Definition
VSO is the calibrated stalling speed, or the minimum steady flight speed, at which the airplane is controllable in the landing configuration — meaning gear down, flaps fully extended, power at idle, and the airplane at its most forward center of gravity at maximum landing weight. It is one of the reference stall speeds published by the manufacturer and is the basis for many approach and landing speed calculations.
Plain English
The slowest speed the airplane can fly without stalling when it is set up to land — wheels down, flaps all the way out, and engine pulled back to idle.
Context Anchor
Seen in airplane manuals, landing speed discussions, and performance charts when choosing safe approach and landing speeds.
Derivation
The 'V' stands for velocity, the 'S' for stall, and the 'O' for the landing configuration (think of it as the 'zero' or fully-out position of flaps and gear — the configuration used at the end of the flight). Knowing this makes the family of V-speeds easier to keep straight: VS is the general stall speed, VSO is stall in the landing configuration, VS1 is stall in a specified clean configuration.
Why Pilots Care
Pilots use VSO to set safe approach speeds and avoid stalls close to the ground during landing.
Intuition Check
Do not read VSO as the speed you should fly during landing. It is the stall-speed reference in the landing setup; normal approach speeds are higher.
Example Sentence 1
With full flaps and the gear down, the airplane's VSO is 50 knots, so the pilot flies final approach at about 65 knots.
Example Sentence 2
The bottom of the white arc on the airspeed indicator marks VSO for this airplane.