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Although indicated airspeed (IAS) and true airspeed (TAS) are the speeds most commonly used in aviation, references to calibrated airspeed (CAS) and equvialent airspeed (EAS) are quite often encountered.
Remembering the relationship between the various speeds and understanding the conversion from one to another can be facilitated by use of the acronym "ICE T" in which the letters represent Indicated, Calibrated, Equivalent and True. The relationships between the speeds are as follows:
- Indicated Airspeed is the speed shown on the airspeed indicator.
- Calibrated Airspeed is indicated airspeed corrected for position installation error.
- Equivalent Airspeed is calibrated airspeed corrected for compressibility.
- True Airspeed is equivalent airspeed corrected for temperature and pressure altitude.