What could cause a vertical speed indicator to display zero?

A vertical speed indicator (VSI) measures the rate of climb or descent of an aircraft. When the aircraft is in level flight, it is neither climbing nor descending, which results in a vertical speed of zero. This state indicates that the aircraft is maintaining a constant altitude, meaning there is no change in height over time.

In contrast, during a climb or descent, the VSI would register a positive or negative value, respectively, indicating the rate of ascent or descent. While an active autopilot system could be managing the aircraft's altitude, it would not specifically influence the VSI to read zero unless the aircraft is indeed in level flight. Therefore, the condition for displaying zero on the VSI is clearly identified when the aircraft is in level flight, reinforcing the understanding of the instrument's function.