Aircraft Magnetism — Page 500, Lesson 503

I want to walk you through the heart of aircraft magnetism now — the part where the compass stops being a perfect instrument and starts being a real one, with all its imperfections. That imperfection is called deviation, and it's the single most important thing you'll manage when you fly with a magnetic compass. Let me define it precisely. The compass needle would accurately define the magnetic meridian — that's the true north-south line of the Earth's magnetic field — were it not for the aircraft's own internal magnetism deflecting it. So deviation is the angular difference measured between the direction taken up by a compass needle and the magnetic meridian. In plain terms: it's how far your compass is wrong, in degrees, because the aircraft itself is pulling the needle away from where it should point. Now, deviation is named easterly or westerly, and the naming rule is exact. It depends on whether the North-seeking end of the compass needle lies to the East or to the West of the magnetic meridian. So if the needle's north end sits east of the true magnetic meridian, that's easterly deviation. If it sits west, that's westerly deviation. The sign convention follows directly from that. Let me show you the relationship with a small table, because this is where pilots get confused. We have Compass Heading, Deviation, and Magnetic Heading. Look at the first example: Compass Heading 095, deviation minus 5, gives Magnetic Heading 090. That's deviation west — and notice the phrase attached to it: "Compass Best." The second example: Compass Heading 090, deviation plus 5, gives Magnetic Heading 095. That's deviation east — and the phrase is "Compass Least." So here's the rule you must internalize. When deviation is west, the compass reads more than the magnetic heading — the compass is "best," meaning it reads high. When deviation is east, the compass reads less than the magnetic heading — the compass is "least," meaning it reads low. The mnemonic is built right into the table: West is Best, East is Least. West deviation, compass best — reads higher. East deviation, compass least — reads lower. And the arithmetic works out: 095 minus 5 is 090, 090 plus 5 is 095. Now, how do we actually find out what the deviation is on our 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. So you have a reference compass on the ground — a datum compass — that you trust, and you compare it against the compass inside the aircraft. This comparison is carried out in an area selected specifically for this purpose — a compass swing base, a place free from magnetic interference. Let me lay out the aims of a compass swing, because there are three, and each one matters. First: to observe or determine the deviations — the differences between Magnetic North, observed on a landing compass, and Compass North, observed in the aircraft — on a series of headings. So you swing the aircraft through a range of headings and record how far off the compass is at each one. Second: to correct or remove as much deviation as possible. You physically adjust the compass to cancel out the error you found. Third: to record the residual deviation which is left after the compass has been adjusted. You can't remove all of it — some always remains — and that residual is what you record and use in flight. Finally, I want to give you the physical source of all this. The magnetic deviation observed during a compass swing can be said to be derived from Hard Iron and Soft Iron magnetism. Hard iron magnetism is the permanent magnetism in the aircraft's structure — it stays fixed. Soft iron magnetism is induced — it 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, later be resolved into two further combined components, called coefficients B and C. Those coefficients are the mathematical way we break the total deviation down so we can correct it systematically. We'll get into exactly what B and C represent when we dig deeper, but for now, know that every deviation you see traces back to hard iron, soft iron, and ultimately those two coefficients. So the picture is complete: deviation is the compass's error caused by the aircraft's own magnetism, named east or west by which side of the meridian the needle sits, corrected through a compass swing using a datum compass, with the goal of observing, correcting, and recording the residual — and the whole thing stems from hard iron and soft iron magnetism resolved into coefficients B and C.