TheDirectIndicatingCompass — Page 30, Lesson 25

I want to walk you through the turning errors of the Direct Indicating Compass — specifically the magnitude of those errors, the effect of liquid swirl, and a summary that ties it all together for the pilot. Let's start with the magnitude of turning errors. Several factors affect how severe these errors become. They are worst at high latitudes, where the vertical component of the Earth's magnetic field — which we call Z — is strong, and the horizontal component — H — is weak. That's the key relationship: strong Z, weak H makes turning errors large. Other relevant variables include the rate of turn, the duration of the turn, the speed of the aircraft, the headings involved, and the design of the compass itself. So it's not just one thing — many factors combine to determine how big the error will be. Now let's look at liquid swirl. This is an effect I mentioned earlier in the chapter. During a turn, the liquid inside the compass bowl — the fluid that surrounds the magnet assembly — is in contact with the inside of the bowl. As the bowl rotates with the aircraft, that liquid tends to be dragged around with the bowl. This produces small eddies — little whirlpools — that drift inward from the circumference and deflect the magnet assembly in the direction of the turn. So the liquid swirls, and it rotates the magnet assembly with it, in the same direction as the aircraft's turn. What does that mean for the pilot? When turning through north in the northern hemisphere, the liquid swirl effect will increase the magnitude of the turning error. Why? Because in that turn, the magnet assembly already turns in the same direction as the aircraft — the swirl adds to that, making the error bigger. When turning through south in the northern hemisphere, the situation is different. In that case, the magnet assembly turns in the opposite direction to the aircraft. The liquid swirl effect will reduce the size of the turning error — it works against the natural error, partially cancelling it. In the southern hemisphere, the swirl effect is in the opposite sense — so the pattern flips. Here's an important note: at the magnetic equator, there is no vertical component Z in the Earth's field. That means the only source of turning error is liquid swirl. And with most compasses, that effect is only slight. Now let's bring it all together with the summary of turning errors. Turning errors are maximum when passing through magnetic north or magnetic south. They decrease to zero when passing through east or west. The error increases with an increase in magnetic latitude — the further you are from the equator, the bigger the error. At the magnetic equator, the only turning error is due to liquid swirl — as we just covered. Now here's the practical piloting rule. Whenever you turn through the nearer pole — that's north in the northern hemisphere, or south in the southern hemisphere — here's what happens: - The aircraft and the compass rotate in the same direction - The compass will be sluggish — it lags behind - The pilot should undershoot the turn — roll out early - Liquid swirl will increase the turning error Whenever you turn through the further pole — that's south in the northern hemisphere, or north in the southern hemisphere: - The aircraft and the compass rotate in the opposite direction - The compass will be lively — it leads ahead - The pilot should overshoot the turn — roll out late - Liquid swirl will reduce the turning error So as a pilot, you need to know which hemisphere you're in, whether you're turning toward the nearer or further pole, and then compensate accordingly — undershoot or overshoot your rollout. That's the practical takeaway from these turning errors.