The Direct Indicating Compass — Page 134, Lesson 154

Let’s pick this up right where the compass behaviour through the turn gets interesting. We’ve already seen what happens when you turn from 045° to 315° in the northern hemisphere — the compass lags, it under-reads, and you have to undershoot the heading to roll out correctly. Now I want to walk you through the two other turning cases, because each one has its own distinct behaviour, and you need to know exactly which way the error goes in each. First, let’s make sure we’re clear on two terms that are easy to mix up. Undershoot means turning through a smaller angle than the required heading — you roll out early, before the compass reads the target. Under-read is different: it means the numerical heading the compass indicates is too small, lower than the actual heading. These are two separate ideas, and confusing them is a classic trap. Now, turning from 315° to 045° in the northern hemisphere. Picture the aircraft turning right, passing through 000°M — that’s magnetic north. As it passes through north, the magnet’s centre of gravity is displaced from beneath the pivot point, away from the north pole, because of the vertical component of the earth’s magnetic field. That displacement is the key — it tilts the magnet assembly so its centre of gravity is no longer directly under the pivot. Because of inertia, the magnet assembly gets thrown out of the turn, and it rotates clockwise. Here’s the critical bit: the aircraft is turning right, which is also clockwise, and the magnet assembly is rotating clockwise too — same direction. Whenever the magnet rotates clockwise, the compass under-reads. So if the pilot stops the turn at 045° indicated, the actual heading will be numerically larger — say 065°. To hit the correct heading, you must stop the turn early, undershooting the indication — roll out around 025° — and the compass will catch up and settle on 045° once the wings are levelled. Now the third case: turning from 135° to 225° in the northern hemisphere, turning right through south, passing through 180°M. Again, the magnet’s centre of gravity is displaced from beneath the pivot point — but this time away from the nearer pole, which is the north pole, again due to the vertical component of the earth’s magnetic field. Because of inertia, the magnet assembly is thrown out of the turn, and this time it rotates anticlockwise. The aircraft is turning clockwise — right — but the magnet assembly is rotating anticlockwise, the opposite direction. That opposite rotation is what changes the behaviour. When the magnet rotates anticlockwise, the compass will over-read — the indicated heading is numerically larger than the actual heading. So if you stop the turn at 225° indicated, the actual heading will be smaller, and you’d need to overshoot the indication to achieve the correct heading. That’s the mirror image of the north case. So the pattern to hold onto: through north in the northern hemisphere, the magnet rotates clockwise and the compass under-reads — you undershoot. Through south, the magnet rotates anticlockwise and the compass over-reads — you overshoot. The direction of the magnet’s rotation, driven by inertia and the displaced centre of gravity, tells you which way the error goes. Those figures show the geometry — the pivot, the north reference, and the heading arcs for each turn. Take a moment to trace the magnet’s rotation direction in each one, because that’s the whole story.