Aircraft Magnetism — Page 508, Lesson 506

Right, let's pick this up with the table we've just been looking at. I want you to see what that table is actually telling us, because it's the heart of understanding how a magnetic source inside the aircraft affects the compass. Look at the heading column, in degrees Celsius — that's the compass heading, 000, 045, 090, all the way around to 000 again. Next to it we have the deviation, and then the directive force. Now, the key thing to grasp here is the pattern. At 000, heading north, the deviation is zero and the directive force is at its maximum. At 090, heading east, the deviation is at its maximum — East, plus some — and the directive force is approximately equal to Earth's. At 180, heading south, deviation is zero again, but now the directive force is at its minimum. And at 270, heading west, deviation is maximum again, but this time it's West, minus some, and the directive force is approximately Earth's. So you see the rhythm: deviation peaks at east and west, and is zero at north and south. The directive force peaks at north, and bottoms out at south. That's the signature of a magnetic source aligned along the fore-and-aft axis of the aircraft — the body of the aircraft. Now, what we've examined here is known, for compass swinging purposes, as Coefficient B. Think of Coefficient B as that component which is resolved along the body of the aircraft — the longitudinal axis, nose to tail. 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, depending on the polarity of the source. Now, equally, we should be able to see that if a further magnetic source is resolved to the right wing — that is, along the lateral axis, wingtip to wingtip — we would achieve a positive cosine curve along the same lines. That one is more usually described as Coefficient C. So B is the sine component along the body, C is the cosine component along the wings. Now, the combination of Coefficients A, B, and C are resolved during the compass swing. Coefficient A is a mechanical function yet to be discussed — we'll get to it in a moment. But the point is, 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 — residual errors we can't fully eliminate. Let me show you what I mean with the figures. — that's Figure 30.1, showing the push of the blue pole. And and show the headings 000°C and 180°C, and how the deviations caused by the blue pole behave. and are Figures 30.5 and 30.6, which illustrate the sine and cosine curves we've been talking about. Now let's move on 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 — this 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. If it's displaced, you correct it by loosening the bolts holding the compass body — or, in the case of the RIMC, the Remote Indicating Magnetic Compass, the detector unit — and carefully turning it until the correct heading is in place. Coefficient B — this correction is required because of magnetic deviating forces acting upon the DRMC — the Direct Reading Magnetic Compass — 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. That makes sense, doesn't it? Because B's deviation peaks at east and west. Coefficient C — the correction is required for the same reason: magnetic deviating forces acting upon the DRMC or the detector unit, giving errors known as deviation. Again, firstly calculate the error to be removed, or the heading you wish to make the compass read. But this time, it's done on a Northerly or Southerly heading — because C's deviation peaks at north and south. So you 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'll double the error instead of removing it. Now, 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 to tie it all together: B is the sine component along the aircraft's body, C is the cosine component along the wings, A is the mechanical lubber line error. We correct B on east/west headings, C on north/south headings, and A by physically turning the compass body or detector unit. Then we do a check swing on eight or twelve points to produce the compass card showing residual deviations. That's the complete picture of coefficient correction.