Aircraft Magnetism — Page 200, Lesson 242

Let’s pick this up right where the mathematics left off. We’d just seen that a magnetic source aligned with the aircraft’s fore-and-aft axis produces a sine curve of deviation as you swing through the headings. Now I want you to see the mirror image: if that same magnetic source is resolved to act along the right wing, the deviation it produces traces a positive cosine curve instead. Sine for the longitudinal axis, cosine for the lateral axis — that distinction is the whole skeleton of coefficient correction, so hold onto it. Now we move to the practical business of correcting the coefficients. The principle is identical no matter which compass system you’re dealing with, and it breaks down into three named corrections: A, B, and C. Let’s start with Coefficient A. This one is purely a mechanical problem — a displaced lubber line. The lubber line is the fixed reference mark on the compass that you align with the aircraft’s fore-and-aft axis to read your heading. If it’s not aligned, the compass reads a false heading even when the magnet itself is perfect. The fix is mechanical: you loosen the bolts holding the compass body — or, in the case of a Remote Indicating Magnetic Compass, the RIMC, you loosen the bolts on the detector unit — and carefully turn the whole unit until the correct heading is in place. No magnetism involved, just geometry. Now Coefficient B. This correction exists because of magnetic deviating forces acting upon the DRMC — that’s the Direct Reading Magnetic Compass — or upon the detector unit, producing errors we call deviation. The procedure: first calculate the error to be removed, or more precisely, calculate the heading you wish to make the compass read. And here’s the key operational detail — this correction is performed on an easterly or westerly heading. That’s the heading where the B-coefficient deviation is most apparent, so that’s where you swing to correct it. Coefficient C is described in almost identical words — correction required because of magnetic deviating forces acting upon the DRMC or the detector unit, giving errors known as deviation. Same first step: calculate the error to be removed, or the heading you want the compass to read. But the critical difference is the heading you do it on: northerly or southerly. So B is corrected on east–west, C is corrected on north–south. That’s the contrast you must keep straight. You’ll notice B and C read very similarly, and that’s deliberate — but I want to stress one thing: you must apply the sign of the correction properly. Get the sign wrong and you’ll double the error instead of removing it. That sign discipline is what separates a clean swing from a botched one. Once the compass swing is complete, you don’t just walk away. You have to verify your work, and that verification is called the check swing. It’s carried out using eight or perhaps twelve points of the compass — so every 45 degrees, or every 30 degrees — and from that data you derive a compass card that gets placed in the aircraft. That card shows you the residual deviations — the small errors you were unable to resolve within the essentially horizontal procedure. Alternatively, those residual deviations can be presented as a graphical table or a curve constructed from the swing information. Either way — card, table, or curve — the result allows you to place a Compass Deviation Card near the compass in the aircraft, so the pilot can apply the remaining corrections in flight. Finally, the accuracy limits. Under CS OPS-1, the European regulations, your compasses must, after correction, sit within these bounds: the Direct Reading Magnetic Compass must be within plus or minus 10 degrees, and the Remote Indicating Compass must be within plus or minus 1 degree. That’s a ten-to-one difference in tolerance — the remote system is held to a far tighter standard because it feeds the autopilot and flight instruments, so it has to be that much more precise. So the whole arc is: sine and cosine curves tell you which axis the deviating source acts along; coefficient A fixes a mechanical misalignment; B and C remove the magnetic deviation on the correct headings with proper sign; the check swing at eight or twelve points reveals what’s left; and the deviation card carries that residual information into the cockpit — all held to the CS OPS-1 accuracy limits.