Aircraft Magnetism — Page 200, Lesson 239

Let's start with the core idea: the compass needle wants to point to magnetic north, but the aircraft itself is a giant lump of metal with its own magnetism. That internal magnetism pulls the needle away from true magnetic north. That pull-away is called deviation. Deviation is defined as the angular difference measured between the direction the compass needle actually takes up and the magnetic meridian. The magnetic meridian is simply the line of magnetic north—the direction the needle would point if nothing interfered. So deviation is the error, in degrees, between where the needle points and where it should point. Now, deviation is named 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. Let me give you a concrete example from the table. Suppose the compass heading reads 095, and the deviation is minus 5. That's westerly deviation, and the magnetic heading is 090. So the compass is showing 095, but the true magnetic heading is 090—the needle is being pulled 5 degrees to the west. That's why it's called "Compass Best" in the table: the compass reads higher than the magnetic heading. Now flip it. Compass heading 090, deviation plus 5, magnetic heading 095. That's easterly deviation, and the table calls it "Compass Least"—the compass reads lower than the magnetic heading. So the sign convention is: westerly deviation is negative, easterly deviation is positive. That's the relationship you need to hold onto. Now, how do we actually find out what the deviation is on a given aircraft? 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. That's a reference compass set up on the ground, away from the aircraft's influence. This comparison is carried out in an area selected specifically for this purpose—a place where there's no external magnetic interference to corrupt the readings. So what are the aims of a compass swing? There are three. First, to observe and determine the deviations—the differences between magnetic north, observed on the landing compass, and compass north, observed in the aircraft—on a series of headings. You swing the aircraft through a range of headings and record the error at each one. Second, to correct and remove as much deviation as possible. Third, to record the residual deviation—that's the deviation left over after the compass has been adjusted. You can't eliminate it entirely, so you document what remains. One more important point. The magnetic deviation observed during a compass swing is said to be derived from hard iron and soft iron magnetism. Hard iron is permanent magnetism—it stays fixed regardless of the aircraft's orientation. Soft iron is induced magnetism—it changes as the aircraft's heading changes relative to the Earth's field. And this total field, for our purposes, can later be resolved into two further combined components, called coefficients B and C. Those coefficients are how we mathematically break down the total deviation into manageable parts for correction. So to tie it together: deviation is the compass error caused by the aircraft's own magnetism, named easterly or westerly by which side of magnetic north the needle sits. We measure it with a compass swing, comparing the aircraft compass against a datum compass, and we aim to observe, correct, and record the residual error. And that error comes from hard iron and soft iron magnetism, which we later resolve into coefficients B and C.