Airspeed Indicator Markings
The airspeed indicators in light aircraft are color coded to emphasize important airspeed limitations.
Green Arc: Normal Operating Range
The green arc shows the normal operating range of the airplane. The speed at the bottom of the green arc, abbreviated Vs1, is the stall speed with the flaps and landing gear retracted, power at idle, and the airplane at maximum gross weight. The top of the green arc shows the high end of the normal operating range, the maximum structural cruising speed, abbreviated Vno.
Yellow Arc: Caution Range
The yellow arc represents the caution range—speeds appropriate only in smooth air. The top of the yellow arc coincides with Vne, the never-exceed speed of the airplane.
Red Line: Never-Exceed Speed
A red line near the top of the airspeed range marks Vne. Exceeding this speed even in smooth air could damage the airplane structure.
White Arc: Flap Operating Range
The white arc shows the range of speeds in which it's safe to extend full flaps. The upper limit of the white arc is called Vfe, maximum flap extended speed. Extending the flaps at higher speeds could cause structural damage. The lower limit of the white arc, abbreviated Vso, is the stalling speed or minimum steady flight speed at maximum gross weight with the flaps and landing gear in the landing position.
Other Airspeed Markings
Multiengine airplanes (except large aircraft) have two additional airspeed markings.
Airspeed indicators on large aircraft like the Boeing 737-400 don't have these markings because the speeds they represent vary considerably depending on aircraft weight, power settings, and other factors. Pilots calculate these speeds before each takeoff and use markers called "bugs" on the airspeed indicator as reminders of those speeds under current conditions.