The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in miles per hour (MPH), knots, or both. The ASI measures the pressure differential between static pressure from the static port, and total pressure from the pitot tube. This difference in pressure is registered with the ASI pointer on the face of the instrument.
The ASI has standard color coded markings for the safe operation within the limitations of the aircraft. At a glance, the pilot can determine a recommended speed (V speeds) or if he needs to make any speed adjustments. Single and multi-engine aircraft have common markings. 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. The yellow arc cautions that flight should be conducted in this range only in smooth air, while the red line (VNE) at the top of the yellow arc indicates damage or structural failure may result at higher speeds