The Direct Indicating Compass — Page 134, Lesson 154

Right, let's pick this up with the direct indicating compass. We've already covered the basics of how the magnet aligns with the earth's field, so now we're looking at the real-world errors that occur during turns. We've just been discussing turning from 045° to 315° in the northern hemisphere. The key point there was that when you turn through north, the compass is sluggish and under-reads. So, if you stop the turn when the compass indicates 315°, your actual heading will be numerically smaller, something like 295°. To get the correct heading, you have to stop the turn early, at about 335°. This is what we call undershooting the required heading. Now, I want to be very precise about terminology here, because it's a classic trap. 'Undershoot' means you're turning through a smaller angle. You stop the turn before the compass reaches the target. This is completely different from 'under-read', which means the numerical heading indicated on the compass is too small. They sound similar, but they're describing different things. Undershoot is an action by the pilot; under-read is a fault of the indication. So, if the pilot deliberately undershoots and rolls out when the compass reads about 335°, he should observe that when the wings are levelled, the compass will 'catch up' and settle on 315°. The error disappears once the bank is removed. Now, let's move on to the next scenario: turning from 315° to 045° in the northern hemisphere. This is a right turn through north. As the aircraft passes through 000°M, the magnet's centre of gravity is displaced from beneath the pivot point, away from the north pole, due to the vertical component of the earth's magnetic field. Because of inertia, the magnet assembly will be thrown out of the turn, and it rotates clockwise. Now, here's the critical bit: the aircraft and the magnet assembly are again rotating in the same direction, but this time, they're both rotating clockwise. Because they're rotating in the same direction, the compass will again be sluggish. And here's the rule to remember: whenever the magnet rotates clockwise, it will under-read. So, if the pilot stops the turn at 045° indicated, the actual heading will be numerically larger, something like 065°. Therefore, the turn must be stopped early, at about 025°, or the pilot should undershoot the indication, to achieve the correct heading. Let's look at the diagram for this. Now, let's consider the final scenario: turning from 135° to 225° in the northern hemisphere. This is a right turn through south. As the aircraft passes through 180°M, the magnet's centre of gravity is displaced from beneath the pivot point, but this time it's displaced 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 will be thrown out of the turn, but this time it rotates anticlockwise. So now we have a contrast: the aircraft is turning clockwise, to the right, but the magnet assembly is rotating anticlockwise. They're rotating in opposite directions. Let's look at the diagram for this. So, to summarise the pattern we're building: turning through north in the northern hemisphere, the magnet and aircraft rotate in the same direction, causing sluggishness and under-reading. Turning through south, they rotate in opposite directions, which produces a different error. We'll explore exactly what that error is in the next part.