Earth Magnetism — Page 62, Lesson 66

Let's pick up right where the magnetic field leaves off and move into the practical problem every pilot faces: the compass doesn't point where you think it does. We've already dealt with variation — the difference between True North and Magnetic North. Now we have a second, separate correction, and it's called Deviation. And the reason we need it is that the aircraft itself is full of magnetic influences. Metal structure, electric currents running through the wiring — all of that makes the aircraft act partially as a magnet itself. So here's the picture. The compass needle is mainly attracted to the Magnetic North Pole, but it's partially attracted to the magnetism within the aircraft. That aircraft magnetism deflects the needle away from Magnetic North to a new direction, and we call that direction Compass North. The difference between Magnetic North and Compass North is what we call Deviation. Let me give you the formal definition, because you'll need it word-for-word. Deviation is defined as the angle measured at a point between the direction indicated by a compass needle and the direction of Magnetic North. Now, how do we name it? Deviation is termed East or West according to whether Compass North lies to the East or West of Magnetic North. So if the needle is pulled to the east of Magnetic North, that's East deviation. Pulled to the west, that's West deviation. But there's a second way to quote it, and this is where pilots often get tangled up. Deviation can also be quoted as plus or minus. For instance, a deviation of -3° is exactly the same as 3°W deviation. Plus is East, minus is West. Why the plus and minus convention? Because deviation is seen as a correction to be applied to the compass in order to establish Magnetic heading. So when deviations are quoted as plus or minus, they are to be applied to the compass heading in order to give magnetic heading. That's the key operational rule: you take your compass heading, you apply the deviation with its sign, and you get magnetic heading. Let me make sure the sign convention is crystal clear, because it's easy to flip. Minus means West, plus means East. So if your compass reads a heading and the deviation is -3°, you subtract 3° to get magnetic heading, and that's the same as saying 3° West deviation. If it's +3°, you add 3°, and that's 3° East. Now, before we leave this, I want to tie it back to something we touched on earlier — the dip angle. You remember we talked about the angle of dip, the vertical tilt of the magnetic field. At Oxford, the angle of dip is about 66°. But here's the clever bit: the simple Direct Reading Compass uses a pendulous suspension, and that reduces the dip of the magnetic compass assembly down to about 2°. So the needle is nearly horizontal, even though the field itself is steeply inclined. But — and this is the important warning — that small residual angle of about 2° is still enough to give the well-known turning and acceleration errors. So even though we've mechanically reduced the dip, we haven't eliminated its effects. Those errors are a direct consequence of that residual dip. And one more subtlety about the relationship between H — the horizontal component of the field — and the dip angle. I want to be honest with you: the relationship is not quite as simple as the diagram suggests. That's because of irregularities in the pattern of the Earth's field, and because the total magnetic force T changes with position and time. So the neat geometric picture is a useful model, but the real field is messier. So to summarise where we are: we have True North, we have Magnetic North, and now we have Compass North. Variation corrects True to Magnetic. Deviation corrects Compass to Magnetic — applied with the plus/minus sign convention, plus East, minus West. And the whole reason deviation exists is that the aircraft's own magnetism pulls the needle away from Magnetic North.