Which instrument is crucial for determining an aircraft's vertical speed?

The Vertical Speed Indicator (VSI) is essential for determining an aircraft's vertical speed because it specifically measures the rate of climb or descent in feet per minute (fpm). The VSI works by comparing the static pressure inside the aircraft with the static pressure outside. It indicates whether the aircraft is ascending, descending, or maintaining level flight.

As the aircraft climbs or descends, the changing pressure in the static system is translated into vertical speed, allowing pilots to make informed decisions about altitude changes during flight. This information is critical, especially during approaches and landings, where precise vertical speed control is necessary for safety and performance.

The altimeter measures altitude rather than vertical speed, while the turn coordinator helps indicate the rate of turn, and the heading indicator shows the aircraft's directional heading. Therefore, while all these instruments are important for various aspects of flight, none fulfill the specific function of measuring vertical speed like the VSI does.