What would cause this type of fluctuation on my altimeter?

[pburger]

I had my pitot-static system completely re-plumbed earlier this year. As part of that project, my altimeter was sent out for overhaul (it was leaking).

I have been getting some weird fluctuations, but only occasionally. The fluctuations eventually settle down, but it’s a bit unnerving and it is definitely new behavior. See attached video.

What could be causing this?

Altimeter fluctuations

 

[Bell206]

What could be causing this?

Does the needle oscillate at the 0 mark or only at the bottom of the scale?

Define occasionally -- at certain specific flight condition, i.e, in climb or decent only, etc. or randomly regardless of flight conditions?

 

[Dan Thomas]

Does the needle oscillate at the 0 mark or only at the bottom of the scale?

Define occasionally -- at certain specific flight condition, i.e, in climb or decent only, etc. or randomly regardless of flight conditions?

Check the VSI below the altimeter. He's climbing, and the altimeter settles down as he's levelling off.

I would suspect water in the static line somewhere. Maybe in the static bottle, if the airplane has one. The air leaving the altimeter in the climb has to bubble upward through the water, or is forcing it out a drop at a time or into an upstream part of the system.

 

[mondtster]

As Dan suggested, my first guess would be water in the system.

 

[pburger]

I had to dig back in the photo/video files a bit. The work on the system was done in June of this year (2023). Here is a video taken on the way to Oshkosh this year:
altimeter fluctuations - July 2023

I doubt there was water in the lines because this was during one of the hottest summers in memory, so the chance for condensation was low, and all the lines were new, and I think the hard lines from the pitot/static blade were blown out. Although I'm not saying I rule that out - I just think it would be unlikely to have water in the lines that soon after all lines being replaced. I'm more likely to suspect the altimeter because it was sent out for overhaul. Perhaps there's some kind of mechanical dampener that isn't working right?

These oscillations occurred in level flight, and were when the pointer was at the 12 o-clock position.

I found a text I sent to my avionics tech that did the work, but there was no response from him.

This has only happened on a few occasions, and only for a little while each time. It is definitely not normal, and not something I ever recall seeing.

 

[Bell206]

These oscillations occurred in level flight, and were when the pointer was at the 12 o-clock position.

Were your static ports replaced or messed with as part of your re-plumbing?
Have you performed a recent static leak check since you noted the bounce?

Given it only happens at random times with no other apparent system issues might be hard to pin down but not impossible.

Once you verify there is no water in the system anywhere, one method to T/S is to disconnect the altimeter at the back of the instrument and cap the static line leaving the altimeter open to the ambient. I usually would make a short test line and connect to the instrument to keep trash out. Then go fly. If your bounce returns its the indicator and talk to the shop. If no bounce look for an intermittent leak in the static system maybe using a wiggle test with the system under test.

 

[Dana]

I once had an altimeter with a partially stripped gear that behaved like that.

 

[Magman]

I just finished an Annual on a newly painted 172. The drain hole on the

bottom of the the PitotTube was painted over. My guess is it happens to

others.

 

[Domenick]

Model of plane?
Some Cherokees had two push-button pitot drains at the floor on the pilot's bulkhead.

 

[brcase]

As noted in the the video your VSI shows climbing and a level off and the oscillation seemed to go away as you leveled off. I assume that is actually what was happening.
You can see a slight oscillation of the VSI also. Would be interesting to see the ASI as well. Is it oscillating also?

My 1st thought was water in the system also, I have seen this in a Cherokee after a horizontal rain storm (windy) filled the blade type pitot static cover with water.
But we noticed it only during altitude changes as the water was in the static system.

Brian
CFIIG/ASEL