Plain English Meaning

The slowest speed the airplane can fly, with power off, in a defined clean configuration, before it stalls. Below this speed, the wing can no longer produce enough lift and the airplane stops flying normally.

Full Definition

VS1 is the calibrated power-off stalling speed, or the minimum steady flight speed at which the aircraft remains controllable, in a specified configuration. The configuration is defined by the manufacturer and is typically a clean configuration (flaps up, gear up if retractable). VS1 is shown on the airspeed indicator as the bottom of the .