The Direct Indicating Compass — Page 129, Lesson 146

Let’s start with the big picture. The direct indicating compass is a magnetic compass, and its job is to keep the north-seeking end of the magnet pointing at magnetic north. But here’s the catch: it’s not a perfect instrument. It suffers from large errors during linear acceleration, linear deceleration, or during a turn. So whenever the aircraft speeds up, slows down, or banks into a turn, the compass can show you a heading that is simply wrong. Why does that happen? Let’s think about the physical setup. The compass magnet assembly is suspended from a pivot, and it hangs down like a pendulum. In its normal, steady state, the centre of gravity of that magnet assembly sits almost directly below the pivot. That’s the key phrase: almost directly below. Because the magnet has residual dip — it tilts down toward the magnetic pole — the centre of gravity is not exactly under the pivot. So the magnet hangs at a tilt. Now, most manoeuvres that move the centre of gravity away from that normal position, away from almost directly below the pivot, will produce an error. But there’s an important exception. If the manoeuvre displaces the centre of gravity north or south of its usual position, so that the centre of gravity and the pivot are still in the plane of the magnetic meridian — that is, still in the vertical plane that contains magnetic north and south — then the magnet assembly merely changes its north-south tilt angle. It tilts a bit more or a bit less, but it does not rotate in azimuth. No rotation in azimuth means no change in the heading shown, so no error. That’s the exception: displacement along the north-south line, within the magnetic meridian plane, gives no error. There’s another crucial condition. Turning and acceleration errors only occur where there is a significant vertical component in the earth’s magnetic field. That vertical component is called Z. Near the magnetic equator, the field is essentially horizontal, so Z is very small. That means, except for a small liquid swirl effect during turns, these errors are essentially non-existent near the magnetic equator. So the errors are tied to the vertical component of the earth’s field, not just to the manoeuvre itself. Now let’s look at what happens during a specific manoeuvre. Consider an aircraft being accelerated towards the west. The magnet assembly is attached to the aircraft at the pivot point. So when the aircraft accelerates west, the pivot is dragged west with the aircraft. But the magnet’s inertia acts at its centre of gravity. Inertia is the tendency of a body to maintain its state of uniform motion — to keep doing what it was doing. So while the pivot is being accelerated west, the magnet’s centre of gravity, through inertia, tries to stay where it was, to maintain its previous state of uniform motion. The result is that the magnet rotates relative to the pivot. In this specific case, accelerating west, the magnet rotates anticlockwise. And because the compass card is attached to the magnet, the card rotates with it, and an incorrect heading is shown. Let me tie that to the figure. Figure 10.3 shows a pendulously suspended magnet in the northern hemisphere, with residual dip. The key observation in that figure is that the vertical line through the pivot point is closer to the nearer magnetic pole — the north pole — than the magnet’s centre of gravity is. So the pivot line and the centre of gravity are not aligned; the centre of gravity hangs slightly behind, toward the south. That offset is what makes the inertia effect produce a rotation rather than just a tilt. So the whole story is this: the compass magnet should keep pointing at magnetic north whether you’re flying straight or turning. But when the centre of gravity is displaced from magnetic north — when the north-seeking end of the magnet is no longer aligned with magnetic north — the inertia of the magnet, acting at the centre of gravity, causes the magnet to rotate, and the attached compass card shows a wrong heading. The error is a direct consequence of the magnet’s inertia acting at a point that is not aligned with the pivot, combined with a significant vertical component of the earth’s field. That’s the foundation. Next we’ll look at the specific errors during acceleration and deceleration in more detail, and then the turning errors. But for now, hold onto these three ideas: the centre of gravity sits almost, but not exactly, below the pivot; errors need a vertical component Z in the earth’s field; and inertia acting at the centre of gravity is what rotates the magnet when the aircraft accelerates or turns.