The Direct Indicating Compass — Page 129, Lesson 146

Let's start with the direct indicating compass and the errors that plague it during acceleration and turning. First, the core idea: a direct reading compass is just a magnet, free to swing, with a compass card attached to it. The north-seeking end of that magnet should always point at magnetic north, whether the aircraft is flying straight or turning. If it does, the card shows the correct heading. The whole problem we're about to discuss is that during acceleration, deceleration, or a turn, the magnet gets displaced from magnetic north, and the card then shows an incorrect heading. Now, why does that happen? The key is the centre of gravity of the magnet assembly. Most manoeuvres that move that centre of gravity away from its normal position — which is almost directly below the pivot — will produce an error. But here's the subtle part: if the manoeuvre displaces the centre of gravity north or south of its usual position, so that the CG and the pivot are still in the plane of the magnetic meridian, then the magnet assembly just changes its north-south tilt angle. There's no rotation in azimuth, and consequently no error. So the error only appears when the displacement is east or west, not north or south. There's another important condition. These turning and acceleration errors only occur where there's a significant vertical component in the earth's field. That vertical component is called Z. Near the magnetic equator, Z is essentially zero, so the errors are non-existent there — except for a small liquid swirl effect during turns. So the errors are a northern or southern hemisphere phenomenon, not an equatorial one. Let me walk you through the geometry with Figure 10.3. It shows a pendulously suspended magnet in the northern hemisphere, with residual dip. The magnet hangs from a pivot point. In the northern hemisphere, the north-seeking end of the magnet dips downward, so the centre of gravity is not directly below the pivot — it's offset. The vertical line through the pivot point is now closer to the nearer, northern magnetic pole than the magnet's centre of gravity is. That offset is what sets up the error. Now consider the aircraft, and therefore the magnet assembly, being accelerated towards the west. The magnet is attached to the aircraft at the pivot point. So when the aircraft accelerates west, the pivot is dragged west with it. But the magnet's inertia acts at its centre of gravity, and inertia tries to maintain the state of uniform motion. So the magnet's CG resists being dragged along. The result is that the magnet rotates — in this case anticlockwise — and an incorrect heading is shown on the card. So the sequence is: acceleration west → pivot moves west → magnet's inertia at the CG resists → magnet rotates anticlockwise → wrong heading displayed. The same logic applies in reverse for deceleration, which Figure 10.6 shows, and for turns, which Figure 10.4 covers. The fundamental cause is always the same: the inertia of the magnet acting at its centre of gravity, combined with the vertical component Z of the earth's field, displacing the north-seeking end from magnetic north. That's the heart of acceleration and turning errors in the direct indicating compass.