A Diagnostic Mystery

[Ernie Martin]

A noise has developed and I wonder if anyone may be able to help.

It starts 10 - 15 minutes into a flight. It seems to be coming from behind the instrument panel and sounds somewhat as if someone is crinkling aluminum foil.

It's been sounding now for three one-hour trips, but I didn't hear it until the last trip, in part because it's hard to hear it with headphones on (my wife heard it on the first two one-hour trips, knew it was a new sound, but assumed I could hear it and didn't mention it until now).

With my mechanic we've inspected the front engine compartment and the electronics bays and found nothing amiss. We then made two diagnostic flights and the consensus seems to be a failing gyro bearing, but there are some indications -- some of them counter-indications -- that I'd like to throw out to see if anyone has other thoughts:

1. It comes and goes, gets louder and quieter. It's mostly a steady sound but it can get weaker or even go entirely away for a few seconds.

2. Lowering flaps can make it go away. It might be the different airflow, or the fact that we've slowed down, or, because I tend to climb to bleed speed before lowering flaps, the change in attitude.

3. It emits electrical noise, When the sound is on, it's best heard without headphones, but it emits electrical noise that can be heard as weak white noise on the headphone; the headphone noise goes away when the sound goes away.

4. Turning power off does not stop it. Turning off the master switch and then the avionics power switch has no effect (we did it only for 15 - 30 seconds and are considering a longer turn-off to see if it's a bearing of an electrical gyro that might take a while to stop).

5. Different/lower noise on the ground. On the ground, with the master on, nothing happens for about 10 minutes after a cold start, and then a typical gyro-bearing-going-bad noise starts. However, this noise is much weaker that in flight.

I can't get an instrument guy to look till next week, and would welcome any ideas.

Thanks,
Ernie

[n86121]

Sounds to me perhaps like an intermittent electrical short arching? Look for wires chafing each other or against any metal. Just a thought.

D

[Ernie Martin]

Thanks, David, but remember it continues after both the master and avionics power switches are turned off, suggesting it isn't arcing.

Having lived in The D.C. area for 20 years, have followed your travails with your lovely airport.

Ernie

[stackj]

Ernie,

I thought at first you may be getting noise from an avionics fan, but I don't think that would continue when you shut the master off. I think it would spool down rather quickly. It would probably continue with the avionics master off. My blower comes on with the aircraft master switch.

If you have access to a stethoscope you might try holding it to the face of each of your gyros to see if that helps you define the source.

[n86121]

To isolate the source of a sound you can even use any old rubber hose.
One end near your ear, and you will be hearing whatever is near the other end of the hose.

Crude but effective.

D

[Ernie Martin]

Thanks for all responses. Did two test flights with a stethoscope. Can't pinpoint where it's coming from. But it sure looks as if speed or attitude affects the noise, which disappears when you climb from cruise altitude. Tomorrow my A&P and avionics guy will take it for a spin.

Ernie