The Cessna 120-140 Association Forum

Was curious if anyone has experienced a sudden fluctuating altimeter. I have a vintage Cessna unit and suspect an internal component failure as airspeed and vsi are working normally. If overhaul an option, any recommendations for reputable shops. Thanks

Rudy’s in Arkansas.

Can you describe the fluctuations? Try disconnecting the static line from the altimeter. If the problem goes away, it's not the instrument.

While flying, 100 foot needle began swinging at 500 foot intervals. This movement has since stopped with needles now stopped at essentially ground level elevation at takeoff. Adjusting to atmospheric pressure has no affect. Suspect internal diaphragm has failed

While flying, 100 foot needle began swinging at 500 foot intervals. This movement has since stopped with needles now stopped at essentially ground level elevation at takeoff. Adjusting to atmospheric pressure has no affect. Suspect internal diaphragm has failed

So are you saying it no longer responds to changes in altitude? Then, yes, it must be broken, or the static inlet is completely blocked. I would still disconnect the static line and see if there is any change.

There is no diaphragm. it has a sealed "can" called an aneroid that compresses and expands lengthwise, and a tricky gear train that moves the needles.

The spring that holds the Aneroid capsule can suffer from hysteresis, if it gets to bad the Altimeter can jump around a bit, that may be a place to start?
Si

Determined that the aneroid wafer assembly has failed and in need of replacement for unit. Aircraft quality instruments in Wichita claims no longer repairable. Unit is a vintage Garwin 22-980. I would be interested in checking other repair shops however and am looking for any recommendations. I would think wafer could Be replaced with newer component. Also, if a member has an old Garwin altimeter they would like to sell, this could also help rectify problem. Thanks

https://rudyaircraftinstruments.com/home-page

https://www.berkshireinstrument.com

I've used both, but the second one has moved twice since then. I'm not sure, but I think it is the same guy, though.

Rudy's is really good with older units, but you still may be buying a new unit. I suffered a similar issue about 3 years ago. "Non-repairable". (while flying at 4500 feet, the altimeter wound up to 7000 feet in about 10-15 seconds Didn't realize I'd hit the JADO switch )

I bit the bullet and bought a new unit. Make sure the barometric knob is in the right spot. some are at 6 O clock and some at 7-7:30 range. If your altimeter is "really old" the knob will be at 6 O clock.