Hi! Has anyone ever encountered a magnetic compass that goes unreliable over certain altitudes? When flying straight and level at 10,500 feet, my compass rotates (about the pitch axis) so the numbers and marks are at the bottom of the window, and a lot of bubbles are visible up at the top, and it bounces about this axis a lot more than normal. It's a lot like what I'd expect if I were pointing straight down, but the other instruments show that, if this is the case, I'm in a very strong updraft.

I haven't ruled out hypoxia yet... it does only happen above a really high altitude. At 7,500' it was okay. The two recent times I've been up at 10,500' I have this problem. I haven't spent much time in between.

You have an air bubble trapped under the card. As you climb, the bubble expands to the point that it binds the card.

I have a method of refilling compasses that gets all the air out. I submerge the entire compass in fluid in a bell jar and pull a vacuum on it. That causes all the bubbles to expand, and float out. When the bell jar is opened to atmospheric pressure, the remaining bubbles collapse to pinhead size. The fluid ends up clear, and the card floats straight.

Thanks MikeMl, that makes sense!