What happens to the vertical speed indicator (VSI) with a blocked static port?

When the static port of an aircraft is blocked, the vertical speed indicator (VSI) is affected because it relies directly on changes in atmospheric pressure to function correctly. The VSI measures pressure differences between the static air pressure inside the instrument and the ambient static pressure outside the aircraft.

When the static port is blocked, the VSI can no longer sense changes in altitude accurately. Initially, when the blockage occurs, the VSI may still show a momentary indication based on the last known static pressure. However, as the aircraft continues to climb or descend, the pressure outside the aircraft changes while the pressure inside the VSI remains constant due to the blockage. Eventually, this leads to a condition where the VSI will indicate "0" because the pressure equalizes, meaning it can no longer show a rate of climb or descent. This situation is critical for pilots to understand since relying on the VSI under these circumstances can lead to incorrect assumptions about the aircraft's altitude changes.

This explains why the correct answer is that the VSI will indicate "0" once pressure equalizes.