Which instrument specifically displays vertical speed to the pilot?

The Vertical Speed Indicator (VSI) is specifically designed to display the rate of climb or descent of the aircraft in feet per minute. It measures the change in altitude over time and presents this information to the pilot, allowing for effective altitude management during various phases of flight.

The VSI works by utilizing an aneroid capsule that responds to changes in atmospheric pressure as the aircraft ascends or descends. This mechanical or electronic device provides an immediate visual representation of whether the aircraft is climbing, descending, or in level flight, all essential information for maintaining safe altitude control.

In contrast, an altimeter measures the current altitude of the aircraft but does not indicate the rate of ascent or descent. A Machmeter measures the speed of the aircraft relative to the speed of sound, while a Flight Director provides guidance commands to the pilot but does not directly measure vertical speed. Thus, the VSI is the only instrument among the options that defines and displays vertical speed accurately.