The Direct Indicating Compass — Page 134, Lesson 159

Right, let's get into the meat of the Direct Indicating Compass. We've already covered the basics of how it works, so now we're looking at the magnitude of turning errors and the factors that affect them. First, let's talk about what makes these errors worse. The severity of turning errors is affected by several factors, but they are at their worst at high latitudes. Why? Because at high latitudes, the vertical component of the Earth's magnetic field, which we call Z, is strong, while the horizontal component, H, is weak. Remember, the compass aligns with the horizontal component, so when H is weak, the magnet assembly is more easily disturbed. Other variables that affect the severity include the rate of turn, the duration of the turn, the speed of the aircraft, the specific headings involved, and the design of the compass itself. Now, let's look at a specific effect called liquid swirl. I mentioned this earlier. During a turn, the liquid that is in contact with the inside of the compass bowl tends to be dragged around with the bowl. This produces small eddies in the liquid that drift inwards from the circumference. These eddies deflect the magnet assembly in the direction of the turn. So, the liquid swirls and rotates the magnet assembly with it, in the same direction as the aircraft's turn. This has a very specific consequence. When you are turning through north in the northern hemisphere, the liquid swirl will increase the magnitude of the turning error. That's because, in that scenario, the assembly is already turning in the same direction as the aircraft. Conversely, when turning through south in the northern hemisphere, where the assembly turns in the opposite direction to the aircraft, the swirl effect will reduce the size of the turning error. In the southern hemisphere, the swirl effect is in the opposite sense. And here's a key point: at the magnetic equator, where there is no vertical component Z in the Earth's field, liquid swirl is the sole source of turning error. With most compasses, this effect is only slight. Let me summarise the turning errors for you. They are at their maximum when passing through magnetic north or south, and they decrease to zero when passing through east or west. The error increases with an increase in magnetic latitude. And, as I just said, at the magnetic equator, the only turning error is due to liquid swirl. Now, let's look at the practical rules for the pilot. Whenever you turn through the nearer pole—that's north in the northern hemisphere, or south in the southern hemisphere—the aircraft and the compass rotate in the same direction. The compass will be sluggish. Because of this, the pilot should undershoot the turn, meaning you roll out early. And remember, liquid swirl will increase the turning error in this case. Whenever you turn through the further pole—south in the northern hemisphere, or north in the southern hemisphere—the aircraft and compass rotate in the opposite direction. The compass will be lively. Here, the pilot should overshoot the turn, meaning you roll out late. And liquid swirl will reduce the turning error. Finally, let's reinforce the equator rule. At the magnetic equator, there is no turning error because there is no "dip". And let's recap the direction of the error itself. It is a displacement of the magnets in a clockwise direction, when viewed from above, which causes the compass to under-read. A displacement in an anticlockwise direction causes the compass to over-read. Keep that relationship clear in your mind, and the rest of the turning error logic will follow.