What is this aircraft noise?

Serendipitous Sid

Serendipitous Sid

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Dan
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We are not all that far from both Gatwick and a radio beacon, so we do get a fair number of airliners passing overhead. From time to time, some of them make a noise which I can't fathom out. A neighbour describes it as a muffled "honk" of sorts, as if the aircraft is catching its breath.

It only happens as they reduce power, and it only happens with some of them. And unfortunately that's all I've got to go on. Any ideas?
 
I can't hear it from here :rolleyes:. But would be interested to know what it sounds like. Thought you may have had a recording of it :).
 
Alas no recording and I couldn't find anything on YouTube either. Another neighbour reckons it sound as if the aircraft does a hiccup. The engine note increases in both volume and pitch for a second or so. Like a jet engine version of a motorcycle changing down without the throttle being blipped ...
 
It possibly might be engine settings changing automatically to help maintain/adjust airspeed and rate of descent for the approach - such as electronic control units changing the scheduling of say something like the variable stator vanes in the compressor section of the engine, and therefore causing the change in sound. Different airliners with different engines will do similar, but might sound different.

They'll probably also power down the engines for noise abatement over particular areas, and power/spool them back up close to landing, so power is readily available in the event of having to overshoot, without a potentially dangerous delay for power.
 
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I am not far from a major airport and hear that noise, too. I have ruled out undercarriage association since nothing was happening there.
Possibly some engine controls affecting engine airflow or operation of flight control surfaces.
First time I heard it I thought something was operating a bit on the rough side. Heard it many times now.
 
They'll be dumping evidence of the chemtrails they were spraying. :)

Or maybe it has something to do with the turbo cutting in/out? if such a thing even happens.
 
It's every drone ever sold going through the engines. Those irresponsible operators.

Is it the autopilot adjusting power to cope with air movement?
 
I know the sound. I've heard it often.

I've always thought it's part of the process of preparing for landing.

Probably an auto function.
 
Pretty much Boyfalldown. It'll be keeping the aircraft on a glide slope provided through picking up the signals from beacons on the ground. So if the aircraft becomes a little steep on the approach, through rising convective air for example, it can, amongst other things, reduce power to increase the rate of descent, in order to re-intercept the correct glide angle, and so on.
 
huffy said:
I did a bit of googling and came up with this page

http://www.pprune.org/archive/index.php/t-146694.html

The 4th posting by 747focal sounds possible describing a "Tuba effect"

"addition of a tone will happen when a High Pressure Bleed Valve opens during spool down on approach. Then you get the tuba effect"
Click to expand...

That's it, Mark! It's been really bugging me because whatever it was, it's sudden, it only lasts a second or so, and it's not just your normal spooling-down sound,

Another mystery solved!
 
Glad to be of help Dan, the answer is not as exciting as some of the other suggestions but that's often the way. :)
 
Serendipitous Sid said:
That's it, Mark! It's been really bugging me because whatever it was, it's sudden, it only lasts a second or so, and it's not just your normal spooling-down sound,

Another mystery solved!
Click to expand...

Yip - I'll buy that (y)
 
I live on the LHR flight path, just about where the undercarriage is opened and if you are unlucky that some poor African can chosen the cheap way to fly from JHB ends up as a frozen corpse on the street. we usually hear that sound - I think it's an RB211/Trent thing as it seems to happen a lot when B777s come over - given that most operators have 777s that's what I base my postulation on.

I could be completely wrong and it might apply to the GE-engine aircraft as some of BAs 777s are fitted with GE90s.
 
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