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Along with the altimeter and variometer, the airspeed indicator is a member of the pitot-static system of aviation instruments, so named because they operate by measuring pressure in the pitot and static circuits.
Airspeed Indicators work by measuring the difference between static pressure, captured through one or more static ports; and stagnation pressure due to "ram air", captured through a pitot tube. This difference in pressure due to ram air is called impact pressure.
Airspeed Indicators in many Sailplanes, 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 colored 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.
Wings & Wheels carries Winter ASI's. Winter GmbH & Co. KG manufactures precision flight instruments. Formed in 1931, today they still produce top quality flight instruments relying on 3 key factors: Experience, Qualification, and Quality management. They offer instruments in 80mm (3.125”) and 57mm (2.25”). You can choose the Zero in the 6 or 12o'clock position. Airspeed indicators that wrap around 1 1/2 turns or only one turn. Units can be in Knots, MPH, or KPH.