Which altitude instrument measures the rate of climb or descent?

The Vertical Speed Indicator (VSI) is the correct instrument for measuring the rate of climb or descent in an aircraft. It provides the pilot with critical information on how quickly the aircraft is ascending or descending, typically expressed in feet per minute. This is crucial for maintaining the desired flight profile and ensuring safe operational maneuvers during climbs, descents, and turns.

The VSI works by measuring the difference between static pressure outside the aircraft and the pressure inside a diaphragm within the instrument. Changes in altitude affect this pressure differential, enabling the VSI to provide an accurate rate of vertical motion. This characteristic makes the VSI indispensable for pilots when managing altitude changes.

Other instruments like the altimeter, while essential for determining the aircraft's current altitude above sea level, do not provide any information regarding the climb or descent rate. The radar altimeter measures height above the terrain directly below the aircraft, but it too does not inform about climb or descent rates. The magnetic compass provides directional information and is unrelated to altitude measurements. Therefore, the VSI stands out as the specific tool that accurately reflects the rate of climb or descent.