21
$\begingroup$

I've lived for years about eight miles NW of Boston's Logan Airport. Although I can't say we get a lot of aircraft noise, once in a while, due to winds or air traffic, there will be a long line of planes traveling right over my house, perhaps 6000-8000 feet up, descending into Logan. At night you can sometimes see three in a row, with a fourth one turning on its landing lights.

Most of the time the overhead planes sound mostly like a roar with some jet whine. However, once in a while a plane will suddenly start making a very strange sound like an enormous blown Coke bottle. Often the pitch will descend during a half dozen seconds, stay constant for a dozen seconds, and then rise again until the sound disappears.

My best guess is that the sound is caused by the pilot lowering the wheels during their descent, and that it only happens on certain models of aircraft. As the wheel well covers open, the air blowing across the wells sounds like that giant Coke bottle, with the pitch decreasing as the opening increases. On certain models, the covers will then close once the wheels are down, causing the pitch to go back up.

(Edit: it's been pointed out that gear isn't dropped until 2000-3000 feet, so in my case that isn't the sound. So, I've removed all those videos, cool though they may be.)

Is my guess right? What model/manufacturer of large jet would make this sound?

Improve this question
$\endgroup$
10
  • 5
    $\begingroup$ If it is at 6000-8000 feet, then it won't be the landing gear. It's probably first flap extension. During flaps transition some aircraft make funny noises. If they are lined up for the runway when they are above your house, and the runway is eight miles away, they are about 2000-3000 feet heigh and it may be the gear. $\endgroup$
    – DeltaLima ♦
    Commented Dec 16, 2015 at 10:45
  • 5
    $\begingroup$ You can find it out for yourself by using a flight tracking website like flightradar24. Just note the exact time that you hear the noise and correlate that to the flight tracks at the website. It shows the aircraft type. $\endgroup$
    – DeltaLima ♦
    Commented Dec 16, 2015 at 10:47
  • $\begingroup$ @DeltaLima Planes can't be on final approach as they come over us, as closer towns would scream. They circle over the Atlantic as they descend. $\endgroup$
    – Daniel Griscom
    Commented Dec 16, 2015 at 12:33
  • $\begingroup$ @DeltaLima Can flight tracking websites give true real-time data? I remember when they had to be delayed 5 minutes for fear of assisting terrorists. $\endgroup$
    – Daniel Griscom
    Commented Dec 16, 2015 at 12:35
  • 3
    $\begingroup$ FR24 gives live data for the yellow planes, the orange planes are delayed by 5 minutes. $\endgroup$
    – DeltaLima ♦
    Commented Dec 16, 2015 at 12:41

5 Answers 5

Reset to default
26
$\begingroup$

Short Answer:

The sound is caused by airflow across holes on the underside of the wings of certain aircraft.

Long Answer:

I too have noted the sound phenomenon that you describe. This video has a great example between 0:26-52 (link starts playback at 0:26 where the noise starts):

I will readily admit that this answer is mostly just a compilation of what I have learned researching this. Your question intrigued me and I've enjoyed learning about it!

As I discovered as I started researching this, many others are familiar with the noise as well. This article on Airliners.net had a lot of discussion about it, starting with a lot of interesting—but ultimately flawed—theories.

But, that thread led me to the answer to your question, which is: the sound is caused by the "circular opening used to equalise the pressure in wing fuel tanks", according to Euractiv.com

This article on AirportWatch further explains:

The Airbus 320 series of aircraft, [...] have been known for many years [...] to have a particularly irritating high pitched whine. This is caused by air rushing across the under-surface of the wing, where there are Fuel Over Pressure Protector (FOPP) cavities. This generates noise, in the same way as blowing air over the mouth of a bottle. Every A320 series aircraft emits a signature howling noise while approaching to land. It is heard most when the plane is travelling at around 160 knots, and the frequency is around 500-600Hz, which is close to peak sensitivity of the human ear.

That being said, there is apparently a fix to that problem, as both the aforementioned links describe. As Euractiv.com explains, the fix is a vortex generator:

a 5-cm triangular piece of aluminium sheet metal upstream of the two vents on each wing, in order to divert the air flow and stop the whistle. A decade of research went into the vortex generator, which stems from efforts to mask a similar whistle produced when air blows over the gun ports of warplanes.

This image shows both the FOPP hole, and the VG that softens the sound:

A320 FOPP and VGImage Courtesy of Lufthansa[Image Courtesy of Lufthansa]

Read the articles linked, interesting stuff therein!

Improve this answer
$\endgroup$
2
$\begingroup$

I think we just meet with the same curiosity I had. I hear that sound you describe also, and I already reduced the observation on it happening with Boeing 757-200's only. Like the ones operated by United. I happened to have found the answer, this is exact technical explanation.:

http://www.airliners.net/forum/viewtopic.php?t=445103

It's the 2.5 stage bleed on the PW 2037. This bleeds excess air from between the 2nd and 3rd stage of compressor blades for engine stability. It is a very loud sound similar to that of an compressed air gun used to blow off dirt from something. This sound is also very common when the PW powered 757 lands and comes out of reverse thrust .It's a different sound than a afterburner or jet exhaust because the air coming from is relatively cold. Air from jet exhaust or an afterburner is very hot and as it mixes with the cold air around it it makes a distinctly different sound than it would if it was cold. This subject has been discussed at length in the Tech Ops forum if you want more info.

DL757Md

Improve this answer
$\endgroup$
2
  • $\begingroup$ From your description this doesn't sound like a "blown bottle" as the question describes. And can you link to the relevant threads that you refer to? $\endgroup$
    – fooot
    Commented May 21, 2018 at 21:01
  • $\begingroup$ Thanks for the answer. I've been meaning to return to this question for a while, because frankly I don't do a good job of expressing what I'm looking for. The critical point is that the pitch descends AND THEN RISES. It isn't a Doppler effect; the pitch seems to be dependent on some control surface. But again: I have to state this better. (Thanks again.) $\endgroup$
    – Daniel Griscom
    Commented May 21, 2018 at 21:35
0
$\begingroup$

My best guess is that the sound is caused by the pilot lowering the wheels during their descent

This was what I was going to say immediately. I lived about the same distance from the south runway at Pearson and wondered what this was until I saw a 737 drop gear right above me.

I would describe the sound as a sort of "pfff-ewwwww" over the first second, and dropping in pitch until it ends in a constant low rumble after perhaps 5 seconds. Is that it?

Improve this answer
$\endgroup$
3
  • $\begingroup$ Thanks for the answer, but it's been pointed out to me that gear isn't dropped until the planes are quite a bit closer to the ground. $\endgroup$
    – Daniel Griscom
    Commented May 25, 2018 at 19:29
  • $\begingroup$ Uhh, I was about the same distance as you, 8 miles right? I could see them dropping their gear above my home. $\endgroup$
    – Maury Markowitz
    Commented May 25, 2018 at 19:39
  • $\begingroup$ Then we're in different situations. In my case, the pitch will often go down for a few seconds, hold steady for a few seconds, and then go back up in pitch, shifting over an octave either way. $\endgroup$
    – Daniel Griscom
    Commented May 25, 2018 at 23:41
0
$\begingroup$

It’s the speed brakes being deployed and then retracted.

The Airbus A320 series makes the exact sound you’re describing. Other medium/large types will make a similar sound.

The reason you hear it in roughly the same place is because approach controllers tend to follow a pattern of speed and altitude changes when sequencing aircraft towards the airport.

Speed brakes are deployed to expedite those speed and altitude changes.

Improve this answer
$\endgroup$
1
-1
$\begingroup$

I think what you hear is the "buzzsaw" caused by tip noise from the fan or the compressor which is only directed out of the engine from certain angles. Here's a video of the sound from inside

but because the aircraft is passing you hear it for only a few seconds.

Improve this answer
$\endgroup$
3
  • $\begingroup$ ... but then why/how does the pitch change so strongly/rapidly? $\endgroup$
    – Daniel Griscom
    Commented Sep 11, 2022 at 14:06
  • $\begingroup$ The sound exits the engine from the front and only at particular angles. As the aircraft passes overhead the modulates the sound and the velocity of the aircraft modifies the pitch due to the dopper effect. The sound is also modified by reflections from the ground. $\endgroup$
    – D Duck
    Commented Sep 11, 2022 at 19:02
  • $\begingroup$ It isn't the doppler effect; as the plane passes the relative velocity goes steadily from "rapidly approaching" to "rapidly receding", so the pitch would only go down. In my case, it will go strongly down, then stay stable for a few seconds, and then go up. $\endgroup$
    – Daniel Griscom
    Commented Sep 12, 2022 at 16:08

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .