Which of the following instruments directly indicates altitude changes?

The Vertical Speed Indicator (VSI) is designed specifically to indicate the rate of climb or descent of an aircraft, which directly relates to altitude changes. It measures the vertical speed in feet per minute (FPM) and provides pilots with essential information regarding how quickly they are gaining or losing altitude. The VSI operates using static air pressure and is an important instrument for maintaining a controlled flight path, especially during takeoff and landing, when precise altitude adjustments are critical.

In contrast, the airspeed indicator measures how fast the aircraft is moving through the air but does not provide any information about altitude changes. The engine display is focused on the performance and parameters of the aircraft's engines, and the hydraulic gauge monitors the hydraulic system's pressures rather than altitude or vertical speed. Therefore, the VSI is the most relevant instrument for directly indicating changes in altitude.