What does a vertical speed indicator display?

A vertical speed indicator (VSI) is designed to show the rate at which an aircraft is climbing or descending, typically expressed in feet per minute. This instrument provides pilots with critical information about the aircraft's vertical motion, allowing them to assess whether they are gaining altitude or losing it and at what rate.

Understanding the vertical speed is essential for maintaining safe flight operations, particularly during ascent and descent phases. For instance, if a pilot is approaching an airport for landing, they need to monitor their vertical speed to ensure they are descending at a rate suitable for a safe approach.

The other choices focus on aspects like ground speed, altitude relative to an airport, and approach angles, which are not functions of the vertical speed indicator. Instead, they pertain to different instruments used alongside the VSI to provide a complete picture of the aircraft's performance and position during flight.