The Direct Indicating Compass — Page 129, Lesson 148

All right, let's get into the meat of the direct indicating compass. We've already covered the basic construction and the turning errors, so now we're looking at a specific and very important set of errors: those caused by linear acceleration. Now, the first thing I want you to grasp is the headline rule, because it's a classic exam point. The size of this acceleration error depends on several factors, but the most critical one is the aircraft's heading. The error is at its maximum when you're on an East or West magnetic heading, and it's zero when you're on a North or South magnetic heading. So, maximum on East and West, zero on North and South. Keep that in your mind. But why does this happen? What's the physical cause? The error is caused by inertia acting on the magnet. Now, here's the key detail: the magnet has what we call residual dip. Remember, the magnet isn't perfectly horizontal; it dips due to the vertical component of the Earth's magnetic field, which we label as Z. So, because of this residual dip, the magnet isn't lying flat, and that's what makes it susceptible to inertia. Let's walk through the two classic scenarios to see this in action. First, let's look at an aircraft accelerating on a magnetic heading of 270°M in the northern hemisphere. Think of a take-off on runway 27. The magnet assembly is pendulously suspended, meaning it hangs from a pivot. When the aircraft accelerates forward, the inertia of that suspended magnet causes it to swing back, behind the pivot point. Now, this pivot point is offset to the north of the magnet's centre of gravity. That displacement is crucial. It enables a turning couple, produced by the Earth's vertical component Z, to act on the magnet and rotate the assembly anticlockwise around the pivot. So, the magnet rotates anticlockwise. Now, think about how we measure the heading. It's the angle measured clockwise from the north-seeking end of the magnet round to the aircraft's nose. If the magnet rotates anticlockwise, that angle increases. The compass reading therefore increases, and it indicates an apparent turn towards north. So, the compass might tell you you're now heading, say, 280°, but your real heading is still 270°. The compass is over-reading. It's telling you you've turned further north than you actually have. Now, let's flip it. Let's consider an aircraft accelerating on a magnetic heading of 090°M, again in the northern hemisphere. Same physical principle: the magnet assembly is pendulously suspended, so its inertia causes it to swing back behind the pivot point. That displacement again enables a turning couple from the vertical component Z to act. But this time, because of the geometry, it rotates the magnet assembly clockwise around the pivot. Now, the angle measured clockwise from the north-seeking end to the aircraft's nose reduces. The compass reading therefore decreases, again indicating an apparent turn towards north. So, the compass might show you 080° while your real heading is still 090°. In this case, the compass is under-reading. It's telling you you've turned less than you actually have. So, you can see the pattern. On an easterly heading, accelerating causes an apparent turn to the north and an over-reading. On a westerly heading, accelerating also causes an apparent turn to the north, but an under-reading. The key is that the error always makes the compass indicate a turn towards north during acceleration. And remember, the opposite happens during deceleration. The inertia swings the magnet forward instead of back, so the error reverses, and the compass will indicate a turn towards south. That's the core of the linear acceleration error. The diagrams in the book, Figure 10.4 and Figure 10.6, show the accelerating and decelerating cases beautifully, so it's worth studying those to visualise the swing of the magnet.