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by Jan Zumwalt
rev  2014.11.07


T rue airspeed (TAS; also KTAS, for knots true airspeed) of an aircraft is the speed of the aircraft relative to the airmass in which it is flying. The true airspeed is necessary for accurate navigation of an aircraft. TAS is the true measure of aircraft performance in cruise, thus listed in aircraft specs, manuals, performance comparisons, pilot reports, and every situation when actual performance needs to be measured. It is the speed normally listed on the flight plan, also used in flight planning, before considering the effects of wind.

At sea level in the International Standard Atmosphere (ISA) and at low speeds where air compressibility is negligible, 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.

Above approximately 100 knots, the compressibility of air introduces error and begins to differ significantly from TAS. To overcome this Mach speed may be used. Mach incorporates the above data including the compressibility factor. Large or high performance aircraft use a Air Data Computer to perform this calculation in real time and display the TAS reading directly on the EFIS (electrionic flight information computer).

Since temperature variations are of a smaller influence, the ASI error can be roughly estimated as indicating about 2% less than TAS per 1,000ft of altitude above sea level. For example, an aircraft flying at 15,000ft in the international standard atmosphere with an IAS of 100kt, is actually flying at 126kt TAS.

<-- Enter Indicated Altitude Feet    Meters
<-- Enter Altimeter Setting Inches    hPa
<-- Enter Temperature °C    °F
<-- Enter Indicated or Calibrated Airspeed (KTS or MPH)
True Airspeed (TAS):
Density Altitude (DA):
Pressure Altitude (PA):

Note: standard pressure is 29.92126, if you're trying to test for zero.