Altimeter and VSI opposing indications
OK guys I am not an airline pilot but many years ago I was flying on the cockpit of an airliner, over the Atlantic Ocean.
We were at 30 (or 31),000ft and got clearance to climb to 32,000ft. The aircraft was put into a shallow climb on autopilot at about 400fpm and something odd happened, the vertical speed indicator showed a climb of above number, but the altimeter slowly started winding backwards (downwards), over the next 5-10 seconds, to about 50ft below the initial cruise altitude. The Captain entered a higher VS number of 700fpm into the autopilot and the altimeter then showed the climb normally.
It seemed odd it was showing a climb (and the pitch angle had gone up slightly) but the altimeter showed a brief descent at the same time. Was this some kind of instrument inaccuracy or something to do with the way they work? It happened only about 10 seconds then went back to normal. I'm remembering it right because the Captain pointed it out as well.
It wasn't very turbulent I remember, but there were quite a few clouds around near our level and strong upper level winds.