Dead reckoning
Dead reckoning is navigation from a known position using heading, TAS, time and wind. The exam tests the wind triangle, drift and groundspeed, conversion between true, magnetic and compass, and revising an ETA when conditions differ from forecast.
Core rules
- Track is the path over the ground; heading is where the nose points. Wind causes drift, so heading = track ± drift correction.
- Resolve wind into crosswind and along-track components. Drift ≈ arcsin(crosswind/TAS); groundspeed ≈ TAS cos(drift) ± along-track wind. The 1-in-60 rule gives drift ≈ crosswind/TAS × 60.
- Convert true to magnetic using local variation: with easterly variation subtract it from true; with westerly variation add it. Then apply compass deviation from the card.
- Update DR at each checkpoint: compare actual time and position, revise GS and ETA, and recalculate fuel.
Worked example
Track 045°T, TAS 110 kt, wind 300°/25 kt, variation 18°E. Wind blows toward 120°. Angle between track and wind-to is 75°, so crosswind = 25 sin75° = 24 kt, along-track tailwind = 25 cos75° = 6 kt. Drift = arcsin(24/110) = 12.7°, and the wind is from the left, so heading = 045 − 13 = 032°T. Magnetic heading = 032 − 18 = 014°M. GS = sqrt(110² − 24²) + 6 = 114 kt. For 84 NM, time = 84/114 × 60 = 44 min. The 1-in-60 check gives about 13° drift, consistent.
Traps
Applying wind to heading rather than track; using the wrong altitude wind; reversing the variation sign; treating TAS as GS; and not revising ETA after a heading change.