# Airspeed Indicator

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## 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 ?
The installation of the airspeed indicator went without any problems.
In the above case, had there been a problem with one (or two) engines during takeoff, the aircraft would have been only 10 knots above Vmc-air, and the airspeed indicators would have shown a 15-knot difference.
The Pitot system rapidly froze, and the airspeed indicator dropped to zero.
To the right of the AI is the altimeter, and to the left of the AI is the airspeed indicator.
The first thing I noticed was the airspeed indicator didn't come off the peg at the normal spot during the takeoff roll.
He politely pointed at my airspeed indicator, and I immediately reduced power and retracted the flaps.
As the airspeed indicator sped past 190 knots (approach flap-limit speed) on my side, I took the controls, pulled back the power levers toward flight idle, and retracted the flaps from approach to maneuver.
I looked down at my instrument panel, saw my airspeed indicator waving like a stereo needle, and then echoed, "601, also experiencing severe turbulence and 70-knot airspeed flux.
When I returned my scan to the airspeed indicator, the flashing master-caution light grabbed my attention.
Despite the aircraft being five degrees nose down and accelerating, my airspeed indicator read zero.
Having made the decision to get airborne as quickly as possible, I rechecked the airspeed indicator, looking for the 123 knots we had briefed as the rotate speed.
My left hand was on the emergency-blow handle, and I pulled it when I saw 150 knots on the airspeed indicator.

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