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Thetrueairspeed (TAS; alsoKTAS, forknots true airspeed) of anaircraft is thespeed of the aircraft relative to theair mass through which it is flying. The true airspeed is important information for accurate navigation of an aircraft. Traditionally it is measured using an analogueTAS indicator, but asGPS has become available for civilian use, the importance of such air-measuring instruments has decreased. Sinceindicated, as opposed totrue, airspeed is a better indicator of margin above thestall, true airspeed is not used for controlling the aircraft; for these purposes theindicated airspeed – IAS or KIAS (knots indicated airspeed) – is used. However, since indicated airspeed only shows true speed through the air at standard sea level pressure and temperature, a TAS meter is necessary for navigation purposes at cruising altitude in less dense air. The IAS meter reads very nearly the TAS at lower altitude and at lower speed. On jet airliners the TAS meter is usually hidden at speeds below 200 knots (370 km/h). Neither provides for accuratespeed over the ground, since surface winds or winds aloft are not taken into account.

TAS is the appropriate speed to use when calculating the range of an airplane. It is the speed normally listed on the flight plan, also used in flight planning, before considering the effects of wind.

Theairspeed indicator (ASI), driven by ram air into apitot tube and still air into a barometric static port, shows what is calledindicated airspeed (IAS). The differential pressure is affected byair density. The ratio between the two measurements is temperature-dependent and pressure-dependent, according to theideal gas law.

At sea level in theInternational Standard Atmosphere (ISA) and at low speeds where air compressibility is negligible (i.e., assuming a constant air density), IAS corresponds to TAS. When the air density or temperature around the aircraft differs from standard sea level conditions, IAS will no longer correspond to TAS, thus it will no longer reflect aircraft performance. The ASI will indicate less than TAS when the air density decreases due to a change in altitude or air temperature. For this reason, TAS cannot be measured directly. In flight, it can be calculated either by using anE6B flight calculator or its equivalent.

For low speeds, the data required arestatic air temperature, pressure altitude and IAS (orCAS for more precision). Above approximately 100 knots (190 km/h), the compressibility error rises significantly and TAS must be calculated by the Mach speed. Mach incorporates the above data including the compressibility factor. Modern aircraft instrumentation use anair data computer to perform this calculation in real time and display the TAS reading directly on theelectronic flight instrument system.

Since temperature variations are of a smaller influence, the ASI error can be estimated as indicating about 2% less than TAS per 1,000 feet (300 m) of altitude above sea level. For example, an aircraft flying at 15,000 feet (4,600 m) in the international standard atmosphere with an IAS of 100 knots (190 km/h), is actually flying at 126 knots (233 km/h) TAS.

To maintain a desiredground track while flying in the moving airmass, the pilot of an aircraft must use knowledge of wind speed, wind direction, and true air speed to determine the required heading. See alsowind triangle.

At low speeds and altitudes, IAS and CAS are close toequivalent airspeed (EAS).

${\displaystyle {\rho }_0(EAS{)}^2=\rho (TAS{)}^2}$

TAS can be calculated as a function of EAS and air density:

${\displaystyle \mathrm{T}\mathrm{A}\mathrm{S}=\frac{\mathrm{E}\mathrm{A}\mathrm{S}}{\sqrt{\frac{\rho }{{\rho }_0}}}}$

where

${\displaystyle \mathrm{T}\mathrm{A}\mathrm{S}}$ is true airspeed,

${\displaystyle \mathrm{E}\mathrm{A}\mathrm{S}}$ is equivalent airspeed,

${\displaystyle {\rho }_0}$ is the air density at sea level in theInternational Standard Atmosphere (15 °C and 1013.25 hectopascals, corresponding to a density of 1.225 kg/m³),

${\displaystyle \rho }$ is the density of the air in which the aircraft is flying.

TAS can be calculated as a function ofMach number and static air temperature:

${\displaystyle \mathrm{T}\mathrm{A}\mathrm{S}=a_{0}M\sqrt{\frac{T}{T_0}},}$

where

${\displaystyle a_0}$ is the speed of sound at standard sea level (661.47 knots (1,225.04 km/h; 340.29 m/s)),

${\displaystyle M}$ is Mach number,

${\displaystyle T}$ is static air temperature inkelvins,

${\displaystyle T_0}$ is the temperature at standard sea level (288.15 K).

For manual calculation of TAS in knots, where Mach number and static air temperature are known, the expression may be simplified to

${\displaystyle \mathrm{T}\mathrm{A}\mathrm{S}=39M\sqrt{T}}$

(remembering temperature is in kelvins).

Combining the above with the expression for Mach number gives an expression for TAS as a function ofimpact pressure, static pressure and static air temperature (valid for subsonic flow):

${\displaystyle \mathrm{T}\mathrm{A}\mathrm{S}=a_0\sqrt{\frac{5T}{T_0}\left[{\left(\frac{q_c}{P}+1\right)}^{\frac{2}{7}}-1\right]},}$

where:

${\displaystyle q_c}$ is impact pressure,

${\displaystyle P}$ is static pressure.

Electronic flight instrument systems (EFIS) contain anair data computer with inputs of impact pressure, static pressure andtotal air temperature. In order to compute TAS, the air data computer must convert total air temperature to static air temperature. This is also a function of Mach number:

${\displaystyle T=\frac{T_{\text{t}}}{1+0.2M^2},}$

where

${\displaystyle T_{\text{t}}=}$ total air temperature.

In simple aircraft, without an air data computer ormachmeter, true airspeed can be calculated as a function ofcalibrated airspeed and local air density (or static air temperature and pressure altitude, which determine density). Some airspeed indicators incorporate aslide rule mechanism to perform this calculation. Otherwise, it can be performed usingthis applet or a device such as theE6B (a handheld circularslide rule).

• United States Department of the Air Force and United States Navy Department. 1989.Air Navigation : Flying Training. Washington D.C.? Air Training Command in accordance with AFR 5-6 : For sale by the Supt. of Docs. U.S. G.P.O.
• Clancy L. J. 1978.Aerodynamics, Section 3.8. New York London: Wiley : Pitman.ISBN 978-0-470-15837-1,978-0-273-01120-0
• Kermode Alfred Cotterill. 1972.Mechanics of Flight, Chapter 2. 8th (metric) ed. London: Pitman.ISBN 978-0-273-31622-0,978-0-273-31623-7
• Gracey William. 1980.Measurement of Aircraft Speed and Altitude. Washington: NASA. Archived fromoriginal Fri, 04 Sep 2020 00:07:41 +0000 at theWayback Machine (11.1 MiB).

• A free windows calculator which converts between various airspeeds (true / equivalent / calibrated) according to the appropriate atmospheric (standard and not standard!) conditions
• Android application for airspeed conversion in different atmospheric conditions
• True, Equivalent, and Calibrated Airspeed at MathPages
• Newbyte airspeed converter
• avc.obsment.com - True airspeed calculator.
• Calculate True Airspeed, Mach, Pitot Tube Impact Air Pressure and more at luizmonteiro.com

• Airspeed

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