Now, the accuracy limits — Page 508, Lesson 508

I want to walk you through what happens after a compass swing is completed, and the rules that govern whether the result is acceptable. First, the results of a compass swing can be shown in two ways: a graphical table, or a curve constructed from the information you obtained during the swing. Either set of calculations allows you to place a Compass Deviation Card near the compass in the aircraft. That card is the little placard that tells you the correction to apply for each heading. Now, the accuracy limits. Under CS Ops-1, which is the European regulation, the aircraft's compasses must, after correction, be within the following limit: the Direct Reading Magnetic Compass must be within plus or minus 10 degrees. That's the tolerance you have to meet. Next, we need to consider how deviation changes with a change of magnetic latitude. The key formula here is tan dip equals Z divided by H. Let me define those terms. H is the horizontal component of the Earth's magnetic field — the directive force that pulls the compass needle toward magnetic north. Z is the vertical component of the Earth's magnetic field. Dip is the angle the total field makes with the horizontal — at the magnetic poles the field points straight down, so dip is 90 degrees, and at the magnetic equator dip is zero. The changes in deviation due to change of magnetic latitude have to be considered in two parts: first with regard to hard iron, and second to vertical soft iron. Let's start with hard iron. The hard iron deviating force remains constant regardless of change of latitude. That's because hard iron is permanently magnetized — it doesn't change with the external field. However, H varies with latitude. H is maximum at the Equator and zero at the poles. So the smaller the directive force H, the greater the maximum deviation caused by the hard iron deviating force. Think of it this way: if the Earth's horizontal field is weak, the compass needle has less restoring force to fight against the hard iron disturbance, so the deviation is larger. Therefore, hard iron deviating effect increases towards the poles and is minimum at the magnetic equator. And importantly, the sign of the deviation will be the same in both hemispheres. To summarize that: maximum deviation due to hard iron magnetism is inversely proportional to the value of H, which changes with change of magnetic latitude. Now vertical soft iron. The directive force at the compass position is H, whilst the magnetizing agent of any VSI component is Z. VSI stands for vertical soft iron — soft iron that gets magnetized by the vertical field. The maximum deviation due to VSI magnetism will vary inversely as H, just like hard iron. But additionally, the VSI maximum deviation will vary directly as Z. Z has no effect on hard iron deviation — that's a contrast to remember. The maximum deviation produced by VSI magnetism increases towards the magnetic poles. And here's the elegant part: max deviation equals Z over H, which equals tan dip. So maximum deviation varies directly with tan dip. That's the relationship that ties it all together — as you move to higher latitudes, dip increases, tan dip increases, and the VSI deviation grows. Finally, let's look at the occasions for swinging the compass. These are the situations that call for a new compass swing. When compass components are installed or replaced. Whenever the accuracy of the compass is in doubt. After a maintenance inspection if required by the schedule. After a significant aircraft modification, repair, or replacement involving magnetic material. When carrying unusual ferromagnetic payloads — ferromagnetic means material that's strongly attracted to magnets, like iron or steel. When the compass has been subjected to significant shock. If the aircraft has been struck by lightning. After significant modification to aircraft radio or electrical systems. After the aircraft has been given a new theatre of operations if the move involves a large change of magnetic latitude. And finally, if the aircraft has been in long-term storage standing on one heading — because sitting in one direction for a long time can magnetize the structure in that direction. So the big picture: after the swing, you document the results on a deviation card, you check you're within the plus or minus 10 degree limit, and you understand that if the aircraft moves to a different latitude, the hard iron and vertical soft iron effects will change in predictable ways — hard iron inversely with H, and VSI directly with tan dip.