Definition
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 at the most forward center of gravity. On the airspeed indicator, VSO is marked by the lower end of the white arc.
Plain English
The slowest speed the airplane can fly in landing configuration — gear down, flaps down, power off — before it stalls. It's the bottom of the white arc on the airspeed indicator.
Context Anchor
Seen in landing, descent, stall, and airspeed indicator discussions; it is often the lower end of the white arc on the airspeed indicator.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed — used throughout aviation for defined performance speeds (V-speeds). The 'S' stands for 'stall.' The 'O' is for the landing configuration, sometimes remembered as 'gear and flaps Out' or as the zero-power, fully-configured state.
Why Pilots Care
Approach speed is normally calculated as 1.3 times VSO; knowing the value ensures adequate stall margin during landing.
Intuition Check
The last character is the letter O, not the number zero. VSO is not an approach speed by itself; normal approach speeds are chosen above it.
Example Sentence 1
The instructor pointed to the lower end of the white arc on the airspeed indicator and said, 'That's VSO — the slowest you can fly in the landing configuration before the wing stalls.'
Example Sentence 2
Landing distance tables in the handbook are based on the published VSO for the airplane.