Something interesting occurs to me. Many of the anecdotes about the squealing on flights says the noise goes away when yawning. Yawning lifts the soft palate and equalizes ambient pressure behind the eardrum in the sinuses. Air pressure is lower in a pressurized airplane cabin than on the ground and is usually more dynamic as well. I wonder if an air pressure gradient forms in the ear canal when in the air that exacerbates the ANC feedback issues that users (myself included) are experiencing on the ground. I feel like there must be some interaction between the multiple microphones due to vibration transmitted through the body of the Airpods.
Since it seems to be an ANC issue it should be fixable in software.
I discovered another way to remove this problem - Initiate a screen recording and it's gone. I was doing this to report a TestFlight app bug on a flight I was experiencing and wow did the ANC improve so much.
That's pretty peculiar... the ANC worked still it just got rid of this issue? I don't have any guesses why that would be.
The acoustic importance mismatch may result in the wrong gain?
And now realizing a day later impedance got autocorrected to importance somehow :(