The Direct Indicating Compass — Page 134, Lesson 157

All right, let's pick this up right where the turning errors left off. We've just seen what happens when you turn through north in the northern hemisphere. Now I want to walk you through the exact opposite case, and then we'll finish with the special cases of turning through east and west. So, we just established that when you turn through north in the northern hemisphere, the magnet gets displaced and rotates in the opposite direction to the aircraft. That makes the compass 'lively' and it over-reads. Now let's look at turning from 135° to 225°, which takes you through south in the southern hemisphere. This is the mirror image. As the aircraft turns right through 180° magnetic, the magnet's centre of gravity is displaced from beneath the pivot point, away from the nearer pole, which is the south pole. Because of inertia, the magnet assembly is thrown out of the turn, and it rotates clockwise. Now, here's the key contrast: the aircraft and the magnet assembly are now rotating in the same direction, both clockwise. And that means the compass will again be sluggish. So we have a clean rule forming. Whenever the magnet rotates anticlockwise, the compass over-reads. Whenever it rotates clockwise, it under-reads. Let's make sure that's crystal clear with the numbers. In the southern hemisphere turn through south, the magnet rotates clockwise, so the compass under-reads. That means if the pilot stops the turn at 225° indicated, the actual heading will be numerically larger, such as 245°. So to achieve the correct heading, the turn must be stopped early, such as 205°, or you undershoot. Now, contrast that with the northern hemisphere turn through north we just did. There, the magnet rotated anticlockwise, so the compass over-read. If you stopped at 225° indicated, the actual heading was numerically smaller, like 205°. So you had to stop the turn late, such as 245°, or overshoot, to get the right heading. One more thing to remember about that southern hemisphere case: when you level the wings, the compass will 'catch up' and settle on the correct 225°. So the error is only present while the turn is being made. Now, let's move on to the special case of turning through east or west, the headings of 090° and 270° magnetic. Here's the crucial difference. When you're on those headings, the magnets are not horizontal, but their tilt is north-south. That means the tilt is in the vertical plane of the magnetic meridian, through the pivot. Because of that geometry, there is no rotational couple acting round the pivot. And if there's no couple, there's no turning error. So the turning errors are zero when passing through east or west. Finally, I want to give you a practical note on steering. In the northern hemisphere, it's easier to steer a southerly heading than a northerly one. There are two reasons. First, on south, the compass does not indicate the wrong direction of turn, which it can do on north. And second, the 'lively' nature of the indications on south reduces the risk of over-correcting small steering errors. So to tie it all together: the turning error is all about the magnet's centre of gravity being displaced by inertia, and whether that makes the magnet rotate with or against the aircraft. Same direction means sluggish and under-reading; opposite direction means lively and over-reading. And the whole thing disappears at east and west.