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What's that sound?

My plane makes a sound like this during climb.

Typically starts between FL150 and FL220. Sometimes only for a few seconds like in the above recording. Sometimes for a few minutes. Resembles an alarm horn and scares my already scared pax even more :-) Seems to originate in the front area.
What’s your guess?

Last Edited by loco at 25 Feb 08:49
LPFR, Poland

How does the stall warning sound on your a/c?

Sounds like the horn of the ‘get your oxygen mask on’ warning. Pressurized cabin leaking or relief valve blocked?

Last Edited by at 25 Feb 09:21

Hi,
pure speculation but.. could be a gear door that has a slight gap turning the gear cavity to a sounding box.

ESG..., Sweden

Some more recordings:

It’s not a stall horn. Stall warning simply says “stall”.
It’s not a depressurization warning. Plane automatically descends when that happens.
Cabin alt is solid when this happens.

Mechanics suspected door seal, but I’m now pretty sure it’s not the door.

Don’t really know how to solve this puzzle.

Last Edited by loco at 25 Feb 09:51
LPFR, Poland

To test the seal hypothesis (and the start of the sound reminds me of what I had when my door seal was punctured), can you play with the pressure differential or is it fully automated in the 930? In my case, I was getting that from a Delta P of 4 to 4.5 bars, and lowering the cabin altitude to go ASAP from 4 to 5 would make it stop, but that wasn’t gentle on the ears.

EGTF, LFTF

The recordings show different frequencies of the noise. Is that based on position? Is the TAS identical in the recordings? Does it change with TAS/AOA/altitude? Is it reproduceable?

It could be an aerodynamic buzz.

Last Edited by mh at 25 Feb 10:38
mh
Aufwind GmbH
EKPB, Germany

Peak is initially at 866Hz, reducing to 857Hz during the duration.

It also starts and ends suddenly, so I doubt it is aerodynamic… But it isn’t electronically generated, since the frequency varies. Perhaps oscillation in an outflow valve, or something like that? Pressure regulating systems can suffer from that sort of thing. The frequency is high which would suggest a high pressure is involved so perhaps something in the oxygen system?

Flutter frequency ought to be dependent on TAS to some extent, though perhaps not to first order. @pilot_dar might have ideas…

Administrator
Shoreham EGKA, United Kingdom

Oscillation in an outflow valve would be aerodynamic…

Last Edited by mh at 25 Feb 11:02
mh
Aufwind GmbH
EKPB, Germany

denopa wrote:

I was getting that from a Delta P of 4 to 4.5 bars, and lowering the cabin altitude to go ASAP from 4 to 5 would make it stop, but that wasn’t gentle on the ears.

I’ll try to observe pressure diff next time. I can maybe use the Max Diff position of the bleed switch to force a rapid change.

LPFR, Poland
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