I believe I understand that there is no specific US FAA requirement for aircraft to carry a Vertical Speed Indicator (VSI). I believe I understand the basic operation of a VSI. It measures the rate of change in static pressure experienced by an aircraft and indicates it as altitude change per minute. And it should indicate zero vertical speed when the aircraft is stationary on the ground. It's main purpose is to indicate (though usually delayed, if changing) the vertical speed of the aircraft.
I ask this question because I recently flew in a light plane that I believe had an out-of-calibration VSI. We were only flying around 3000-3500 ft (1000m) MSL so I don't think this was any sort of outlier case.
Suppose the "calibrated leak" in the diaphragm changes (it becomes more 'leaky') so that the instrument still reads zero on the ground, but only indicates (in an extreme case) half the actual vertical speed. This is quite easily verified in flight by descending at a constant rate for some known amount of time and then comparing that to the altitude change indicated on the altimeter (or better yet, a separate, accurate GPS). I know that the VSI lags the actual altitude change rate by several seconds, but this was the sanity check I performed after the fact using video shot that included the instrument panel of the aircraft.
How often does it happen where the instrument has lost its ability to accurately indicate the rate of change of altitude?
What amount of error would be considered acceptable?
At least this causes annoyance; at worst it could cause a mishap (e.g. for a pilot trying to set the rate of descent on instrument approach to landing).