What happens to the vertical speed indicator if the static vent is blocked?

When the static vent is blocked, the vertical speed indicator (VSI) typically indicates a climb regardless of the actual altitude change. The VSI relies on the change in static pressure to determine whether the airplane is climbing or descending. If the static vent is obstructed, the static pressure inside the instrument becomes trapped and cannot equalize with the external atmospheric pressure changes as the aircraft ascends or descends.

As a result, if the aircraft climbs, the VSI will still show a climb because it cannot sense the decrease in static pressure normally associated with an increase in altitude. Conversely, if the airplane descends, the VSI may continue to show a climb until the static pressure equalizes. Therefore, the VSI will display the wrong information, indicating continuous ascent even when the aircraft may not be gaining altitude. This phenomenon underscores the importance of having a functioning static vent for accurate altitudinal readings and vertical performance assessment.