What happens to the VSI reading when an aircraft climbs?

When an aircraft climbs, the Vertical Speed Indicator (VSI) provides a reading that indicates the rate at which the aircraft is ascending. This increase in the VSI reading occurs because the instrument is designed to measure the change in altitude over time. As the aircraft climbs, there is a corresponding increase in altitude, which the VSI translates into a positive indication of climb rate.

The VSI operates by measuring the difference between static pressure outside the aircraft and the pressure in the VSI’s internal diaphragm. When the aircraft climbs, the outside pressure decreases while the pressure inside the instrument remains relatively unchanged for a brief period, causing the diaphragm to expand and move the needle upward on the VSI dial. Therefore, during a steady climb, the VSI reflects this change, indicating a positive rate of ascent, which is shown as an increase in the reading.

This behavior helps pilots monitor their climb rate effectively, allowing for better altitude management and ensuring compliance with various flight regulations.