Aircraft Magnetism — Page 508, Lesson 506

Let’s pick this up right where the table left off, because that table is the heart of what we call Coefficient B. I want you to look at the numbers we just walked through. On heading 000, deviation is zero and directive force is maximum. On 045, deviation is east, a little bit, and directive force is more than Earth’s. On 090, deviation is east and maximum, and directive force is approximately Earth’s. On 135, deviation is still east, some, but now directive force is less than Earth’s. On 180, deviation is zero and directive force is minimum. On 225, deviation is west, some, and directive force is less than Earth’s. On 270, deviation is west and maximum, and directive force is approximately Earth’s. On 315, deviation is west, some, and directive force is more than Earth’s. And back at 000, zero deviation and maximum directive force. Now, what we have examined here is known, for compass swinging purposes, as Coefficient B. I want you to think of Coefficient B as that component which is resolved along the body of the aircraft — that is, along the fore-and-aft axis, the nose-to-tail line. The forces resolved follow a simple sine curve. In our case here, that sine curve would be ‘positive’, although negative curves occur just as frequently. So the deviation pattern you see in that table — zero at north and south, maximum east at 090, maximum west at 270 — that is the signature of a sine-shaped variation along the aircraft’s longitudinal axis. Equally, we should be able to see that if a further magnetic source is resolved to the right wing, we would achieve a positive cosine curve along the same lines, and this is more usually described as Coefficient C. So B is the sine component along the body, C is the cosine component along the wings. The combination of Coefficients A, B, and C are resolved during the compass swing. Coefficient A is a mechanical function yet to be discussed — I’ll get to it in a moment. These three coefficients, A plus B plus C, are resolved during the swing, and to some extent they can be removed by adjustment. But other factors are at work here, and they will probably leave us with some errors at the end. That’s why we never get a perfect compass — we get residual deviation. Now let’s move to the correction of these coefficients. The principle for correcting coefficients is the same for any system, and it can be summed up as follows. Coefficient A is a mechanical problem of a displaced lubber line. The lubber line is the reference mark on the compass that you align with the aircraft’s fore-and-aft axis to read your heading. If it’s displaced, you correct it by loosening the bolts holding the compass body — or, in the case of the RIMC, the detector unit — and carefully turning it until the correct heading is in place. So A is purely mechanical alignment, not magnetic. Coefficient B is correction required because of magnetic deviating forces acting upon the DRMC or the detector unit, giving errors known as deviation. The procedure: firstly, calculate the error to be removed — or, more correctly, the heading you wish to make the compass read — and this will be done on an Easterly or Westerly heading. So B is corrected while you’re on an east or west heading. Coefficient C is correction required because of magnetic deviating forces acting upon the DRMC or the detector unit, giving errors known as deviation. Same wording, but the key difference: firstly, calculate the error to be removed, or the heading you wish to make the compass read, and this will be done on a Northerly or Southerly heading. So C is corrected on a north or south heading. We can see that the correction for B and C are very similar, but we must remember to apply the sign of the correction properly to ensure an accurate correction to our compass system. Get the sign wrong and you double the error instead of removing it. When the compass swing is completed, we of course have to check our work. This ‘check swing’ is carried out using eight or perhaps twelve points of the compass, to allow us to derive a compass card that will be placed in the aircraft. This compass card indicates to us the residual deviations that we have been unable to resolve within the essentially horizontal procedure. Alternatively, the residual deviations affecting the compass after the completion of a compass swing may be shown by the use of a graphical table or a curve constructed from the data. So the full picture: A is mechanical alignment of the lubber line, B is the sine component along the aircraft body corrected on east/west headings, C is the cosine component along the wings corrected on north/south headings, and after the swing we do a check swing on eight or twelve points to build the compass card showing residual deviation.