Great-circle planning on a north Atlantic crossing
A flight is planned from 60°N 030°W to 60°N 010°W. The crew is comparing a rhumb-line track with a great-circle track and considering the effect of a jet stream forecast of 250°/80 kt at flight level. The aircraft cruises at Mach 0.80, and the magnetic variation along the route changes from 20°W in the west to 5°W in the east. The crew must also consider contingency fuel for a possible diversion to a northern airfield.
1. Compare the rhumb-line and great-circle distances and tracks between the two points, and explain which the crew should plan.
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Both points lie on the same parallel, 60°N, so the rhumb line runs due east along the parallel: track 090° true throughout, with distance = 20° of longitude × cos60° × 60 nm ≈ 600 nm.
The great circle between two points on the same parallel in the northern hemisphere bulges poleward: the initial track is less than 090° (north of east) and the final track greater than 090°. Its distance is shorter than the rhumb line - for this geometry the saving is modest, of the order of a few tens of miles. The crew should plan the great circle because it is shorter, but must recognise that the changing true track along the route, combined with variation changing from 20°W to 5°W, means the magnetic track changes markedly and must be recomputed at each waypoint.
2. Explain how the jet stream and variation change affect the practical navigation and fuel planning.
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The jet stream at 250°/80 kt gives a strong tailwind component on an eastbound great-circle track, raising ground speed well above true airspeed and reducing trip fuel; the crew should use the forecast wind at each waypoint rather than a single average. On a westbound flight the same wind would be a severe headwind, and the great-circle route choice might be modified to avoid the core.
Variation changing from 20°W to 5°W over the route means that a constant magnetic heading would produce a curving true track; the crew must convert true tracks to magnetic at each leg using the local variation, or fly true tracks with a gyro or GNSS reference. For contingency fuel, the northern diversion airfield may be far from the great-circle track, so the crew should identify the equal-time point and ensure the contingency fuel covers the worst diversion, not just the planned destination.
3. What is the main risk in relying solely on the flight-management great-circle solution in this region?
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The main risk is that the FMS great-circle solution assumes the aircraft can follow the coded track and that the wind and temperature forecasts are accurate. In the high latitudes, magnetic variation changes rapidly, and near the magnetic pole the magnetic reference becomes unreliable; a compass-based or magnetic-track display may diverge from the true great-circle track. The crew must therefore monitor true track and GNSS position, cross-check the FMS against independent fixes, and be prepared to revert to true-track navigation if magnetic indications become suspect. The judgement tested is recognising the limits of the magnetic reference at high latitude and not treating the FMS solution as infallible.