General Navigation
General Navigation tests your ability to work with the Earth's geometry, chart projections, tracks and time - not simply to quote definitions but to compute the numbers a flight crew uses. The exam favours short calculations in which one wrong assumption (true versus magnetic, departure versus change of longitude) eliminates every answer option.
Earth and track geometry
- The Earth is treated as an oblate spheroid. A great circle is the shortest distance between two points; a rhumb line cuts every meridian at the same angle.
- Departure (NM) = change of longitude (minutes of arc) × cos latitude. Remember that 1° = 60'.
- Convergence = change of longitude × sin mean latitude. Conversion angle = half the convergence.
Chart projections
- Mercator: the rhumb line is straight, the great circle plots concave towards the nearer pole, and scale expands with secant latitude - measure distance only at the latitude in use.
- Lambert conformal conic: parallels are arcs, meridians are straight converging lines, a great circle is almost straight, and scale is nearly constant between the standard parallels.
- Polar stereographic: meridians radiate from the pole, which suits polar tracks.
Worked example
A and B both lie on 50°N, A at 000°E and B at 020°E. The rhumb-line track is 090°T. Convergence = 20 × sin 50° = 15.3°, so the conversion angle is 7.7°. Eastbound in the northern hemisphere the great circle starts more poleward, that is, on a smaller bearing: initial great-circle track = 090 − 7.7 = 082.3°T. Conversely, from the great-circle track, the rhumb-line track is found by adding the conversion angle.
Traps
- Applying the conversion angle in the wrong direction: sketch both tracks before choosing the sign.
- Mixing magnetic and true: with variation west, magnetic bearings are larger than true.
- Taking a scale bar from the wrong latitude on a Mercator chart.