Definition
A pitot-static flight instrument that displays the airplane's rate of climb or descent in feet per minute (fpm) by sensing the rate of change in static air pressure as altitude changes.
Plain English
A cockpit gauge that shows how fast the airplane is going up or down, measured in feet per minute.
Context Anchor
Seen on the instrument panel during cockpit checks, climbs, descents, and instrument scanning.
Derivation
Straight from the words: it indicates (shows) vertical (up-and-down) speed. The name describes exactly what the instrument does.
Why Pilots Care
Pilots rely on the VSI to maintain assigned altitudes, control climb and descent rates during approaches, and avoid unintended altitude deviations that could lead to terrain conflicts or ATC conflicts.
Intuition Check
Do not confuse vertical speed with forward speed. The VSI does not show how fast the airplane is moving across the ground or through the air; it shows how fast its altitude is changing.
Example Sentence 1
After leveling off at 5,500 feet, the pilot adjusted pitch and power until the VSI settled on zero.
Example Sentence 2
During the ILS approach the VSI showed a steady 700 feet-per-minute descent to the decision altitude.