What is the function of the vertical speed indicator (VSI)?

The vertical speed indicator (VSI) serves a crucial function in flight by indicating the rate of climb or descent of the aircraft. It provides real-time feedback to pilots on how quickly the aircraft is gaining or losing altitude, expressed in feet per minute (fpm). This information is essential for maintaining safe flight operations, particularly during takeoff, landing, and when executing climbs or descents.

By monitoring the VSI, a pilot can adjust the aircraft's pitch and power settings to achieve a desired rate of ascent or descent, helping to ensure that the aircraft remains within safe operating limits and complies with air traffic control instructions. This instrument is vital for visualizing altitude changes, especially in IMC (Instrument Meteorological Conditions), where pilots rely heavily on their instruments for navigation and safety.

In contrast, an indicator of ground speed, altitude display, or engine performance would serve entirely different functions unrelated to the specific flight dynamics indicated by the VSI.