Firstly: a) Indicated Air Speed (IAS) is the reading of a particular air speed indicator & b) Rectified Air Speed (RAS) or Calibrated Air Speed (CAS) is the indicated airspeed corrected for instrument and installation errors & c) Equivalent Air Speed (EAS) is equal to the air speed indicator reading corrected for position error, instrument error and for adiabatic compressible flow for the particular altitude. (EAS is equal to RAS at sea level in standard atmosphere.)
True Air Speed (TAS) is the air speed relative to the surrounding air undisturbed by the aircraft’s motion. It is determined from the RAS (UK) or CAS (USA) by applying a correction for the density error and – at higher speeds – for the effects of compressibility. The density error is caused by deviation of the flight level pressure and temperature from the standard sea level values upon which the calibration of air speed indicators is based.
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