The Direct Indicating Compass — Page 129, Lesson 152

Right, let's get into the turning errors of the direct indicating compass. This is where the compass really starts to misbehave in flight, and it's a classic exam area. We've already covered the linear acceleration errors, where the compass swings due to fore-and-aft acceleration. Turning errors are the same family of problem, but they happen when the aircraft banks and turns. The key headline fact is this: turning errors are maximum when turning through north and south, and zero when turning through east and west. Ignoring liquid swirl, that's your rule of thumb. Now, why does this happen? The basic theory is much the same as for linear acceleration errors. It all comes down to the earth's vertical component of the magnetic field, which we call Z. Because of Z, the compass's centre of gravity, the CG, is displaced from almost beneath the pivot point, away from the nearer pole. So the magnet assembly is hanging slightly off-centre, tilted by the vertical field. Here's the physics of the turn. In a turn, the aircraft accelerates towards the centre of the turn. So an acceleration force acts through the pivot, pointing towards the centre of the turn. But there's an opposing force: the centrifugal force due to inertia, which acts outward through the CG. So you have the pivot being pulled one way and the CG resisting, being thrown the other way. The result is that the magnet assembly tends to 'swing out' from the turn. It rotates around the pivot point, and that rotation produces the turning error. The magnet is no longer aligned with the magnetic meridian; it's been twisted by the forces of the turn. Now, why do we care more about turning errors than acceleration errors? The book gives two solid reasons. First, they are inherently of greater magnitude, because greater displacement of the magnet assembly is likely in turns. Second, turns occur more often and are likely to be more prolonged than linear accelerations. So in practice, you'll see this error more frequently and it'll be bigger. Let's walk through a specific example to make it concrete: turning from 045° to 315° in the northern hemisphere. That's a left-hand turn, passing through 000°M, which is north. So we're turning port, anticlockwise, through north. Remember, the magnet's CG 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, rotating the magnet assembly anticlockwise. Now, imagine there was no turning error. The magnet would remain stationary, and the aircraft would rotate 90° around it. The pilot would see 90° pass beneath the compass's lubber line. The lubber line is the fixed reference mark on the compass that you align with the aircraft's heading. But here's the catch. The aircraft is turning port, and the magnet assembly rotates in the same direction—anticlockwise. So although the aircraft has turned 90° around the compass, the magnet has been displaced and rotated in the same direction by a number of degrees, say 20°. The pilot will therefore only see 70° pass beneath the lubber line. The compass is termed sluggish—it lags behind the actual heading change. And here's the golden rule to remember: whenever the magnet rotates anticlockwise, it will over-read. That's the direction of the error. In this left-hand turn through north, the compass over-reads. So to tie it together: turning through north in the northern hemisphere, turning left, the compass is sluggish and over-reads. The magnet swings out of the turn, rotating anticlockwise, and the pilot sees fewer degrees pass than actually turned. That's the turning error in action.