6.8 Speed and Velocity

6.8.2 Define: 

(a) a Knot (kt);

A knot is a unit of speed representing one nautical mile per hour.

(b) Ground Speed (GS);

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

It is calculated by combining the aircraft's airspeed with the effect of the wind:

• Headwind: Reduces ground speed.

• Tailwind: Increases ground speed.

Examples:

• Airspeed 100 kt + Headwind 20 kt → Ground speed 80 kt

• Airspeed 100 kt + Tailwind 20 kt → Ground speed 120 kt

(c) Indicated Airspeed (IAS);

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

It is read directly from the aircraft’s airspeed indicator, which is calibrated for standard sea-level atmospheric conditions.

IAS is determined using the pitot-static system, which measures the difference between total pressure (from the pitot tube) and static pressure (from the static vents) to calculate dynamic pressure. This dynamic pressure is then converted into an airspeed reading.

IAS is crucial for assessing aerodynamic performance.

(d) Calibrated Airspeed (CAS); 

Calibrated airspeed is the indicated airspeed corrected for instrument and position errors.

• Instrument error results from constructional imperfections in the airspeed indicator.