A vertical speed indicator (VSI) is an instrument used in aircraft to measure the rate of climb or descent. It displays how quickly an aircraft is gaining or losing altitude, typically in feet per minute. The VSI helps pilots maintain a desired altitude and ensures safe flight operations. The VSI operates using a diaphragm that responds to changes in air pressure as the aircraft ascends or descends. When the aircraft climbs, the pressure inside the diaphragm decreases, causing the needle to move upward. Conversely, during descent, the pressure increases, and the needle moves downward, providing real-time feedback to the pilot.