What does a vertical speed indicator display?

A vertical speed indicator (VSI) measures and displays the rate of climb or descent of an aircraft. It indicates how fast the aircraft is gaining altitude or losing altitude measured in feet per minute. This instrument is essential for pilots, providing immediate feedback on the aircraft's vertical movement relative to the ground.

The VSI operates using a diaphragm that reacts to changes in air pressure caused by the aircraft’s ascent or descent. As the aircraft climbs, the air pressure in the instrument decreases, and the VSI needle will point upward, indicating a climb. Conversely, during a descent, the needle points downward. This real-time data is crucial for maintaining safe flight paths, especially during takeoff and landing phases.

The other options relate to different aspects of flight. Ground speed refers to the speed at which the aircraft is moving across the ground and is typically displayed on a separate instrument called a groundspeed indicator. Heading changes are monitored by the heading indicator or magnetic compass, which shows the aircraft's direction relative to magnetic north. Air pressure changes can be detected by a barometer but are not directly indicated by the VSI.