What does a vertical speed indicator (VSI) measure?

A vertical speed indicator (VSI) specifically measures the rate of climb or descent of an aircraft in feet per minute. It provides pilots with critical information on whether the aircraft is ascending or descending, and at what rate, allowing them to make informed decisions regarding altitude changes during flight. This instrument is an essential tool for maintaining control over the aircraft's vertical profile, especially during takeoff and landing phases, where proper altitude management is crucial.

Understanding the climb and descent rate is vital for ensuring safe operation, preventing uncontrolled altitude changes and assisting in the management of airspace requirements. The VSI responds to changes in static air pressure in the surrounding atmosphere – an increase in pressure can indicate descent, while a decrease suggests climb. This information directly supports maintaining optimal performance during different flight stages.

The other choices do not align with the function of the VSI: measuring aircraft speed in knots relates to a different instrument, while the angle of ascent in degrees involves other calculations or gauges, and air pressure variations pertain more to barometric instruments.