What instrument provides a rate-of-pressure change indication and shows deviation from a constant pressure level?

The Vertical Speed Indicator (VSI) is the instrument that provides a rate-of-pressure change indication, which reflects how rapidly a pilot is climbing or descending. The VSI measures the change in static pressure as the aircraft gains or loses altitude. This change is interpreted and shown in feet per minute, indicating whether the aircraft is climbing or descending.

When the aircraft climbs, the pressure decreases, and when it descends, the pressure increases, allowing the VSI to display these variations efficiently. This instrument is vital for maintaining a desired altitude and in managing climb and descent rates during flight.

In contrast, the altimeter measures absolute altitude based on static air pressure but does not indicate the rate of change, while the static port serves as an inlet for static pressure but does not provide any readings itself. The airspeed indicator, on the other hand, measures the speed of the aircraft relative to the surrounding air but does not reveal any information about altitude changes or rates thereof. Thus, the VSI uniquely fulfills the requirement of showing the rate of pressure change and deviation from a constant pressure level.