What is indicated if the VSI is at zero while the static source is blocked?

When the vertical speed indicator (VSI) shows a reading of zero while the static source is blocked, this situation indicates that the flight conditions are stable concerning vertical movement. Specifically, if the VSI is showing zero, it suggests that there is no change in altitude at that moment.

In the event of a blocked static source, the VSI may read zero due to the fact that it measures the rate of climb or descent by comparing static air pressure changes. If the static source is blocked, the instrument may not provide an accurate reading of altitude changes. As a result, if the VSI indicates zero, it can be interpreted that the aircraft is in level flight, because there is no altitude change being detected either upwards or downwards, even though the aircraft may not be in level flight realistically.

Other scenarios, such as being in a steady climb or descent or the VSI malfunctioning, would usually indicate different VSI readings. A steady climb would show a positive VSI, and a descent would show a negative VSI. Thus, a reading of zero suggests a neutral state pertaining to altitude changes, consistent with level flight, despite the malfunction of the static source.