Aircraft Magnetism — Page 508, Lesson 504

I want to walk you through the heart of aircraft magnetism now — the part that actually explains why your compass misbehaves in the air and why we do a compass swing at all. We start with hard iron magnetism. Think of hard iron as metal that has become permanently magnetised — it keeps its magnetism regardless of what's around it. The total force that this permanent hard iron magnetism produces at the compass position can be resolved into three components. The key point is that these three components are fixed for a given aircraft — they will not change with a change of heading. So no matter which way you point the nose, that hard iron force stays the same relative to the aircraft structure. Now, soft iron magnetism is different. Soft iron magnetism is induced in parts of the aircraft structure by surrounding fields — and the most important surrounding field is the Earth's. The Earth's field has both a vertical and a horizontal component. For our purposes, within the constraints of the syllabus, we only consider the vertical soft iron magnetism — abbreviated VSI. Here we introduce two symbols: Z is the vertical component of the Earth's field, and H is the horizontal component. Here's the critical relationship. The component Z has an increasing effect with latitude, because the compass magnets try to follow the Earth's flux lines. So as you fly toward the poles, the vertical pull grows stronger, and therefore VSI magnetism must also vary with latitude. But at the Equator, Z is zero — the vertical component disappears — and that's exactly where the horizontal component H is greatest. So at the Equator, no VSI magnetism is induced at all. Now, when we do a compass swing, we're effectively positioning imaginary magnets to model the real magnetic effects on the compass. But we must remember something important: we use a real system — the compass — to give us aircraft heading, and that readout is itself affected by these magnetic forces we've gone to trouble to discover. So the heading we read is already distorted by the very forces we're trying to measure. From Figure 30.3, we can see that the positioning of these imaginary magnets can vary — even to the extent of having two imaginary magnets affecting our compass. But the effect will be easily resolved by the compass swing, which can cater for any positioning as long as we follow the basic rules. Let me give you a concrete example. Suppose the effect of the blue pole is said to be in the nose — forward of the aircraft compass. Let's walk through what happens as the aircraft turns. Heading North — 000° — 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 — the alignment of the Earth's field — is being augmented by the blue pole; effectively they are pulling together. So on North, no deviation, and the directive force is at its maximum. As the aircraft turns right onto 045°, deviation begins to take place. By 090° — due East — this deviation has become maximum. Then it starts to become less as we approach 180°. Now here's the key to understanding the direction of that deviation. Remember that 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. So on the easterly headings, the blue pole in the nose pulls the red end of the compass needle to the East of Magnetic North — that's an easterly deviation, maximum at 090°. On the remaining headings — from 180° through to 360° — the effects of the blue pole in the nose are as expected: the red end of the compass needle is being attracted to the West of Magnetic North, giving the maximum westerly deviation on 270° — due West. Now, if we plot the deviations caused by the blue pole in the nose against compass heading, we obtain a positive sine curve. That's the signature of a blue pole forward of the compass. But here's the contrast. Had the blue pole been aft of the compass — behind it — we would have obtained a negative sine curve. That would mean that 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 directive force would occur on 180°, and the minimum on 360°. So let me pull the whole picture together. Hard iron magnetism gives you three fixed components that don't change with heading. Soft iron — specifically vertical soft iron — varies with latitude because of the Earth's vertical field Z, and vanishes at the Equator where H is greatest. And when you model the compass swing with imaginary magnets, a blue pole forward gives you a positive sine curve of deviation — maximum easterly at 090°, maximum westerly at 270°, no deviation on North and South — while a blue pole aft flips that to a negative sine curve, with the maximum westerly deviation at 090° instead. That's the foundation of why your compass reads what it reads — and why the compass swing is built to resolve all of it.