Definition
A pitot-static flight instrument that displays the aircraft's rate of climb or descent in feet per minute, with internal accelerometer-driven pumps (or vanes) that compensate for the lag inherent in a standard vertical speed indicator. The IVSI shows pitch and vertical rate changes almost immediately when the aircraft begins to climb or descend, rather than waiting for the static pressure differential to build up across a calibrated leak.
Plain English
An instrument that shows how fast the airplane is climbing or descending, and reacts right away when you change pitch instead of taking several seconds to catch up.
Context Anchor
Seen in instrument flying when monitoring climbs, descents, altitude corrections, and the instrument scan.
Derivation
Instantaneous comes from the Latin instans, meaning 'present moment' or 'happening at once.' The name highlights what makes this instrument different from a regular vertical speed indicator: it shows the change in vertical motion at the moment it happens, rather than a few seconds later.
Why Pilots Care
It lets the pilot make immediate pitch corrections to hold altitude or a desired vertical speed, which is especially useful in instrument conditions where small errors can grow quickly.
Grounding Statement
When the airplane starts climbing or descending, the IVSI is designed to show that change almost at once rather than several seconds later.
Intuition Check
“Instantaneous” does not mean the instrument is perfect or predicts the future. It means the indication responds faster than a standard vertical speed indicator.
Example Sentence 1
When the pilot lowered the nose slightly, the IVSI immediately showed a 200 foot-per-minute descent, allowing a quick correction back to level flight.
Example Sentence 2
During the missed approach, the IVSI confirmed an immediate positive rate of climb after full power was applied.