Which instrument is primarily used to determine vertical speed in flight?

The Vertical Speed Indicator (VSI) is specifically designed to measure and display the rate of climb or descent of an aircraft in feet per minute or meters per second. It provides pilots with crucial information regarding how quickly the aircraft is ascending or descending relative to a fixed point. This capability is essential for safe navigation and altitude management during flight operations.

The VSI operates based on the difference in static pressure as the aircraft changes altitude, thus providing an accurate representation of vertical speed. This information is vital for maintaining a stable flight profile and ensuring that changes in altitude are made safely and efficiently.

The other instruments serve different functions: the airspeed indicator measures horizontal speed, the altimeter reports altitude based on atmospheric pressure, and the attitude indicator shows the aircraft's orientation relative to the horizon, but none of these directly indicate vertical speed like the VSI does.