Definition
An instrument that measures how quickly atmospheric pressure is changing, rather than measuring the pressure itself. In the cockpit, the vertical speed indicator (VSI) is the rate-of-pressure change instrument: as the aircraft climbs or descends, outside air pressure changes, and the VSI displays that rate of change as feet per minute up or down.
Plain English
It's an instrument that watches how fast the outside air pressure is changing and uses that to tell you how fast you are climbing or descending.
Context Anchor
Seen in discussions of the vertical speed indicator, which uses changing air pressure to show climb or descent rate.
Derivation
The name describes exactly what the instrument does: it measures the rate (how fast) at which pressure is changing. Because air pressure decreases with altitude in a predictable way, a steady rate of pressure change corresponds to a steady rate of climb or descent.
Why Pilots Care
Allows precise control of climb and descent rates for altitude accuracy, traffic separation, and stabilized approaches.
Grounding Statement
As an airplane climbs, outside air pressure drops; as it descends, outside air pressure rises, and the speed of that pressure change tells the instrument how fast the airplane is moving up or down.
Intuition Check
This instrument does not directly feel the airplane moving upward or downward. It measures how fast air pressure changes and uses that to show climb or descent rate.
Example Sentence 1
The vertical speed indicator is a rate-of-pressure change instrument, so it shows climb and descent rates by sensing how quickly static pressure is changing.
Example Sentence 2
During an ILS approach the pilot adjusted pitch to keep the rate-of-pressure change instrument at 700 feet per minute descent.