AI pitch bounce

I just noticed today a tendency for my vacuum AI to vibrate a few
millimeters in its pitch axis occasionally, then the movement damps out.
The roll and pitch indication seem to be OK, and the vacuum DG is working
fine. The vacuum guage is at 5.0, right where it belongs. I'm wondering if
this is an early failure indication.

--
Regards,
Mike

http://mywebpage.netscape.com/amountainaero/fspic1.html

I just noticed today a tendency for my vacuum AI to vibrate a few
millimeters in its pitch axis occasionally, then the movement damps out.
snip
I'm wondering if
this is an early failure indication.


An instrument shop owner told me once that it is. How long it will take to
fail is the question. This tech also told me that if you put any gimbaled
instrument down on a surface and could hear the case touch the surface you have
damaged the bearings.

John Dupre'

On Sat, 27 Nov 2004 17:05:54 -0700, Mike Noel wrote:
I just noticed today a tendency for my vacuum AI to vibrate a few
millimeters in its pitch axis occasionally, then the movement damps out.
The roll and pitch indication seem to be OK, and the vacuum DG is working
fine. The vacuum guage is at 5.0, right where it belongs. I'm wondering if
this is an early failure indication.


My AI has done this for as long as I have owned the aircraft (coming
up on 4 years). Of course the gyro shop said, "Needs overhaul"
so last year I sprung for the $200 overhaul. No difference whatsoever.
Although it does spin up a lot faster now. Used to take several minutes,
now it snaps to attention within 5-10 seconds of engine start.

The needle on the vacuum gauge seems to vibrate in sync with the AI,
The face of the gauge also seems to be loose, so my latest theory
is a leaky gauge causing variations in vacuum level. If this is
still the case at the next annual, I may try replacing the gauge.

"Mike Noel" wrote in message ...
I just noticed today a tendency for my vacuum AI to vibrate a few
millimeters in its pitch axis occasionally, then the movement damps out.
The roll and pitch indication seem to be OK, and the vacuum DG is working
fine. The vacuum guage is at 5.0, right where it belongs. I'm wondering if
this is an early failure indication.

It's caused by engine or prop vibration. The instrument panel
shock mounts might be old and hard, or the prop may need balancing.
Either way, the vibration is reaching the indicator. Next time you
fly, play with RPM settings to see if you can make the indicator
resonate with vibratory fequencies.

Dan