[1] The ASI has standard colour-coded markings to indicate safe operation within the limitations of the aircraft.
For instance, the green arc indicates the normal operating range of the aircraft, from VS1 to VNO.
The white arc indicates the flap operating range, VSO to VFE, used for approaches and landings.
[1]: 8–9 The ASI in multi-engine aircraft includes two additional radial markings, one red and one blue, associated with potential engine failure.
The radial blue line indicates VYSE, the speed for best rate of climb with the critical engine inoperative.
If the static ports are blocked but the pitot tube remains open, the ASI will operate, but inaccurately.
Alternatively, a rule of thumb is to add 2 percent to the CAS for every 1,000 ft (300 m) of altitude gained.
Thus, a pilot of a jet aeroplane needs both an airspeed indicator and a Machmeter, with appropriate red lines.
An ASI will include a red-and-white striped pointer, or "barber's pole", that automatically moves to indicate the applicable speed limit at any given time.
For a particular configuration, it is a constant independent of weight, bank angle, temperature, density altitude, and the center of gravity of an aircraft.