Gridded Charts — Page 464, Lesson 464

Let’s pick this up right where the grid concept leaves off, because now we’re going to put it into practice — actually steering the aircraft by a grid heading, and then dealing with the magnetic compass and the gyro. First, I want you to picture a rhumb line track — that’s a straight line on a chart that crosses all meridians at the same angle. In Figure 27.14 we have such a track, and after correcting for drift, we need to maintain a True heading of, say, 080°(T). Now, the chart shows isogonals — those are lines joining points of equal magnetic variation. To the west of the 3.5°W isogonal, the variation is, to the nearest degree, 3°W. To the east of it, the variation is 4°W. Similarly, at the 4.5°W isogonal, tracking eastwards, the variation changes from 4°W to 5°W. So here’s the practical flying problem. The pilot starts by flying a Magnetic heading of 083°(M) in order to maintain a True heading of 080°(T). Why 083? Because the variation is 3°W, and to convert a True heading to a Magnetic heading, you add westerly variation — so 080 plus 3 gives 083. As he crosses the 3.5°W isogonal, the variation steps up to 4°W, so he alters Magnetic heading to 084°(M). Then, as he crosses the 4.5°W isogonal, variation becomes 5°W, and he alters Magnetic heading to 085°(M) — all the while maintaining that same True heading of 080°(T). Now, here’s the key transfer of the whole idea. If a chart is constructed with isogrivs, then the same principles can be used to maintain a Grid heading when using a magnetic compass. An isogriv is the grid equivalent of an isogonal — it’s a line joining points of equal grid variation, that is, the difference between Grid North and Magnetic North. So instead of correcting for True variation, you correct for grid variation, and you fly a steady Grid heading. Let me show you what a printed grid chart actually looks like. Figure 27.15 shows one. The blue graticule is latitude and longitude — that’s the familiar lat/long graticule. The grey graticule is the grid — the grid graticule. The blue dotted lines are isogonals, and the grey ones are isogrivs. So you have two superimposed systems: the geographic one in blue, and the grid one in grey. Now, the important observation from that chart: the rate at which the isogrivs are changing is much the same as the rate at which the isogonals are changing. That means it is no more difficult, in many parts of the world, to fly a Grid heading using a magnetic compass than it is to fly a True heading when using a magnetic compass. In other words, the correction workload is identical — you’re just reading a different set of lines. But then we come to the reason we might be using Grid in the first place — steering by gyro. One of the reasons we may be using Grid is because the aircraft is at high latitudes. In this case, a magnetic compass may be unusable in some parts of the world because the route — well, the excerpt cuts off there, but the point is that at high latitudes, the magnetic compass becomes unreliable, and that’s exactly when the grid system, steered by a directional gyro, becomes the tool we rely on. So to tie it together: the grid gives us a fixed reference frame at high latitudes, and whether we steer by magnetic compass or by gyro, we use the isogrivs to make the same kind of heading corrections we just did with isogonals. That’s the whole mechanism — same logic, different graticule.