Let's pick up right where the compass swing leaves off — Page 508, Lesson 508

Let's pick up right where the compass swing leaves off. Once the swing is complete, we don't just walk away. We have to record what we found, and we have to know the legal limits that the corrected compass must meet. First, the results. The deviations we measured during the swing can be shown in one of two ways: as a graphical table, or as a curve constructed from the information we obtained. Either set of calculations allows us to place a Compass Deviation Card near the compass in the aircraft. That card is the pilot's reference — it tells us what residual deviation remains on each heading, so we can apply it when steering. 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: a Direct Reading Magnetic Compass must be within ±10 degrees. That's the tolerance. After a swing, if the compass is off by more than 10 degrees on any heading, it doesn't meet the standard. But here's the deeper problem. A compass that's perfect at one location won't stay perfect. Deviation changes with magnetic latitude — how far north or south of the magnetic equator you are. And to understand that, we need a formula: tan dip = Z / H Let me define those symbols. 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 field. And dip is the angle of inclination — the angle the total field makes with the horizontal. At the magnetic equator, dip is zero, so Z is zero. At the poles, dip is 90 degrees, so H is zero. Now, the changes in deviation due to change of magnetic latitude have to be considered in two parts: first with regard to hard iron, and secondly to vertical soft iron. Let's take hard iron first. The hard iron deviating force remains constant regardless of change of latitude. Hard iron is permanently magnetized — it doesn't change strength. But H, the directive force, varies with latitude. H is maximum at the Equator and zero at the poles. So here's the key relationship: 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, a fixed piece of hard iron has relatively more influence over the compass needle. So 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. If hard iron pulls the compass to the east in the northern hemisphere, it pulls it to the east in the southern hemisphere too. 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. This is different. The directive force at the compass position is H, whilst the magnetizing agent of any VSI component — that's Vertical Soft Iron — is Z. Soft iron isn't permanently magnetized; it's magnetized by the field it sits in. So the maximum deviation due to VSI magnetism will vary inversely as H — same as hard iron, weaker directive force means more relative influence. But it also varies directly as Z — the stronger the vertical field, the more the soft iron gets magnetized. Z has no effect on hard iron deviation, only on soft iron. So the maximum deviation produced by VSI magnetism increases towards the magnetic poles. And here's the elegant part. We can combine those two relationships. Since max deviation = Z / H, and tan dip = Z / H, the maximum deviation varies directly with tan dip. That's the whole story in one line — the deviation from vertical soft iron grows as the tangent of the dip angle grows, which happens as you move toward the poles. Now, when do we actually have to swing the compass? There's a list of occasions, and I want you to know each one. We swing when compass components are installed or replaced. We swing whenever the accuracy of the compass is in doubt. We swing after a maintenance inspection if required by the schedule. We swing after a significant aircraft modification, repair, or replacement involving magnetic material. We swing when carrying unusual ferromagnetic payloads — that exclamation mark is there for a reason, because a load of steel cargo can wreck your compass. We swing when the compass has been subjected to significant shock. We swing if the aircraft has been struck by lightning. We swing after significant modification to aircraft radio or electrical systems. We swing after the aircraft has been given a new theatre of operations if the move involves a large change of magnetic latitude. And finally, we swing if the aircraft has been in long term storage standing on one heading — because sitting in one orientation for months can let the Earth's field induce a new magnetic state in the structure. So the full picture: the swing gives us data, we record it on the deviation card, we check it against the ±10 degree limit, and we understand that the correction is only valid for the latitude where we did the swing. Move far enough north or south, and the hard iron and vertical soft iron effects change — hard iron inversely with H, vertical soft iron with tan dip — and the compass needs swinging again.