Definition
The maximum speed at which an airplane may be operated in smooth air. Above this speed, the airplane must only be flown in smooth air, and even then with caution, because turbulence or abrupt control inputs could cause structural damage. On the airspeed indicator, VNO is marked by the upper end of the green arc and the lower end of the yellow (caution) arc.
Plain English
The fastest speed you should normally fly at. You can go faster, but only if the air is smooth, because rough air at higher speeds could damage the airplane.
Context Anchor
Seen on the airspeed indicator and in the airplane’s operating limitations, especially when checking speed ranges for descent or cruise.
Derivation
The 'V' comes from the French 'vitesse,' meaning speed. The subscript 'NO' stands for 'normal operation.' So VNO literally means 'speed, normal operation' -- the top of the normal operating range.
Why Pilots Care
Exceeding this speed in turbulence or rough air can overstress the airframe and lead to structural damage.
Intuition Check
VNO is not the same as VNE, the never-exceed speed. VNO is the top of normal operations; above it is a caution range, not a normal cruising range.
Example Sentence 1
Cruising in light chop, the pilot reduced power to keep the airspeed below VNO.
Example Sentence 2
During the multiengine checkout the instructor reminded the student to treat VNO as the upper limit for normal operations.