Aircraft Magnetism — Page 200, Lesson 242

Right, let's pick this up with the correction of coefficients. We've already seen how magnetic sources in the aircraft create those cosine curves of deviation as you swing the compass. Now we're going to fix them. The principle for correcting coefficients is the same for any system, whether it's a Direct Reading Magnetic Compass or a Remote Indicating Compass. Let's take them one at a time. First, Coefficient A. This is a mechanical problem, not a magnetic one. It's caused by a displaced lubber line. The lubber line is the fixed reference mark on the compass that you align the aircraft's heading with. If it's physically out of position, the compass will read incorrectly on every heading. The correction is purely mechanical: you loosen the bolts holding the compass body, or in the case of the RIMC, the detector unit, and carefully turn it until the correct heading is in place. Then you tighten it back down. Now, Coefficient B. This is a magnetic problem. It's the correction required because of magnetic deviating forces acting upon the DRMC or the detector unit, giving errors known as deviation. The procedure is to first calculate the error to be removed, or more correctly, the heading you wish to make the compass read. And this is done on an easterly or westerly heading. So you fly or align on east or west, and you adjust the B magnets to remove the deviation there. Coefficient C is very similar. It's also correction required because of magnetic deviating forces acting upon the DRMC or the detector unit, giving errors known as deviation. Again, you first calculate the error to be removed, or the heading you wish the compass to read. But this time, it's done on a northerly or southerly heading. So you can see, B and C are very similar in principle, but we must remember to apply the sign of the correction properly to ensure an accurate correction to our compass system. The sign matters because the deviation on east may be opposite in sense to the deviation on west, and you have to compensate in the correct direction. Now, once the compass swing is completed, we have to check our work. This 'check swing' is carried out using eight or perhaps twelve points of the compass. That allows us to derive a compass card that will be placed in the aircraft. This compass card indicates to us the residual deviations — the 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 information obtained. Either set of calculations will allow for the placing of a Compass Deviation Card near to the compass in the aircraft. Finally, let's look at the accuracy limits. In accordance with CS OPS-1, the European Regulations, the aircraft's compasses must, after correction, be within the following limits. For a Direct Reading Magnetic Compass, the limit is plus or minus 10 degrees. For a Remote Indicating Compass, the limit is much tighter, plus or minus 1 degree. That's the standard you have to meet after correction. So to summarise the whole picture: Coefficient A is a mechanical alignment fix on the lubber line, B is corrected on east or west, C is corrected on north or south, and after the swing you do a check swing on eight or twelve points to build the deviation card, and you must be within those CS OPS-1 limits.