Definition
An airspeed equal to 1.8 times the airplane's stalling speed in the landing configuration (gear and flaps extended). For small airplanes, this is the maximum airspeed at which an instrument approach should be flown unless the airplane is being operated at a higher airspeed for a specific reason.
Plain English
A speed worked out by taking the slowest speed at which the airplane can still fly with gear and flaps down, then multiplying it by 1.8. It sets a sensible upper limit for how fast a small airplane should fly during an instrument approach.
Context Anchor
Seen in discussions of slow-speed flight, approach speeds, and small-airplane handling near the lower end of the speed range.
Derivation
VSO comes from V (velocity) and S (stall), with the subscript O meaning 'in the landing configuration' (gear and flaps fully extended). The 1.8 is simply a multiplier — 1.8 times that stall speed.
Why Pilots Care
Provides a safe margin above stall speed while operating in the slow-flight regime, preserving control effectiveness.
Grounding Statement
If an airplane’s VSO is 50 knots, then 1.8 VSO is 90 knots.
Intuition Check
Do not read 1.8 VSO as a fixed speed for every airplane. It is a multiplier: first know that airplane’s VSO, then multiply by 1.8.
Example Sentence 1
The instructor reminded the student that the approach should be flown at or below 1.8 VSO in their small training airplane.
Example Sentence 2
At 1.8 VSO the airplane exhibited predictable handling qualities with adequate aileron and rudder response.