Airspeed indicator
The airspeed indicator or airspeed gauge is an instrument used in an aircraft to display the craft's airspeed, typically in knots, to the pilot. In its simplest form, an ASI measures the difference in pressure between the air around the craft and the increased pressure caused by propulsion. The needle tracks pressure differential but the dial is marked off as airspeed.
The airspeed indicator is used by the pilot during all phases of flight, from take-off, climb, cruise, descent and landing in order to maintain airspeeds specific to the aircraft type and operating conditions as specified in the Operating Manual.
During instrument flight, the airspeed indicator is used in addition to the Artificial horizon as an instrument of reference for pitch control during climbs, descents and turns.
The airspeed indicator is also used in dead reckoning, where time, speed, and bearing are used for navigation in the absence of aids such as NDBs, VORs or GPS.
Airspeed indicators in many Light and Recreational aircraft can only show the pilot Indicated Airspeed (IAS). For True Airspeed (TAS) other components would have to be added by the manufacturer. Airspeed Indicator markings use a set of standardized coloured bands and lines on the face of the instrument. The white range is the normal range of operating speeds for the aircraft with the flaps extended as for landing or takeoff. The green range is the normal range of operating speeds for the aircraft without flaps extended. The yellow range is the range in which the aircraft may be operated in smooth air, and then only with caution to avoid abrupt control movement.
A redline mark indicates VNE, or velocity (never exceed). This is the maximum demonstrated safe airspeed that the aircraft must not exceed under any circumstances. The red line is preceded by a yellow band which is the caution area, which runs from VNO (maximum structural cruise speed) to VNE. A green band runs from VS1 to VNO. VS1 is the stall speed with flaps and landing gear retracted. A white band runs from VSO to VFE. VSO is the stall speed with flaps extended, and VFE is the highest speed at which flaps can be extended. Airspeed indicators in multi-engine aircraft show a short radial red line near to the bottom of green arc for Vmc, the minimum indicated airspeed at which the aircraft can be controlled with the critical engine inoperative and a blue line for VYSE, the speed for best rate of climb with the critical engine inoperative.
...
Wikipedia