What is the primary function of the VSI?

The primary function of the Vertical Speed Indicator (VSI) is to show whether the aircraft is climbing or descending and at what rate. This instrument provides critical feedback to pilots regarding the vertical movement of the aircraft, typically measured in feet per minute. By indicating the rate of climb or descent, the VSI helps pilots maintain situational awareness and make necessary adjustments to flight control to achieve desired altitude changes. Understanding the aircraft's vertical position is essential for safe navigation, especially during takeoff, approach, and landing phases of flight. The VSI does not serve functions such as indicating fuel levels, displaying speed, or tracking navigation waypoints, which are the responsibilities of other instruments.