The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometers per hour (km/h), knots (kn), miles per hour (MPH) and/or meters per second (m/s). The recommendation by ICAO is to use km/h, however knots is currently the most used unit. 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. Color-coded speeds and ranges The ASI has standard color-coded markings to indicate safe operation within the limitations of the aircraft. At a glance, the pilot can determine a recommended speed (V speeds) or if speed adjustments are needed. 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. The ASI in multi-engine aircraft includes two additional radial markings, one red and one blue, associated with potential engine failure. The radial red line near the bottom of green arc indicates Vmc, the minimum indicated airspeed at which the aircraft can be controlled with the critical engine inoperative. The radial blue line indicates VYSE, the speed for best rate of climb with the critical engine inoperative. Operation The ASI is the only flight instrument that uses both the static system and the pitot system. Static pressure enters the ASI case, while total pressure flexes the diaphragm, which is connected to the ASI pointer via mechanical linkage. The pressures are equal when the aircraft is stationary on the ground, and hence shows a reading of zero. When the aircraft is moving forward, air entering the pitot tube is at a greater pressure than the static line, which flexes the diaphragm, moving the pointer. The ASI is checked before takeoff for a zero reading, and during takeoff that it is increasing appropriately. The pitot tube may become blocked, because of insects, dirt or failure to remove the pitot cover. A blockage will prevent ram air from entering the system. If the