The Direct Indicating Compass — Page 500, Lesson 501

We're starting a new topic now: the Direct Indicating Compass, and the very first thing I want to nail down with you is deviation — because it's the single most important error source you'll deal with on this instrument. Deviation is produced by the iron and steel components in the aircraft itself. It's defined as the angle between the local magnetic meridian and the direction in which the compass magnets are actually lying. So think of it this way: the Earth's magnetic field wants to pull your compass needles to point along the magnetic meridian — that's the local magnetic north line. But the aircraft's own iron and steel parts create their own magnetic fields, and they tug on those compass magnets too. The result is that the compass needles don't lie exactly along the magnetic meridian; they're offset by some angle. That angle is deviation. Now, we name deviation by which side of magnetic north the compass magnets point to. If the North-seeking ends of the magnets — and remember, those are the red ends — point to the East of magnetic North, we call that deviation Easterly, or plus. If the North-seeking ends point to the West of magnetic North, we call it Westerly, or minus. So the sign convention is tied directly to the red end of the needle: east of magnetic north is plus, west is minus. Here's the critical operational point: deviation varies with heading. It's not a fixed error. As the aircraft turns, the relationship between the aircraft's iron and steel and the Earth's field changes, so the deviation changes too. That means we can't just measure it once and be done — we have to measure it on a series of different headings. That's done by conducting a compass swing, which is fully covered in the chapter on aircraft magnetism. During the swing, we reduce the deviation as far as possible, and whatever remains after that correction is called the residual deviation. That residual deviation is recorded on a compass deviation card, which is located in the aircraft — so you, as the pilot, can read off the correction for any heading you're flying. Now, there's a very important discipline around how the swing is conducted. During the swing, normal flying conditions should be simulated as far as possible. That means engines running, electrical and radio services switched on, and the aircraft in a level flight attitude. Why? Because all of those things — the engine, the electrical systems, the radios — are sources of magnetic influence, and if they're not in their normal operating state, the deviation you measure won't match what you'll actually have in flight. And then there's the human factor, which is absolutely vital. It's most important that no ferromagnetic objects — and by that I mean things like tools or watches — are placed near the compass, because they would introduce unknown amounts of deviation. A steel wrench left on the glareshield, a wristwatch on your arm — these are ferromagnetic, and they'd corrupt the reading completely. Furthermore, ferromagnetic payloads — cargo that's magnetic — should be stowed as far away from the compass as permissible within the loading limits. And here's the extreme case: with exceptionally large ferromagnetic loads, a compass swing may have to be carried out before flight with the load aboard. So if you're carrying a big magnetic cargo, you don't just trust the old deviation card — you swing the compass with that load in place, because the load itself changes the deviation. Let me show you what this looks like geometrically, because it helps to see the equilibrium. That figure shows the equilibrium in the Northern hemisphere, viewed from the west. You can see how the compass magnet sits relative to the magnetic meridian — that's the deviation angle we've been talking about, the angle between the local magnetic meridian and the direction the compass magnets are lying. So to tie it all together: deviation is an aircraft-induced error, it's named Easterly or plus when the red ends point east of magnetic north, Westerly or minus when they point west, it varies with heading, it's measured on a compass swing under simulated normal flight conditions, the residual is recorded on a deviation card, and you must keep ferromagnetic objects and payloads away from the compass — or swing with the load aboard if it's exceptionally large.