Aircraft Magnetism — Page 508, Lesson 504

We're starting a new topic now: Aircraft Magnetism. This is the heart of why a compass behaves the way it does in an aircraft, and it's all about two very different kinds of magnetism. Let's get into it. First, we have Hard Iron Magnetism. Think of this as permanent magnetism. The total force at the compass position produced by this permanent hard iron magnetism can be resolved into three components. These three components are fixed for a given aircraft, and they will not change with a change of heading. So, no matter which way you point the nose, these three forces stay constant in their relationship to the aircraft. Now, the second kind is Soft Iron Magnetism. This is induced magnetism. It's induced in parts of the aircraft structure by surrounding fields, and the most important of those surrounding fields is the Earth itself. 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, which we call VSI. We use the letter Z for the vertical component of the Earth's field, and H for the horizontal component. Here's the key relationship: the component Z has an increasing effect with latitude. Why? Because the compass magnets try to follow the Earth's flux lines. As you move toward the poles, the flux lines dip more vertically, so the vertical component grows stronger. Therefore, VSI magnetism must also vary with latitude. But here's the beautiful part: Z is zero at the Equator, where the horizontal component H is greatest. So, at the Equator, no VSI magnetism is induced at all. Now, when we examine the effective positioning of the imaginary magnets found when completing a compass swing, we must remember something crucial. We use a real system—the compass—to give us aircraft heading. And that readout is affected by these magnetic forces we've gone to the trouble to discover. The positioning 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's look at a specific example. Imagine the effect of the blue pole is said to be in the nose, or 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—the alignment of the Earth's field—is being augmented by the blue pole. Effectively, they are pulling together. As the aircraft turns right onto 045°, deviation begins to take place. By 090°, this deviation 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, 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°. Now, here's the mathematical beauty. If the deviations caused by the blue pole in the nose are plotted against compass heading, a positive sine curve is obtained. Had the blue pole been aft of the compass, a negative sine curve would have been obtained. That would mean that on a heading of 090°, the deviation would reach a maximum westerly value instead of a maximum easterly value. The changes in directive force would also be revised—the maximum occurring on 180° and the minimum on 360°. So, to tie it all together: hard iron is permanent and fixed; soft iron is induced and varies with latitude. And the compass swing resolves all of it, no matter how the imaginary magnets are positioned. That's the foundation of aircraft magnetism.