Definition
VS0 is the calibrated power-off stalling speed, or the minimum steady flight speed at which the aircraft remains controllable, in the landing configuration. The landing configuration means flaps fully extended, landing gear down (if retractable), and engine power at idle. VS0 is shown on the airspeed indicator as the lower end of the white arc.
Plain English
The slowest speed the airplane can fly without stalling when it is set up to land — flaps down, gear down, throttle at idle. Fly slower than this and the wing stops producing enough lift to keep flying.
Context Anchor
You will see VS0 in aircraft performance-speed tables and on some airspeed indicators, usually tied to the lower limit for landing-configuration flight.
Derivation
The 'V' comes from the French vitesse, meaning 'speed,' which is why nearly all aircraft speed references begin with V. The 'S' stands for stall. The '0' (sometimes written as a subscript) indicates the configuration — zero flap retraction, meaning everything hanging out for landing.
Why Pilots Care
VS0 sets the baseline for safe approach speeds and helps prevent stalls during the landing phase.
Intuition Check
VS0 is not a recommended landing speed, and it does not mean zero airspeed. It is the stall or minimum controllable speed for the aircraft in its landing setup, so pilots normally fly with a margin above it.
Example Sentence 1
On final approach, the pilot targeted 1.3 times VS0 to ensure a safe margin above the stall with full flaps and gear down.
Example Sentence 2
With the airplane in landing configuration, any speed below VS0 risked an immediate stall.