General Navigation
General Navigation is where careless units cost the most marks. The exam tests Earth geometry, chart projections, the triangle of velocities and the plotting skills behind an accurate heading.
Earth and charts
- The Earth is treated as an oblate spheroid; a great circle is the shortest track, a rhumb line cuts every meridian at a constant angle.
- Mercator: rhumb lines are straight, scale expands with latitude, great circles curve poleward. Lambert conformal conic: great circles plot as almost straight lines, standard parallels, used for most topographical charts. Polar stereographic serves high latitudes.
- Convergency = change of longitude × sin mean latitude; it determines the track change along a great circle.
Triangle of velocities
- Heading and TAS form the air vector; wind direction and speed the wind vector; the resultant is track and groundspeed.
- Drift = track minus heading; positive drift means the wind is from the left.
- The 1-in-60 rule: 1 nm of track error in 60 nm is 1°; so error in nm × 60 / distance = degrees.
Worked example
After 80 nm you are 4 nm right of track. Track error = 4 × 60 / 80 = 3°. You must also converge on a destination 40 nm ahead: closing angle = 4 × 60 / 40 = 6°. Total heading change to the left = 3 + 6 = 9°. If 9° is beyond a sensible heading change, revise the plan rather than the heading.
Magnetic work
- Variation converts true to magnetic; deviation converts magnetic to compass. TVMDC: True to Magnetic, add westerly variation ('variation west, magnetic best'; 'east is least').
Traps
Adding and subtracting variation the wrong way round; using groundspeed where the question wants TAS; treating a rhumb-line distance as a great-circle distance; and forgetting that Mercator scale changes with latitude, so a measured distance must use the scale at that latitude.