What does the vertical speed indicator communicate to a pilot?

The vertical speed indicator (VSI) specifically provides crucial information about the aircraft's rate of ascent or descent. It measures how quickly the plane is climbing or descending, typically displayed in feet per minute (fpm) on the instrument. This helps pilots maintain an optimal climb or descent profile, ensuring they can execute safe maneuvers, adjust flight paths, and adhere to air traffic control instructions effectively.

Understanding the vertical speed is essential, especially during takeoff and landing phases, where precise altitude changes are critical for safety. The VSI utilizes a diaphragm that expands or contracts as the aircraft alters its altitude, thereby allowing the indicator to show the vertical speed accurately.

This capacity to quantify how quickly the aircraft is moving up or down is indispensable in maintaining situational awareness during flight, making option C the correct choice.