The airspeed indicator (ASI) or airspeed gauge is a flight instrument indicating the airspeed of an aircraft in kilometres per hour (km/h), knots (kn or kt), miles per hour (MPH) and/or metres per second (m/s). The recommendation by ICAO is to use km/h, however knots (kt) 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.

Colour-coded speeds and ranges

The ASI has standard colour-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 V<sub>S1</sub> to V<sub>NO</sub>. The white arc indicates the flap operating range, V<sub>SO</sub> to V<sub>FE</sub>, 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 (V<sub>NE</sub>) at the top of the yellow arc indicates damage or structural failure may result at higher speeds.

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 pitot opening is blocked, but the drain hole is open, the system pressure will drop to ambient pressure, and the ASI pointer will drop to a zero reading. If both the opening and drain holes are blocked, the ASI will not indicate any change in airspeed. However, the ASI pointer will show altitude changes, as the associated static pressure changes. If both the pitot tube and the static system are blocked, the ASI pointer will read zero. If the static ports are blocked but the pitot tube remains open, the ASI will operate, but

inaccurately.

See also

  • Acronyms and abbreviations in avionics
  • Flight instruments
  • Global Positioning System
  • Indicated airspeed
  • ICAO recommendations on use of the International System of Units
  • Position error
  • Speedometer
  • V speeds

References

Further reading

Installing and flying the Lift Reserve Indicator, article and photos by Sam Buchanan http://home.hiwaay.net/~sbuc/journal/liftreserve.htm

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