Rule 39C

Triangle of Velocities


Track:    It is the actual path traversed over the ground.  It may be track required, track desired or Track Made Good (TMG).

–    The Americans use the term True Course for  Track required.

–    It is indicated by two arrows on the track line.

Ground Speed:  The actual speed of the aircraft over the ground, covered or expected to be covered in a specified time.

Heading: The direction of the longitudinal axis of the aircraft from a known reference direction is called aircraft heading. It may be True heading (HdgT), Magnetic heading, (HdgM), Compass heading (HdgC) or Grid heading (HdgG).

–    It is indicated by a single arrow on the heading line

Wind Direction: The direction from which the wind is blowing. The direction is always True direction.

–    It is indicated by three arrows on the wind vector.

Wind Speed:  The speed at which the wind is blowing.

Wind Velocity (W/V):  Wind direction and speed when represented as vector is called wind velocity

Triangle of Velocities: The six components, Heading & True Air Speed, Track & Ground Speed and Wind Direction & Wind Speed form the 3 sides and 3 angles of a triangle. This triangle is called triangle of velocities.



















–    In this triangle the heading and track are the directions towards progress of the aircraft were as wind direction is the direction from which wind is blowing.

–    In Navigation this triangle is continuously solved to know the position of the aircraft.

–    In pre-flight planning the triangle of velocities is solved for every leg from the four known  elements ie.  Track required, TAS and forecast W/V. Heading and Ground Speed are calculated.

–    If any of the four elements are known, other two elements may be calculated.

Drift: It is the angular difference between Heading and Track, measured in degrees port or starboard of the heading.

Bearing:  The direction of one object from another, measured from a reference direction clockwise through 360º is called bearing.

–    It may be True, Magnetic, Compass, Grid or Relaltive Bearing depending upon the reference direction chosen.

Relative Bearing (RB):  The direction measured with respect to the longitudinal axis of the aircraft is called Relative Bearing.

Radial: The magnetic direction emanating from a radio station is called radial.

–    It is always magnetic.

–    Reciprocal of the radial is called anti-radial


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