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What is the exact formula for True Airspeed (TAS)?

Asked by: 16289 views , , ,
Commercial Pilot, General Aviation, Instrument Rating, Private Pilot

Hello!, I always use the E6b flight computer to get the TAS, but i'd like to know the exact formula i mean in case i don't have the e6B .

There's a basic formula for TAS:  IAS + 2% * Altitude/1000  but this does not work with non-standard temperatures and in all altitudes.

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2 Answers



  1. Nathan Parker on Nov 09, 2012

    Generally, TAS = SQRT(air density sea level /air density at altitude) * CAS.

    However, you don’t have any means of obtaining air density in flight. There are some formulas that can compute this for you, but I don’t think any of them are practical for use in flight.

    The 2% rule-of-thumb is probably good enough considering the accuracy of the other factors that influence your flight performance.

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  2. John D. Collins on Nov 09, 2012

    For speeds well below the speed of sound, the value for IAS is approximately the same as EAS (Effective Airspeed). The relation between the TAS and EAS is the EAS divided by the square root of the density ratio of air. The density ratio of air is the ratio between the density of the air at a given pressure altitude and temperature divided by the density of the air at sea level with standard temperature and pressure. So, for speeds that most of us fly at, the TAS equals the IAS divided by the square root of the density ratio of air. In the book “Basic Aerodynamics for Naval Aviators” which you can download from this site, on page 13, figure 1.6 is a Density Altitude chart which allows you to lookup the pressure and temperature at any pressure altitude and read off the ratio of one divided by the square root of the density ratio. You multiply this value times the IAS to determine the TAS. Better yet, keep your e6B handy.

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