V/SO is defined as the

• A. Stalling speed or minimum steady flight speed in the landing configuration
• B. Normal operating range
• C. Maximum flap extended speed
• D. Maximum landing gear extended speed

Answer: (A)

V_S0 is the stall speed in the landing configuration—the minimum steady flight speed at which the airplane can maintain level flight with the landing setup (gear down and any landing devices like flaps or spoilers arranged for landing). This speed defines the lower limit of the normal operating envelope because below it the wing cannot produce enough lift to support the aeroplane’s weight, so a stall occurs unless you change the configuration or increase speed. The other options describe different limits: the normal operating range is a speed band, not a single speed; the maximum flap extended speed is the upper limit with flaps out; and the maximum landing gear extended speed is the limit with gear down.