"cpu" wrote in message
om...
Yesterday I flew a cessna 172 in the hard IFR. When I penetrated
apparently a heavy cumulonimbus rain cloud area, the VSI and altimeter
started to oscillate and bounce +/- 250 FPM (ALT oscilated 200~300 ft
up and back).

You had water in the static line. It is a common problem, especially when
doing things like penetrating thunderstorms. Opening the alternate static
port can help the problem.

If the alternate static port does not fix the problem, then smashing the
gauges will not help, either.