# Airspeed Indicator

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Related to Air speed indicator: Attitude indicator, Vertical speed indicator

## airspeed indicator

[¦er‚spēd ‚in·də‚kād·ər]
(engineering)
A device that computes and displays the speed of an aircraft relative to the air mass in which the aircraft is flying.

## Airspeed Indicator

an instrument used in aviation to measure the flight speed of an aircraft (airplane or helicopter) relative to the ambient air. The determination of the airspeed V is necessary in handling an airplane (since the lift from the wing is proportional to the square of the airspeed), as well as for navigational purposes—for example, to calculate the flight path of the airplane.

An airspeed indicator has three main parts: a Pitot-static tube, a duct, and a needle gauge. The Pitot tube measures the static pressure ps and the dynamic (total) pressure Pd. The difference between them equals the dynamic pressure—that is, 0.5 ρ V2, where ρ is the density of the air. The scale is graduated in units of airspeed to correspond to the deformation of the sensing element—the manometric (aneroid) capsule—of the airspeed indicator. When measuring flying speeds above 800 km/hr, a correction is made to take into account the compressibility of air.

The readings of the instrument are directly proportional to the value of ρ, which depends on the pressure p and the temperature T of the ambient air. If they are assumed to be constant(p = 101,325 newtons per sq m = 760 mm of mercury; T = 288° K), then the instrument will show the indicated (instrument) airspeed. If, however, the readings are corrected for their variation with altitude (this is done automatically by a device within the mechanical linkage between the sensing element and the needle), the instrument will show the true airspeed. In practice, airspeed indicators have two needles (combined airspeed indicators), one of which shows instrument readings, and the other the true airspeed.

A. L. GORELIK

References in periodicals archive ?
O'Dell told his passengers and crew: "It looks as if we've got a failed air speed indicator on the captain's side which, I'm sure you will realise, is fairly crucial for flying.
A Singapore Airlines Airbus A340-300 reportedly suffered problems with its air speed indicator and recorded dramatic rudder swings that were not commanded by the pilots, during a flight to Dhaka, Bangladesh.
The commissioner was almost killed four years ago when the landing gear on his two-seater Piper aircraft failed to lower due to a faulty bolt and his air speed indicator failed.
But their plans were shelved as the pilot of the Britannia Boeing 757 dramatically aborted takeoff because the air speed indicator had broken down.
After studying the An-148's flight data recorder, the Interstate Aviation Committee said that Sunday's crash near Moscow occurred after the pilots saw conflicting data on the plane's two air speed indicators. The flawed readings came because the pilots failed to turn on the heating unit for the plane's pressure measurement equipment prior to takeoff, the committee said.
In choppers, air speed indicators are unreliable during low- visibility take- offs.

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