Dead Reckoning: Track, Heading, Wind and Drift
The Vectors
True track is the path over the ground measured from true north; heading is the direction the aircraft nose points. True airspeed (TAS) is the aircraft's speed through the air; groundspeed (GS) is its speed over the ground. Wind velocity is the direction the wind blows from, at its speed.
The Triangle of Velocities
Heading and TAS form one vector, wind another, and their resultant is track and groundspeed. Drift is the angle between heading and track, positive to the right. Solve by the wind triangle: draw the heading/TAS line, add the wind vector, and read track and groundspeed.
Computations
Track made good is tracked by comparing successive fixes. If drift is known, track = heading + drift (right positive). Wind components: headwind/tailwind along track, crosswind across it. Groundspeed = TAS ± headwind component.
Traps
Wind direction is always the direction from which the wind blows, so a wind of 270° comes from the west. Drift sign errors are common: a wind from the left drifts the aircraft right of heading. Also distinguish variation (true to magnetic) from drift (heading to track).
Worked Example
TAS 120 kt, heading 090°T, wind 180°/20 kt (from the south). The wind is a pure crosswind, so drift = arcsin(20/120) = 9.6° to the right, giving track 099.6°T; the along-track component is zero so groundspeed is about 118 kt (√(120² − 20²)).