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Airspeed Indicator

 

The airspeed Indicator is a very sensitive differential pressure gauge, indicating the difference between the pitot pressure and the static pressure. Indication errors may arise from the instrument and from the placement of static and pitot ports. Instrument errors are noted on the ground by comparing readings to a standard manometer. Position errors are checked with special in-flight calibration equipment.

Indicated Airspeed (IAS) is reading shown on the airspeed indicator uncorrected for variations in the atmospheric density or the position and instrument error.

Calibrated Airspeed (CAS) is the indicated airspeed corrected for position and instrument error.

True Airspeed (TAS) is the actual speed of the aircraft moving through the air. A proper set of reference conditions is necessary and the TAS is CAS corrected to standard conditions of 15 degrees Celsius and 29.92 inches of Hg. At higher altitudes the air is less dense so that the airplane has to move faster to create the same pressure difference as at low altitudes. True airspeed therefore increases with altitude.