whoooo from a 737, but I hear it almost every time from the 320.
I don't find it loud or annoying but it is different.

In the past I have been told that a lot of the noise we think is coming from the engines is actually coming from the airframe.

Ruscoe

RE: Explain Curious 737 Engine Noise Overhead.

Posted: Tue Jun 23, 2015 4:07 am
by OMP777X
As I have already posted in the "A320 whine on approach" thread, I feel the need to clear the air here, too!

The "Curious 737 Engine Noise" that's been descirbed above is not an engine noise at all, nor is it happening on the 737. It is in fact coming from A320's when they open up the fuel vents on final approach. I'm so glad that this has been determined to be what it is, since LH has developed the solution to it with Airbus, and I hope that all A320 operators install that modification before long. I'd hate to keep having to listen to them shriek like a tin whistle being played with by a child each time they come in for a landing!

Best,

OMP777X