What is measured by the vertical velocity indicator?

The vertical velocity indicator (VVI) is specifically designed to measure the rate of climb or descent of an aircraft. When an aircraft is climbing, the VVI shows a positive rate, indicating the altitude is increasing; conversely, when descending, the VVI shows a negative rate, indicating the altitude is decreasing. This measurement is crucial for pilots in maintaining a safe and controlled ascent or descent, allowing them to manage their flight path effectively and ensure a smooth operation during these phases of flight.

Other measurements such as altitude, time, or speed are indicated by different instruments. For instance, altitude is measured by an altimeter, time is typically tracked with a watch or onboard timer, and speed is indicated by an airspeed indicator. Therefore, the VVI's focus on rate of climb or descent is the defining characteristic that is correct in this context.