Two factors peculiar to polar areas that make steering more difficult than usual are magnetic compass unreliability and geographic meridians converging at acute angles. The combined effect of these two factors makes steering by conventional methods difficult if not impossible. Each factor is examined below.
Unreliability of Magnetic Compass
Maintaining an accurate heading in high latitudes is difficult when a magnetic compass is used as the heading indicator. Built to align itself with the horizontal component of the earth’s magnetic field, the compass instead must react to the strong vertical component that predominates near the magnetic poles. Here, the horizontal component is too weak to provide a reliable indication of direction. As a result, compass performance becomes sluggish and inaccurate. The situation is further aggravated by the frequent magnetic storms in the polar regions that shift the magnetic lines of force.
But even if these conditions did not exist, the mere proximity to the magnetic pole would sharply reduce compass usefulness. While the aircraft may fly a straight course, the compass indicator would swing rapidly, faithfully pointing at a magnetic pole passing off to the left or right. To cope with the unreliable magnetic compass, we use gyro information for our heading inputs.
Problem of Converging Meridians
The nature of the conventional geographic coordinate system is such that all meridians converge to the pole. Each meridian represents a degree of longitude; each is aligned with true north (TN) and true south. On polar charts, the navigator encounters 1 degree of change in true course for each meridian crossed; thus, the more closely the aircraft approaches a pole, the more rapidly it crosses meridians. Even in straight-and-level flight along a great circle course, true course can change several degrees over a short period of time. You are placed in the peculiar position of constantly altering the aircraft’s magnetic heading in order to maintain a straight course. For precision navigation, such a procedure is clearly out of the question. Notice in Figure 14-1 that the course changes 60° between A and B and much nearer the pole, between C and D, it changes 120°.
The three polar projections most commonly used in polar areas for grid navigation are the transverse Mercator, the polar stereographic, and the polar gnomonic. The transverse Mercator and polar stereographic projections are used inflight, the polar gnomonic is used only for planning. The Lambert conformal projection is the one most commonly used for grid flight in subpolar areas. The division between polar and subpolar projections varies among the aeronautical chart series. For example, the division is at 70° of latitude for the JN series, and at 80° of latitude for the Operational Navigation Chart (ONC) series charts.