Double Track Error Method

This is the recommended method if the position fix shows the aircraft to be less than halfway along the leg – hence the reason for marking that midpoint on the chart. The procedure is as follows:

Using the diverging 10° drift lines estimate the track error – the difference in degrees between the track required and the track made good. The track error is also referred to as the opening angle.

For example let’s say that after passing the SHP and on our way to CEB7, we find ourselves 5° left of the required track. i.e. the track made good is 141° magnetic. Double the track error and add or subtract that value from the planned heading to arrive at the heading to regain track or the intercept. If the drift was to the right of track the new heading must be to the left of the original heading and, conversely, if drift was to the left the new heading must be to the right of the old. The time we must remain on this new heading, until intercepting the required track, is roughly equivalent to the time flown on the original heading.

For example track error is 5° left and original heading 146° magnetic, thus the heading to regain track is 146° + 10° = 156° magnetic and we fly that for the same time flown on the original heading. After it is visually evident that the required track has been reached, or the required time has passed, subtract the track error and turn onto the new heading to maintain the required track.

Theoretically this method doesn’t work if the position fix is past the halfway point because the point at which the required track is finally regained would be past the waypoint.