What occurs to the VSI when the static port is blocked?

When the static port is blocked, the Vertical Speed Indicator (VSI) will read zero. This occurs because the VSI relies on changes in static pressure to determine the rate of climb or descent. When the static port is blocked, the pressure inside the instrument becomes stale—it no longer accurately reflects the atmospheric pressure outside the aircraft as the plane climbs or descends.

As the aircraft changes altitude, the VSI cannot detect these changes in ambient pressure since it is unable to receive new static pressure data. As a result, it indicates that there is no vertical speed, hence reading zero. Pilots must be aware of this limitation as it can significantly impact situational awareness during flight. If the VSI reads zero even when the aircraft is climbing or descending, it indicates a malfunction related to the static system.