Indicated airspeed

Indicated airspeed (IAS) is the airspeed read directly from the airspeed indicator on an aircraft, driven by the pitot-static system. IAS is directly related to calibrated airspeed (CAS), but includes instrument errors and position error.

An aircraft's indicated airspeed in knots is typically abbreviated "KIAS" for "Knots-Indicated Air Speed" (vs. "KCAS" for calibrated airspeed and "KTAS" for true airspeed).

IAS and V speeds

Unless an aircraft is at a sufficient sea level under International Standard Atmosphere conditions (15°C, 1013 hPa, 0% humidity) and no wind, the IAS bears little relation to how fast an aircraft is moving in reference to the ground; however, because the air pressure and density affect IAS/CAS and an aircraft's flight characteristics in exactly the same way, IAS and CAS are extremely useful for controlling an aircraft, and the critical V speeds are usually given as IAS.

In aneroid instruments the indicated airspeed drops-off with increasing altitude as air pressure decreases, and this leads to an apparent falling-off of airspeed at higher altitudes. For this reason never exceed speeds (abbreviated "V"NE) are often given at several differing altitudes in the aircraft's operating manual, the "V"NE IAS figure falling as height is increased, as shown in the sample table below.

{| class="toccolours" border="1" cellpadding="4" style="border-collapse:collapse"
- bgcolor="#dedede"
Diving below
mph IAS
-
30,000 ft
370
-
25,000 ft
410
-
20,000 ft
450
-
- valign="top"
15,000 ft
490
-
10,000 ft
540
-)

Ref: "Pilot's Notes for Tempest V Sabre IIA Engine" - Air Ministry A.P.2458C-PN

IAS and navigation

For navigation, it is necessary to convert IAS to ground speed (GS) using several steps:

* correct IAS to calibrated airspeed (CAS) using an aircraft-specific correction table;
* convert CAS to equivalent airspeed (EAS) by allowing for compressibility effects (not necessary at slow speed or low altitude);
* convert EAS to true airspeed (TAS) by allowing for differences in density altitude; and
* convert TAS to ground speed (GS) by allowing for the effect of wind.

With the advent of GPS receivers and other advanced navigation equipment that allows pilots to read ground speed directly, these complex calculations are becoming unnecessary.


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Look at other dictionaries:

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  • indicated airspeed — /ˌɪndəkeɪtəd ˈɛəspid/ (say .induhkaytuhd airspeed) noun the reading of an airspeed indicator which, at high altitudes, will be lower than the true airspeed owing to the lower air density …   Australian English dictionary

  • indicated airspeed — See airspeed …   Military dictionary

  • Airspeed — is the speed of an aircraft relative to the air. There are several different measures of airspeed: indicated airspeed, calibrated airspeed, equivalent airspeed and true airspeed.It is measured within the flying aircraft with an airspeed indicator …   Wikipedia

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  • airspeed indicator — An instrument which displays the indicated airspeed of the aircraft derived from inputs of pitot and static pressures …   Military dictionary

  • indicated air speed — The uncorrected reading of the air speed indicator. It is calibrated to reflect standard atmospheric adiabatic compressible flow at sea level uncorrected for air speed system errors. It is equal to 1/2 ρV2 , where ρ is density and V is true air… …   Aviation dictionary


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