Editing True airspeed (section)

==Calculating true airspeed==
===Low-speed flight===
At low speeds and altitudes, IAS and CAS are close to [[equivalent airspeed]] (EAS).

<math>\rho_0 (EAS)^2 = \rho (TAS)^2</math>

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

<math>\mathrm{TAS} =\frac { \mathrm{EAS}}{\sqrt{\frac{\rho}{\rho_0}}}</math>

where
:<math>\mathrm{TAS}</math> is true airspeed,
:<math>\mathrm{EAS}</math> is equivalent airspeed,
:<math>\rho_0</math> is the air density at sea level in the [[International Standard Atmosphere]] (15&nbsp;°C and 1013.25 hectopascals, corresponding to a density of 1.225&nbsp;kg/m<sup>3</sup>),
:<math>\rho</math> is the density of the air in which the aircraft is flying.

===High-speed flight===
TAS can be calculated as a function of [[Mach number]] and static air temperature:

<math>\mathrm{TAS} ={a_0} M\sqrt{T\over T_0},</math>

where 
:<math>{a_0}</math> is the speed of sound at standard sea level ({{convert|661.47|kn|km/h m/s}}),
:<math>M</math> is Mach number,
:<math>T</math> is static air temperature in [[kelvin]]s,
:<math>T_0</math> is the temperature at standard sea level (288.15&nbsp;K).

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

<math>
\mathrm{TAS} = 39M\sqrt{T}
</math>

(remembering temperature is in kelvins).

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

<math>\mathrm{TAS} = a_0\sqrt{\frac{5T}{T_0}\left[\left(\frac{q_c}{P} + 1\right)^\frac{2}{7} - 1\right]},</math>

where:
:<math>q_c</math> is impact pressure,
:<math>P</math> is static pressure.

[[Electronic flight instrument system]]s (EFIS) contain an [[air data computer]] with inputs of impact pressure, static pressure and [[total 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:

<math>
T = \frac{T_\text{t}}{1 + 0.2M^2},
</math>

where
:<math>T_\text{t} = </math> total air temperature.

In simple aircraft, without an air data computer or [[machmeter]], true airspeed can be calculated as a function of [[calibrated airspeed]] and local air density (or static air temperature and pressure altitude, which determine density).  Some airspeed indicators incorporate a [[slide rule]] mechanism to perform this calculation. Otherwise, it can be performed using [http://www.newbyte.co.il/calc.html this applet] or a device such as the [[E6B]] (a handheld circular [[slide rule]]).