Can anyone identify this altimeter problem?

[peter-h]

I have uploaded a 25MB video here.

The #1 altimeter oscillates slightly, by some 20ft either side.

Initially this is during a climb, on autopilot. After the aircraft levels off, around 4300ft, it continues to wobble a little.

The #2 altimeter is always stable.

The VSI is fairly stable.

The curious thing is that this oscillation is not always there. It is intermittent.

This altimeter might be expensive. It is an encoding one, used by the KFC225 autopilot for altitude capture. Unless the problem is easy to fix, I would probably choose to buy a new one with an exchange of the original.

 

[Dr. O]

Stiction in the gear train due to wear.... (yes, it is a word)....
Not worth fooling with with... Junk it or trade it...

denny-o

 

[Kelvin]

Stiction in the gear train due to wear.... (yes, it is a word)....
Not worth fooling with with... Junk it or trade it...

denny-o

Learned something new right off the bat this morning!

 

[peter-h]

Stiction I can see, but stiction due to wear?

As far as I can tell, referring to a list of original avionics serials from 2001, it is a KEA-130A P/N 066-3064-05 or 066-03064-0005, supposedly equivalent to United Instruments P/N 5035PB-P159H.

South East Aerospace list it here.

It has done 1000hrs airborne time, BTW.

 

[John Collins]

Peter,

I expect you only see this on the climb. My guess is that it is engine/prop vibration induced. Try changing your RPM slightly. It might be much cheaper to get your prop dynamically balanced.

 

[peter-h]

I expect you only see this on the climb

That's a good point. Yes, I think that is true.

I have had the prop dynamically balanced; I do it every 2 years or so. But the instrument panel sections are rubber mounted and it is probably the panel resonating at just the "right" frequency.

Is there some kind of a "damper" device in the altimeter, which might be worn out / leaked out and which is allowing this to happen? I've had the plane since new and it certainly did not use to do this.

 

[John Collins]

That's a good point. Yes, I think that is true.

I have had the prop dynamically balanced; I do it every 2 years or so. But the instrument panel sections are rubber mounted and it is probably the panel resonating at just the "right" frequency.

Is there some kind of a "damper" device in the altimeter, which might be worn out / leaked out and which is allowing this to happen? I've had the plane since new and it certainly did not use to do this.

Peter,

Since you get your prop balanced frequently, I would concentrate on the rubber mount isolation or change the frequency slightly by adjusting the RPM. I suspect if you swapped positions of the two altimeters, the backup unit would exhibit the same bouncing around of the pointer and the KEA 130A would be solid.

I recall when we had to switch our altimeters to the TSO version many years ago, that one of the consequences was that there was less friction allowed in the TSO version and this kind of vibration would have the effect you see. Isolate the instrument from the vibration or reduce the vibration. I would ignore the vibration if it only occurs in the climb at climb power.

 

[nyoung]

Years ago, the non-encoding altimeter in my Cherokee exhibited a very similar oscillation that (believe it or not) was humidity dependent. VMC conditions, it was fine. IMC it would oscillate +/- 100ft. I promise it wasn't my instrument skills

Given the humidity dependency, several people said it could be water in the static lines, but that was not the case. I ended up replacing the altimeter, and the problem went away.

 

[peter-h]

I did wonder about water but I checked the lines (and I am really careful about static plugs etc), and anyway the #2 altimeter (which is solid) is even more downstream than the #1.