Airspeed Indicator

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airspeed indicator

[¦er‚spēd ‚in·də‚kād·ər]
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.


References in periodicals archive ?
The installation of the airspeed indicator went without any problems.
Airspeed indicator out of calibration, static system leaking or influenced by disrupted airflow near static ports.
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CO-PILOT: The airspeed indicator (sound of switch, end of ringing).
For example, hop into a typical training aircraft and look at the airspeed indicator.
The problem partially stems from decades of misusing airspeed indicators.
My pilot's airspeed indicator erratically fluctuated between 15 to 30 knots.
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Presuming a no-wind situation, our hero now is covering more than a nautical mile per minute, even though the airspeed indicator and the vertical speed indicator both "say" the approach is being flown as if at sea level.
Few pilots need an airspeed indicator to tell them when they're getting on the edge of a main-wing stall.
If you're faced with replacing an airspeed indicator (a source of pitot system leakage), you'll likely have to order a new United model and rest the old one on your bar shelf as a conversation piece.