True airspeed
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Current revision
True airspeed (TAS) is the speed of an aircraft relative to the airmass in which it flies, i.e. the magnitude of the vector difference of the velocity of the aircraft and the velocity of the air. Under zero wind conditions this is equal to the speed over the ground. Under wind conditions an estimation of the wind is used to make a windspeed vector calculation that computes an estimated ground speed from the true air speed and a wind correction angle to maintain the desired ground track.
Aircraft display an indicated airspeed on an instrument called an airspeed indicator. Indicated airspeed will differ from true airspeed at air densities other than some reference density. Air density is affected by temperature, moisture content, and altitude. Indicated airspeed is used in aircraft operation as the aircraft stalling speed and structural limiting speeds are dependant on indicated airspeed, irespective of true airspeed. However, proper navigation via dead reckoning (without constant ground reference) requires the use of true airspeed and wind corrections.
TAS can be calculated as a function of Mach number and static air temperature, or as a function of Mach number and total air temperature.
<math>
\mathrm{TAS} = 39M_a\sqrt{T_{static}+273.25}
</math>
<math>
\mathrm{TAS}=39M_a\sqrt{\frac{\left(T_{total}+273.15\right)}{1+0.2M_a^2}}
</math>
where
<math>TAS=</math> true airspeed in knots
<math>M_a=</math> Mach number
<math>T_{static}=</math> static air temperature in degrees Celsius
<math>T_{total}=</math> total air temperature in degrees Celsius
Electronic flight information systems (EFIS) contain an air data computer with inputs of total air temperature, impact (pitot) pressure and static pressure. Since Mach number is a function of impact and static pressures, an EFIS can compute and display TAS.
In simple aircraft, without an air data computer or Mach meter, true airspeed can be calculated as a function of equivalent 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 with a calculator such as the E6B handheld circular slide rule.