The Direct Indicating Compass — Page 500, Lesson 501

We're starting a new topic now: the Direct Indicating Compass, and specifically the concept of deviation. Let's get into it. Deviation is an error in the compass, and it's produced by the iron and steel components in the aircraft itself. Think of it this way: the compass magnets want to align with the Earth's magnetic field, but the aircraft is full of metal that has its own magnetic influence. So deviation is defined as the angle between the local magnetic meridian and the direction in which the compass magnets are actually lying. The local magnetic meridian is just the direction of magnetic North at your position; the compass magnets are being pulled away from that by the aircraft's metal. Now, we name this deviation based on which way the compass is being pulled. If the North-seeking ends of the magnets—those are the red ends—point to the East of magnetic North, we call that Easterly deviation, and we give it a plus sign. Conversely, if the North-seeking ends point to the West of magnetic North, we call it Westerly deviation, and that's minus. Here's the critical part: deviation is not a fixed number. It varies with the aircraft's heading. Because the aircraft's metal is fixed relative to the airframe, as the aircraft turns, the relationship between that metal and the Earth's magnetic field changes, so the error changes too. That means we can't just measure it once; we have to measure it on a series of different headings. This measurement is done by conducting what's called a compass swing. That's a procedure where the aircraft is aligned on known headings and the compass reading is compared. Now, during that swing, we have to simulate normal flying conditions as closely as possible. That means engines running, electrical and radio services switched on, and the aircraft in a level flight attitude. The reason is that all of those things—the engine, the electrical current, the radio gear—they all generate or influence magnetic fields, and we want the deviation we measure to match what we'll actually see in flight. After the swing, we can't eliminate deviation entirely, but we reduce it as far as possible. What's left is called the residual deviation, and that gets recorded on a compass deviation card, which is located in the aircraft. That card is your reference in the cockpit—it tells you the remaining error on each heading so you can correct for it. Now, there's a very important practical warning here. It's absolutely critical that no ferromagnetic objects—things like tools or watches—are placed near the compass. If they are, they introduce unknown amounts of deviation, which defeats the whole purpose of the swing. Ferromagnetic just means materials that are strongly attracted to magnets, like iron and steel. And it's not just loose items: ferromagnetic payloads, the cargo you carry, should be stowed as far away from the compass as the loading limits will allow. And here's the extreme case: if you have an exceptionally large ferromagnetic load, you may have to carry out a compass swing before flight with that load actually aboard, because the load itself will change the deviation so much. So to tie it together: deviation is the compass error caused by the aircraft's own iron and steel, it's named Easterly or Westerly depending on which side of magnetic North the red ends point, it changes with heading, and we manage it through a compass swing, recording the leftover residual deviation on the card in the cockpit.