Aircraft Magnetism — Page 200, Lesson 240

Let’s start with the two kinds of magnetism that act on an aircraft compass, because everything else in this chapter hangs off that distinction. First, hard iron magnetism. This is permanent magnetism — it’s locked into the aircraft structure, often from manufacturing, from being struck by lightning, or from sitting in a strong magnetic field. The key point is that the total force at the compass position produced by this permanent hard iron magnetism can be resolved into three components. And those three components are fixed for a given aircraft — they do not change with a change of heading. So no matter which way the aircraft points, that hard iron force stays the same relative to the airframe. Now, soft iron magnetism. This 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 field. Soft iron isn’t permanently magnetised; it becomes magnetised while it sits in an external field, and the amount of magnetism depends on the direction and strength of that field at the time. The earth’s field has a vertical component and a horizontal component. For our purposes, within the constraints of the syllabus, we only consider the vertical soft iron magnetism, abbreviated VSI. The vertical component of the earth’s field is called Z, and the horizontal component is called H. Here’s the important relationship: the component Z has an increasing effect with latitude, because the compass magnets try to follow the earth’s flux lines. Think of the earth’s field lines — they dip down toward the poles. Near the poles, the field is steep, so the vertical component is strong. Near the equator, the field is flat, so the vertical component is weak. That means VSI magnetism must also vary with latitude — it’s not constant. And here’s the clean boundary: Z is zero at the equator, where the horizontal component H is greatest. So at the equator, there is no VSI magnetism induced at all. That’s a clean, testable fact — at the equator, vertical soft iron magnetism is zero because the vertical component of the earth’s field is zero. Now, when we do a compass swing — that’s the procedure where we rotate the aircraft through headings and record the compass errors — we use a real system, the compass, to give us aircraft heading. And that read-out is affected by these magnetic forces we’ve been discussing. So when we examine the effective positioning of the imaginary magnets found during a compass swing, we have to remember that we’re looking through the lens of a real compass that is itself being disturbed by these forces. We can see from Figure 16.3 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. Consider the case where the effect of the blue pole is said to be in the nose, or forward, of the aircraft compass. Remember, the compass needle has a red end and a blue end — the red end points toward magnetic north, and the blue end points toward magnetic south. So a blue pole in the nose is a south-seeking pole sitting forward of the compass. Heading north — that is, compass heading 000° — the isolated blue pole is in the same horizontal direction as the earth’s blue pole. The earth’s blue pole is in the south, so the blue pole in the nose is pointing toward the south, same 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 heading north, no deviation, and the directive force is strengthened. As the aircraft turns right onto 045°, deviation begins to take place. By 090° — due east — this deviation has become maximum. It then starts to become less as we approach 180° — due south. Now 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. So on the remaining headings, from 180° 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°. So let me summarise the pattern. With a blue pole in the nose: on heading 000°, zero deviation. On 090°, maximum easterly deviation. On 180°, back toward zero. On 270°, maximum westerly deviation. And on 360°, back to zero. Now, if we plot these deviations against compass heading, we get a positive sine curve. That’s the signature of a blue pole forward of the compass. But what if the blue pole had been aft of the compass — behind it? Then we would get 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 the whole picture flips: forward blue pole gives positive sine, aft blue pole gives negative sine. Forward gives max easterly at 090°, aft gives max westerly at 090°. And the directive force peaks and troughs swap headings as well. That’s the core of hard iron versus soft iron, and the sine-curve behaviour of a single blue pole in the nose versus aft. Take a moment to let that settle — the key is that hard iron is fixed with heading, soft iron varies with latitude, and the blue pole position determines whether you get a positive or negative sine curve of deviation.