Speed and Velocity

A Knot (kt);

• A knot is the standard unit of airspeed, 1 kt = 1 nm/hr.

Groundspeed (GS);

• The speed of an aircraft in flight relative to the ground over which it is travelling.

Indicated Airspeed (IAS);

• Indicated Airspeed (IAS) is the speed read from the airspeed indicator.
• It is a measure of dynamic pressure obtained by subtracting static pressure from total pressure.

Calibrated Airspeed (CAS);

Calibrated airspeed (CAS) is Indicated airspeed corrected for instrument and position error.

Instrument Error is caused by constructional defects in the instrument.

Position Error arises from the fact that the flow of air over and around the static vent and pitot tube is disturbed by the aircraft's movement through the air.

Depending on the location of the pitot-static system, the measured dynamic pressure measured may differ from the actual dynamic pressure due to local induced velocity effects.

The offset between indicated and calibrated airspeed is usually published in the aircraft operating manual in the form of a table.

CAS is therefore more accurate than IAS, and when available, should be used in preference to IAS.