1nm Distance Off Track after 60nm Distance Flown = 1˚ HE*
2nm Distance Off Track after 60nm Distance Flown = 2˚ HE
10nm Distance Off Track after 60nm Distance Flown = 10˚ HE
10nm Distance Off track after 30nm Distance Flown = 20˚ HE
*HE = Heading Error
Basically, (60/Distance Flown) X Distance Off Track = Heading Error OR
(60/D1) X d1 = HE1˚
(Note 1: D1 = Distance Flown. d1 = Distance OFF. HE˚ = Heading Error and D = Total Leg Distance)
Now we know what the HEADING ERROR is to THIS POINT down track i.e. at D1.
If we turned by the amount of HEADING ERROR (in the correct direction of course) we would parallel the track. This would not help us to successfully fly to the next Turning Point or Destination. We would merely fly by the turning point by the distance off d1.
So we DON’T DO THAT. What we DO is:
Make a mental note of the HEADING ERROR (HE1˚)
Keep a mental note of the DISTANCE OFF (d1)
Calculate the distance to the next turning point D2 (D – D1)
Now we have all the information we need to calculate the HEADING ALTERATION to reach the next turning point
The calculation method is EXACTLY the same as the first one:
(60/D2) X d1 = HE2˚
Finally, ADD HE1˚+ HE2˚= HA˚ (Heading Alteration)
Example: Illustrated on the chart above.
Flight from St Mawgan to Chivenor.
Distance = 50nm.
At 30nm you estimate that the aircraft is 4nm right of track*.
There is 20nm to to go.
*This is much easier than drawing 'DRIFT' lines on all the flight plan legs.
[(60/D1) X d1] + [(60/D2) X d1] = HA
[(60/30) x 4] + [(60/20) x 4] = 20
HA = 20-degrees. Alter HEADING 20-degrees to the left.
These calculations can be done on the "Dalton" flight computer. d1 on outer scale against D1 on inner scale. Read H.E. aginst the INDEX (60).
TIP: If you fly an accurate HEADING by LOOKING OUT and maintain CHRONO discipline, then you should not have to calculate a heading alteration very often. You WILL be very close to your intended track.