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6.8 Speed and Velocity

6.8.2 Define: 

(a) a Knot (kt);

The knot is a unit of speed equal to one nautical mile per hour.


(b) Ground Speed (GS);

Ground Speed is the speed of an aircraft relative to the ground.

Ground speed can be determined by the sum of the aircraft's airspeed and the wind speed;

  • a headwind reduces the ground speed, while
  • a tailwind increases the ground speed.


Airspeed 100kts + Headwind 20kts = Groundspeed 80kts.

Airspeed 100kts +   Tailwind  20kts = Groundspeed 120kts.


(c) Indicated Airspeed (IAS);

Indicated airspeed is the speed of an aircraft relative to the air mass it is flying in.

The airspeed is read directly from the airspeed indicator on an aircraft, calibrated to assume standard sea level atmospheric density.

It uses the difference between total pressure and static pressure of the air, provided by the pitot-static system, to measure dynamic pressure.

The system uses the difference between the total pressure (measured by the pitot tube) and the static pressure (measured by the static vents) to determine the dynamic pressure which is converted to an airspeed reading.

IAS is important for the consideration of aerodynamic performance.


(d) Calibrated Airspeed (CAS); 

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