Figure 10.4 Acceleration on 270° M — Page 129, Lesson 150

We're now looking at the acceleration errors of the direct-indicating compass, and I want to walk you through the specific cases shown in these figures, because this is where the behaviour of the instrument becomes really important to understand. Let's start with Figure 10.4, which shows an aircraft accelerating on a magnetic heading of 270°M in the northern hemisphere. Remember, the magnet assembly is pendulously suspended — it hangs below the pivot point. When the aircraft accelerates, the inertia of that hanging magnet assembly causes it to swing backwards, behind the pivot point. Now, because the pivot is offset to the north of the magnet's centre of gravity in this northern hemisphere case, that backward swing creates a turning couple that rotates the magnet assembly clockwise around the pivot. Here's the key consequence: when the magnet assembly is displaced clockwise, the compass reading decreases. So on this 270°M heading, accelerating, the compass will under-read. The aircraft is really still heading 270°M, but the compass might show, say, 260° — an apparent turn towards south. Now let's contrast that with Figure 10.5, acceleration on 090°M in the northern hemisphere. Same acceleration, but now the pivot is offset to the south of the magnet's centre of gravity. The inertia still swings the magnet assembly backwards, but this time that displacement creates a turning couple that rotates the assembly anticlockwise around the pivot. And when the assembly is displaced anticlockwise, the readings increase — the compass over-reads. So the aircraft is really still on 090°M, but the compass might show 100°, again an apparent turn towards south. Now let's look at deceleration, Figure 10.6, on 090°M in the northern hemisphere. When the aircraft decelerates, the inertia of the pendulously suspended magnet assembly causes it to swing forwards, ahead of the pivot point. This displacement enables a turning couple to rotate the assembly anticlockwise around the pivot. So the compass reading increases — it over-reads. The aircraft is really still on 090°M, but the compass shows, say, 100°, indicating an apparent turn towards south. Now let's go to the southern hemisphere, Figure 10.7, acceleration on 270°M. The inertia causes the magnet assembly to swing back behind the pivot point, which is now offset to the south of the magnet's centre of gravity. This displacement enables a turning couple to rotate the assembly clockwise around the pivot. So the compass reading decreases — it under-reads. The aircraft is really still on 270°M, but the compass shows, say, 260°, indicating an apparent turn towards south. Now here's a really important special case, Figure 10.8: acceleration on a northerly heading, 360°M, in the northern hemisphere. The centre of gravity lags and the north-south tilt of the magnet assembly changes, but the magnets are tilting in the vertical plane of the magnetic meridian through the pivot — so no error occurs. Similarly, with deceleration on north/south headings there is again no error, only a reduced north-south tilt due to the inertial forward swing of the magnet assembly. Let me give you the summary of acceleration errors, because this ties everything together. Acceleration errors are zero on north/south magnetic headings in both hemispheres, increasing to maximum on headings 090°M and 270°M. Acceleration causes an apparent turn towards the nearer pole — that's an apparent turn north in the northern hemisphere, and an apparent turn south in the southern hemisphere. Deceleration causes an apparent turn towards the further pole — apparent turn south in the northern hemisphere, apparent turn north in the southern hemisphere. And here are the two rules that govern the reading: whenever the magnet assembly is displaced clockwise, the readings will decrease and the compass will under-read. Whenever the magnet assembly is displaced anticlockwise, the readings will increase and the compass will over-read. Finally, the size of a linear acceleration error depends on four factors: the heading, the magnitude of the acceleration, the design of the magnet system, and the magnetic latitude — which affects the relative strengths of H and Z, the horizontal and vertical components of the Earth's magnetic field. The errors are maximum near the magnetic poles, decreasing to zero at the magnetic equator. So the practical takeaway for you as a pilot: on easterly and westerly headings, acceleration and deceleration will give you false turn indications, and you need to know which way the compass is lying. On north and south headings, you're safe — no error at all.