Intermittent altimeter problem?

[IO540]

Got this yesterday.

During climb to 5000ft, one altimeter appeared to have lost its damping. The needle wobbled about by about 30ft up and 30ft down. The overall indication was correct.

The other altimeter was solid. The VSI was solid. Everything else was fine. The altitude itself was of course steady too.

Pulling the alternate static produced the usual result in all the P/S instruments but didn't change the #1 altimeter behaviour.

After about 10 mins, the problem disappeared and was not seen again.

I don't know if altimeters contain any fluid damping but if the fluid had leaked out how did it get back in again? :O

There are two static vents, one either side of the airframe, and left/right yaw made no difference. The two vents are combined into one pipe anyway.

Any ideas?

[High Wing Drifter]

My understanding is that what you describe can happen when the aneroid capsule looses its elasticity and thus distorts ununiformly (I guess unevenly would have been a better word!), a phenomenon that I believe is called "hysteresis" (sp?). I guess, above a certain altitude the problem may become evident as the capsule expands, below a certain altitude the problem may be masked; pure conjecture on my part.

If the propblem was upstream in the static system then I would have thought, certainly for the type schematics I have seen, that the other static instruments, incl the ASI, would play up too.

[david viewing]

Is there any chance that vibration is a factor - a combination of specific RPM and the altitude putting the aneroid into a specific stiffness state? It's easy to imagine how a certain vibration could sum or difference in the rack that drives the pointer due to friction effects.

[EGBKFLYER]

HWD - For your information:

Hysteresis is a property of systems that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state: that is, systems whose states depend on their immediate history. For instance, if you push on a piece of putty it will assume a new shape, and when you remove your hand it will not return to its original shape, or at least not immediately and not entirely.

Didn't think that ALTs had any damping/ beyond the spring on the aneroid capsule and the capsule itself? Would be interesting to find out what happened.

My idea would be cr*p in the needle or subscale gearing or maybe even a loose needle?