The Direct Indicating Compass — Page 134, Lesson 159

Let’s pick this up right where the turning error story gets practical — the magnitude of those errors, and then the liquid swirl effect, which is the one piece of this that isn’t about the earth’s field at all. First, what makes a turning error worse or milder? The book lists the factors that affect severity. They are worst at high latitudes, where Z is strong and H is weak. Remember Z is the vertical component of the earth’s magnetic field, and H is the horizontal component. So near the poles, the field is steep — lots of vertical pull, very little horizontal pull — and that’s exactly when the compass misbehaves most during a turn. The other variables that matter are 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 where you are — how you turn and what you’re flying in also change the size of the error. Now, liquid swirl. This is a purely mechanical effect, separate from the magnetic dip we’ve been talking about. Picture the compass bowl — the liquid inside it is in contact with the inside wall of the bowl. When you turn the aircraft, the bowl turns with it, and the liquid in contact with that wall gets dragged around with the bowl. That produces small eddies in the liquid, which drift inwards from the circumference — from the outer edge toward the centre — and those eddies 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. Now here’s the key consequence. When you’re turning through north in the northern hemisphere, the magnet assembly is already turning in the same direction as the aircraft — that’s the normal turning error behaviour through the nearer pole. Liquid swirl acts in that same direction, so it increases the magnitude of the turning error. But when you turn through south in the northern hemisphere, the assembly turns in the opposite direction to the aircraft — that’s the further-pole behaviour. There, the swirl opposes the error, so it reduces the size of the turning error. And in the southern hemisphere, the swirl effect is in the opposite sense entirely. There’s a really important special case here. At the magnetic equator, there is no vertical component Z in the earth’s field — so there’s no dip, and therefore no turning error from the magnetic field at all. That means liquid swirl is the sole source of turning error there. And the book notes that with most compasses, that effect is only slight. So at the equator, you’re not free of error — you just have a much smaller, purely mechanical one. Let me give you the summary of turning errors, because this is the part you’ll want to hold onto. Turning errors are 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 — the further you get from the equator, the worse it gets. And at the magnetic equator, the only turning error is due to liquid swirl. Now the operational rules — this is the piloting side. Whenever you turn through the nearer pole — that’s north in the northern hemisphere, or south in the southern hemisphere — four things happen. The aircraft and the compass rotate in the same direction. The compass will be sluggish. The pilot should undershoot the turn, meaning roll out early. And liquid swirl will increase the turning error. Whenever you turn through the further pole — south in the northern hemisphere, or north in the southern hemisphere — the opposite set applies. The aircraft and compass rotate in the opposite direction. The compass will be lively. The pilot should overshoot the turn, meaning roll out late. And liquid swirl will reduce the turning error. So the mnemonic to carry: nearer pole — same direction, sluggish, undershoot, swirl adds to the error. Further pole — opposite direction, lively, overshoot, swirl subtracts from the error. And one final reminder from the book, because it ties the whole thing together. At the magnetic equator there is no turning error because there is no dip. And remember the sense of it: a displacement of the magnets in a clockwise direction when viewed from above causes the compass to under-read, and a displacement in an anticlockwise direction causes the compass to over-read. That’s the direction of the magnet displacement, and it tells you which way the reading is off. Let me show you the geometry of these two turns so you can see the same-direction versus opposite-direction behaviour clearly. — that’s the turn from 045° to 315°, which passes through north. — that’s the turn from 135° to 225°, which passes through south. And shows the turn through south in more detail. Watch how the compass needle leads or lags the aircraft heading in each case, and you’ll see exactly why you undershoot through the nearer pole and overshoot through the further one.