Navigation plan calculations: tracks, headings, wind triangle, ground speed and leg times
Domain 4 is the numerical core of the plan: the wind triangle, drift, ground speed, leg time and total elapsed time.
The method
- Wind angle (WA) = wind direction − track, taken as the smaller angle.
- Wind correction angle: sin WCA = (wind speed × sin WA) / TAS.
- Headwind or tailwind component = wind speed × cos WA; ground speed ≈ TAS × cos WCA − headwind (add a tailwind).
- Leg time = distance / ground speed; total elapsed time = sum of leg times plus taxi and any planned holding.
- True to magnetic: magnetic = true + westerly variation (minus easterly); magnetic to compass: apply deviation.
Altitude conversions
- True altitude differs from indicated altitude by pressure error and temperature error; use the forecast temperatures for level and fuel computation.
Worked example
Track 090°T, TAS 300 kt, wind 180/40, leg 450 NM. WA = 90°, so it is a pure crosswind. sin WCA = 40/300 = 0.133, giving WCA = 7.7°. The wind comes from the right of track, so heading = 090 + 7.7 = 098°T. Ground speed = 300 × cos 7.7° = 297 kt, with no headwind component. Leg time = 450 / 297 = 1.515 h = 90.9 min, about 1 h 31 min. With 5°W variation the magnetic heading is 098 + 5 = 103°M. A second leg of 300 NM at GS 350 kt takes 300/350 = 0.857 h = 51 min, so total air time = 91 + 51 = 142 min.
Traps
- Applying variation in the wrong direction (east is least: magnetic = true − easterly variation).
- Using the full wind speed as a headwind when the wind is not along track.
- Using heading instead of track to start the wind triangle.
- Mixing decimal hours with minutes between legs, producing a wrong total EET for field 16.
- Forgetting that elapsed time starts at take-off, while block time includes taxi.