Aircraft Magnetism — Page 500, Lesson 503

Let’s start with the core idea of this chapter: deviation. The compass needle would point exactly along the magnetic meridian — that is, the true north–south line of the Earth’s magnetic field — if it weren’t for the aircraft’s own internal magnetism deflecting it. That deflection is what we call deviation. So, deviation is the angular difference measured between the direction the compass needle actually takes up and the magnetic meridian itself. Now, deviation is named easterly or westerly, depending on which side the North-seeking end of the needle lies relative to the magnetic meridian. If the North-seeking end of the needle lies to the East of the magnetic meridian, we call it deviation East. If it lies to the West, we call it deviation West. So the naming is purely about which side of the true magnetic line the needle is sitting on. Let me give you a concrete example with numbers, because this is where it gets practical. Suppose your compass heading reads 095, and the deviation is minus 5 degrees West. That means your magnetic heading is 090. So, compass heading minus deviation West gives you magnetic heading. Conversely, if your compass reads 090 and the deviation is plus 5 degrees East, then your magnetic heading is 095. So, compass heading plus deviation East gives you magnetic heading. The sign convention is critical: West deviation is subtracted, East deviation is added, when converting from compass to magnetic. Now, how do we actually find out what the deviation is on a given aircraft? That’s where the compass swing comes in. The basic method is to compare the aircraft’s heading compass reading with the magnetic heading as defined by a high-quality ‘land or datum’ compass. This comparison is carried out in an area selected specifically for this purpose — a place where the magnetic field is clean and undisturbed. So, the aims of a compass swing are threefold. First, to observe and determine the deviations — the differences between Magnetic North, observed on a landing compass, and Compass North, observed in the aircraft — on a series of headings. Second, to correct and remove as much deviation as possible. And third, to record the residual deviation that is left after the compass has been adjusted. That residual deviation is what remains on your compass card, and you’ll use it for corrections in flight. Finally, I want to introduce where this deviation comes from. The magnetic deviation observed during a compass swing can be said to be derived from two sources: Hard Iron and Soft Iron magnetism. Hard iron is the aircraft’s permanent magnetism; soft iron is the induced magnetism from the Earth’s field acting on ferrous material. Together, this total field can, for our purposes, be resolved into two further combined components, which we call coefficients B and C. Those coefficients are how we mathematically break down the total deviation into manageable parts for correction. We’ll get into what B and C specifically represent as we go further, but for now, hold onto this: deviation comes from hard and soft iron, and we resolve that total field into coefficients B and C.