Earth Magnetism — Page 56, Lesson 63

I want to walk you through the concept of magnetic dip angle, which is fundamental to understanding how a magnetic compass behaves in an aircraft. Let’s start with the Earth’s magnetic field. If you look at the lines of magnetic force that surround the Earth, they don’t run perfectly parallel to the surface. Imagine an aircraft at three different positions — call them A, B, and C — at different latitudes. At each location, the lines of force make a different angle relative to the horizontal. That angle is what we call the Angle of Dip, sometimes just called dip. The total magnetic field along the line of force is labelled T. We can break that total field down into two components: a horizontal component H and a vertical component Z. Let’s talk about the vertical component first. The vertical component Z is of no use for finding horizontal direction — in fact, it’s undesirable for two specific reasons. First, the vertical component causes the needle of a direct-reading magnetic compass to dip downward from the horizontal. Compass designers try to correct for this using something called pendulous suspension, but even with that correction, the needle still hangs down to some extent. That means the centre of gravity of the needle is no longer directly below the centre of suspension, and that misalignment is what produces the well-known turning errors and acceleration errors you’ll learn about in detail later. Second, the vertical component of the Earth’s field induces what we call vertical soft-iron magnetism in the aircraft structure itself. That induced magnetism increases the deviation — the error caused by the aircraft’s own magnetic materials interfering with the compass. Now, the horizontal component H is the part that actually matters for navigation. This is the component that the compass needle detects in order to point toward magnetic north. We call it the directive force — it’s the force that directs the compass. At the magnetic equator, the directive force H is at its strongest, approaching the value of the total field T. At the same time, the vertical component Z approaches zero, and so does the angle of dip. As you move away from the equator toward higher latitudes, the directive force H decreases as the angle of dip increases, and vice versa — they are inversely related. When you approach either the North Magnetic Pole or the South Magnetic Pole, the horizontal component H drops to nearly zero strength, while the vertical component Z approaches the value of the total field T. So how do we measure magnetic field strength? It’s measured in units called microteslas, abbreviated µT. There’s a generally accepted figure: when the horizontal component of the Earth’s field drops to 6 microteslas, it becomes too weak for a compass to detect reliably. Now, in practice, the actual detection threshold depends on the design of the particular compass you’re using, but 6 µT is the notional figure that’s normally quoted in aviation. Finally, the maximum possible dip angle is 90 degrees, and that occurs directly overhead the North and South Magnetic Poles. So to summarise: the angle of dip is the angle the total magnetic field makes with the horizontal. The vertical component causes compass dip and induces soft-iron deviation. The horizontal component is the directive force that actually steers the compass needle, and it weakens as you move toward the magnetic poles, becoming effectively unusable below about 6 microteslas.