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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).

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    $\begingroup$ VSI are regulated (when mandatory for IFR) by TSO-C8b, AS394A. I don't know the required check frequency, only this can be done on the ground with a Pitot-Static test set. You will find some tolerances at the end of the document. Note there are also IVSI. $\endgroup$
    – mins
    Jun 9 at 11:01
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    $\begingroup$ @mins - Unfortunately, your linked document could not be opened. Do you have another source for it? Also, since the TSO listed is certifying the part itself, does it also have any regulatory bearing on its use or maintenance? $\endgroup$
    – Dean F.
    Jun 11 at 18:00
  • $\begingroup$ @DeanF. Unfortunately this is a paying (and expensive) specification for manufacturers, but you may glance at page 101 of this old document for an idea of the content. I really have no more information about the process, just the specs. There is a list of related FAA documents at the end of the manual,you may be able to infer more details from them with a bit of luck. $\endgroup$
    – mins
    Jun 11 at 18:55
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If you’re not operating under a Minimum Equipment List that requires one, and the typical General Aviation pilot in the USA is not, then “No” is the answer to your posted question. There is no requirement for a VSI under FAR 91.205 hence no specific VSI inspection requirement is stated.

Note that in the USA prior to conducting flight under Instrument Flight Rules, a static pressure system inspection must have been performed within the preceding 24 months (FAR 91.411) and if your VSI is connected to that system, then it would be a part of that inspection but only as a component of the static pressure system.

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