The Direct Indicating Compass — Page 134, Lesson 157

I want to walk you through the turning errors of the direct indicating compass. We've already covered the basics of how the compass works, so now we're looking at what happens when the aircraft actually turns — and this is where the compass can genuinely mislead you if you don't understand it. Let's start with the northern hemisphere case, turning from 045° to 315°. Here's the key idea: the aircraft and the magnet are now rotating in opposite directions. Think about that. The aircraft turns one way, but the magnet assembly, because of inertia and the way the pivot works, gets displaced and rotates the other way. So even though the aircraft has turned 90° around the compass, the magnet has been displaced and rotated in the opposite direction by a number of degrees — say 20°. The pilot will therefore see 110° pass beneath the lubber line, and the compass is termed 'lively'. Now, that word 'lively' is important. It means the compass indication is moving faster than the actual turn — it's over-reading. Whenever the magnet rotates anticlockwise, it will over-read. Let me make that concrete. If the pilot stops the turn at 225° indicated, the actual heading will be numerically smaller — such as 205°. So the compass is showing you a bigger number than your real heading. That means the turn must be stopped late — such as 245° — or the pilot should overshoot, to achieve the correct heading. You deliberately turn past the target and then come back, because the compass is running ahead of you. Now let's look at the southern hemisphere case, turning from 135° to 225°, passing through south. This is Figure 10.12. Here's the difference. As the aircraft turns right through 180°M, the magnet's centre of gravity is displaced from beneath the pivot point, away from the nearer pole — which is the south pole in the southern hemisphere. Because of inertia, the magnet assembly will be thrown out of the turn, rotating the magnet assembly clockwise. Now the aircraft and the magnet assembly are rotating in the same direction — both clockwise — and therefore the compass will again be sluggish. So we have a contrast here. In the northern hemisphere, the magnet rotates opposite to the aircraft, and the compass is lively. In the southern hemisphere, they rotate together, and the compass is sluggish. And the error direction flips. Whenever the magnet rotates clockwise, it will under-read. So if the pilot stops the turn at 225° indicated, the actual heading will be numerically larger — such as 245°. The compass is showing you a smaller number than your real heading. Therefore the turn must be stopped early — such as 205° — or undershoot, to achieve the correct heading. And here's a crucial point to remember: when the wings are levelled, the compass will 'catch up' and settle on 225°. So the error is a turning error — it exists only while the aircraft is actually turning. Once you roll out and level the wings, the compass corrects itself and reads the true heading. Now let's consider turning through east or west — the magnetic headings of 090° and 270°. This is a special case. When you pass through east or west, the magnets are not horizontal, but their tilt is north-south. That means the tilt lies in the vertical plane of the magnetic meridian, through the pivot. Because of that geometry, there is no rotational couple acting round the pivot. No couple means no turning error. So turning errors are zero when passing through east or west. That's a clean, simple rule to remember. Finally, there's a practical note about steering. It is easier to steer a southerly rather than a northerly heading in the northern hemisphere. There are two reasons. First, because on south, the compass does not indicate the wrong direction of turn as it can on north. Second, because the 'lively' nature of the indications reduces the risk of over-correcting small steering errors. So the liveliness that makes north tricky actually helps you on south — the compass responds quickly, so you don't chase small deviations. Let me pull the whole picture together. The turning error comes from the magnet assembly being displaced by inertia during a turn. In the northern hemisphere, the magnet rotates opposite to the aircraft, the compass over-reads, it's lively, and you must overshoot. In the southern hemisphere, the magnet rotates with the aircraft, the compass under-reads, it's sluggish, and you must undershoot. And through east or west, the error is zero. That's the complete turning error picture for the direct indicating compass.