Definition
A reference wind value equal to 20 percent of an aircraft's stalling speed in the landing configuration (VS0). On the FAA Crosswind and Headwind Component Chart, it represents the maximum recommended crosswind component for an aircraft when a manufacturer-published demonstrated crosswind value is not available.
Plain English
A rule-of-thumb crosswind limit. Take the speed at which your airplane stalls with full flaps and gear down, then take 20 percent of that number. That is the strongest direct crosswind you should plan to land in if the aircraft's manual does not give you a specific figure.
Context Anchor
Seen in performance and crosswind discussions where a wind value is compared with a fraction of the aircraft’s landing stall speed.
Derivation
VS0 is a standard aviation symbol: V for velocity (speed), S for stall, and the subscript 0 for the landing configuration (flaps and gear extended). The 0.2 in front simply means 20 percent of that speed.
Why Pilots Care
Crosswind landings are one of the most common situations where aircraft are damaged. If the airplane's manual does not list a demonstrated crosswind component, this 20 percent guideline gives the pilot a conservative number to plan against, rather than guessing.
Intuition Check
Do not read 0.2 VS0 as “0.2 knots” or as 20 percent of the wind. It means 20 percent of the airplane’s VS0 speed. Also, the last character in VS0 is zero, not the letter O.
Example Sentence 1
With a VS0 of 50 knots, the pilot used 0.2 VS0 to set a personal crosswind limit of 10 knots for the day's training flight.
Example Sentence 2
At 0.2 VS0 the wind component line indicates the airplane can still maintain directional control on the runway.