Aircraft Magnetism — Page 200, Lesson 240

Let’s start with the two big families of aircraft magnetism, because everything else in this chapter hangs off them. First, hard iron magnetism. This is the magnetism that is permanent — it’s locked into the aircraft structure, like a bar magnet that never loses its strength. The total force this permanent magnetism produces at the compass position can be resolved into three components. Those three components are fixed for a given aircraft, and crucially, they do not change with a change of heading. So no matter which way the nose points, that hard iron force stays the same relative to the aircraft. Second, soft iron magnetism. This is induced — it’s not permanent. It’s created in parts of the aircraft structure by surrounding magnetic fields, and the most important surrounding field is the earth’s. Now, the earth’s field has both a vertical and a horizontal component. For our syllabus, we only consider the vertical soft iron magnetism, which we call VSI. Let me define the symbols: Z is the vertical component of the earth’s field, and H is the horizontal component. Here’s the key relationship. The vertical component Z has an increasing effect with latitude, because the compass magnets try to follow the earth’s flux lines — the field lines dip down toward the poles. So VSI magnetism must also vary with latitude. But at the equator, Z is zero — the field is purely horizontal there — and that’s where H, the horizontal component, is greatest. So at the equator, no VSI magnetism is induced at all. Now, when we do a compass swing — that’s the procedure where we rotate the aircraft and record compass errors — we’re effectively positioning imaginary magnets to model the real magnetic forces. But we must remember something important: we use a real system, the compass, to give us aircraft heading, and that read-out is itself affected by these magnetic forces we’re trying to discover. So the compass swing has to resolve the effect of these imaginary magnets, and it can cater for any positioning — even two imaginary magnets affecting the compass — as long as we follow the basic rules. Let me walk you through a concrete example. Suppose the effect of the blue pole — that’s the north-seeking pole of the imaginary magnet — is said to be in the nose, forward of the aircraft compass. Heading north, the isolated blue pole is in the same horizontal direction as the earth’s blue pole, so the needle is not deviated. The directive force — that’s the alignment force of the earth’s field — is being augmented by the blue pole. Effectively, they’re pulling together. Now turn right onto 045°. Deviation begins to take place. By 090°, it has become maximum. Then it starts to become less as we approach 180°. Remember: the blue pole represents a magnetic force which, on this heading, acts along the same line but in opposition to the stronger earth’s field. On the remaining headings, 180° to 360°, the effects of the blue pole in the nose are as expected — the red end of the compass needle is attracted to the west of magnetic north, giving the maximum westerly deviation on 270°. Here’s the beautiful part. If you plot the deviations caused by the blue pole in the nose against compass heading, you get a positive sine curve. Had the blue pole been aft of the compass, you’d get a negative sine curve. That would mean on a heading of 090°, the deviation would reach a maximum westerly value instead of a maximum easterly value. And the changes in directive force would also be revised — the maximum occurring on 180° and the minimum on 360°. So the takeaway: hard iron is fixed, soft iron varies with latitude, and the compass swing resolves the net effect into imaginary magnets whose deviation pattern follows a sine curve — positive if the pole is forward, negative if it’s aft.