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

The vertical speed indicator (VSI) is specifically designed to show the rate of climb or descent of an aircraft. It provides the pilot with crucial information on how quickly the aircraft is ascending or descending, which is important during various phases of flight, especially takeoff and landing. The VSI does this by measuring the change in atmospheric pressure as the aircraft moves vertically. When the aircraft climbs, the pressure decreases, and the VSI reflects this by indicating a positive rate of climb. Conversely, during descent, the pressure increases, and the VSI shows a negative rate. This information is essential for maintaining controlled flight and for making informed decisions about altitude changes.

In contrast, the other options relate to different instruments: altitude is indicated by the altimeter, airspeed is shown by the airspeed indicator, and navigation direction is provided by the heading indicator or magnetic compass. Each of these instruments plays a distinct role in flight management and does not overlap with the functionality of the VSI.