Aircraft Magnetism — Page 200, Lesson 239

Let’s start with the core idea of this whole chapter: deviation. I want you to picture the compass needle sitting in the aircraft. If the aircraft had no magnetism of its own, the needle would point straight along the magnetic meridian — that’s the true north-south line of the Earth’s magnetic field. But an aircraft is full of its own internal magnetism, and that deflects the needle. So deviation is defined as the angular difference measured between the direction the compass needle actually takes up and the magnetic meridian. In other words, it’s the error in your compass caused by the aircraft itself, not by the Earth. Now, deviation gets a name — easterly or westerly — depending on which side of the magnetic meridian the north-seeking end of the needle lies. If the north end of the needle sits to the east of the magnetic meridian, that’s easterly deviation. If it sits to the west, that’s westerly deviation. So the name tells you which way the needle is being pulled off the true magnetic line. Let me give you the relationship that ties this together, because it’s the heart of the matter. We have compass heading, deviation, and magnetic heading. The rule is: compass heading plus deviation equals magnetic heading. Look at the numbers in the figure. First case: compass heading 095, deviation minus 5, gives magnetic heading 090. That’s deviation west — the needle is pulled west of the meridian, so you subtract. Second case: compass heading 090, deviation plus 5, gives magnetic heading 095. That’s deviation east — the needle is pulled east, so you add. So remember the sign convention: westerly deviation is subtracted, easterly deviation is added, when you go from compass heading to magnetic heading. Now, how do we actually find out what the deviation is on any given heading? That’s the compass swing. The basic method is to compare the aircraft’s heading compass reading with the magnetic heading as defined by a high-quality “land or datum” compass. So you have a reference compass on the ground — a datum compass — that gives you the true magnetic heading, and you compare it to what the aircraft’s compass is showing. And this comparison has to be carried out in an area selected specifically for that purpose — a dedicated area, free from magnetic interference, so the datum reading is trustworthy. Let me walk you through the aims of a compass swing, because there are three distinct goals. First, to observe or determine the deviations — the differences — between magnetic north, as observed on the landing compass, and compass north, as observed in the aircraft, on a series of headings. So you swing the aircraft through a set of headings and record the error at each one. Second, to correct or remove as much deviation as possible — you adjust the compass to cancel out the error you found. And third, to record the residual deviation — that’s the deviation that’s left over after the compass has been adjusted. You can’t remove it all, so you document what remains so the pilot can apply the correction later. Finally, there’s an important point about where that deviation comes from. The magnetic deviation observed during a compass swing can be said to be derived from two sources: hard iron magnetism and soft iron magnetism. Hard iron is the permanent magnetism in the aircraft’s structure; soft iron is the induced magnetism that changes with the aircraft’s orientation in the Earth’s field. And this total field — the combination of hard and soft iron — can, for our purposes, be resolved into two further combined components, which are called coefficients B and C. So when you do a compass swing, you’re effectively dealing with these two coefficients — they represent the combined effect of all that aircraft magnetism, and they’re what you correct for. So to tie it all together: deviation is the aircraft-induced compass error, named east or west by which side of the meridian the needle sits, and it follows the rule that compass heading plus deviation equals magnetic heading. The compass swing is the procedure to measure it against a datum compass, correct what you can, and record the residual. And the source of it all is hard and soft iron magnetism, resolved into coefficients B and C.