Earth Magnetism — Page 62, Lesson 66

Let’s pick up with Earth magnetism and move into a practical consequence of the dip angle we just looked at. I want to talk about deviation. First, a quick recap of where we are. We already know that the difference between True North and Magnetic North is called variation. That’s a correction we apply because the magnetic pole doesn’t line up with the geographic pole. But there’s another correction needed, and it’s for the difference between Magnetic North and the direction a compass needle actually points inside an aircraft. Why aren’t they the same? The reason is that the aircraft itself is full of magnetic influences — metal structure, electric currents flowing through wiring, ferrous engine components, and so on. The whole aircraft acts partially as a magnet itself. So the compass needle is mainly attracted to the Earth’s Magnetic North Pole, but it’s also partially attracted to the magnetism within the aircraft. That aircraft magnetism deflects the compass needle away from Magnetic North to a new direction. We call that direction Compass North. The difference between Magnetic North and Compass North is called Deviation. Let me give you the formal definition. Deviation is defined as the angle measured at a point between the direction indicated by a compass needle and the direction of Magnetic North. So it’s an angular error introduced by the aircraft’s own magnetic field. Deviation is termed East or West according to whether Compass North lies to the East or West of Magnetic North. So if the compass points to the east side of Magnetic North, that’s East deviation. If it points to the west side, that’s West deviation. Deviation can also be quoted as plus or minus. For instance, a deviation of minus three degrees is the same as three degrees West deviation. Plus is East, minus is West. The reason for this sign convention is that 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. In other words, you take the reading from your compass — that’s Compass heading — and you add or subtract the deviation to get Magnetic heading. Now, before we leave this page, I want to tie it back to the dip angle we discussed earlier. The excerpt reminds us that the relationship between the horizontal component H and the dip angle is not quite as simple as the diagram might suggest, because of irregularities in the pattern of the Earth’s field and changes with position and time of the total magnetic force T. The angle of dip at Oxford is about 66 degrees. But the use of pendulous suspension in a simple Direct Reading Compass reduces the dip of the magnetic compass assembly to about 2 degrees. However, that small residual angle — that two degrees — is still sufficient to give the well-known turning and acceleration errors. So even though we mechanically reduce the dip effect, we don’t eliminate it entirely, and that leftover tilt is what causes those errors during manoeuvres. So to summarise: variation is the difference between True and Magnetic North, caused by the Earth’s own field. Deviation is the difference between Magnetic North and Compass North, caused by the aircraft’s own magnetic influences. Deviation is named East or West, or given as plus or minus, and you apply it to the compass heading to get magnetic heading.