We're talking about ferromagnetic materials — Page 106, Lesson 128

Let me walk you through what's happening here, because this is the heart of why a compass on an aircraft doesn't just point north — it points somewhere slightly off, and we have to understand why. We're talking about ferromagnetic materials. The word "ferro" means iron, and "magnetic" means, well, magnetic. So ferromagnetic materials are ones that can be magnetized — they can become magnets themselves. The classic example is iron and steel. Now, steel is just iron that's been alloyed — that means mixed — with other substances. The excerpt names them: carbon, cobalt, nickel, chromium, and tungsten. So when you take iron and add a bit of carbon, you get steel. Add cobalt, nickel, chromium, or tungsten, and you're still in the ferromagnetic family. Here's the critical part for an aircraft: these ferromagnetic metals can become magnetized. Think about what happens in an aircraft — you have engines running, electrical currents flowing, all sorts of magnetic fields around. If a piece of iron or steel in the airframe gets exposed to those fields, it can pick up its own magnetism. Once it's magnetized, it becomes a little magnet itself, and that little magnet will pull on the compass needle. The result is what we call deviation — the compass doesn't show the true magnetic heading; it shows a heading that's been bent by all these magnetized bits of metal in the aircraft. Now, the good news — and this is the second half of the excerpt — is that many materials used in aircraft construction are non-magnetic. They don't affect the compass at all. The excerpt gives us the examples: aluminium, duralumin, brass, copper, plastic, and paint. These are non-ferrous — meaning they contain no iron — so they can't be magnetized, and they won't cause deviation. So here's the picture you should hold: the aircraft is built from a mix of materials. The ferromagnetic ones — iron and steel — are the troublemakers; they can become magnetized and bend the compass. The non-ferrous ones — aluminium, duralumin, brass, copper, plastic, paint — are the good citizens; they're magnetically inert and leave the compass alone. When we talk about compass deviation in an aircraft, we're really talking about the influence of those ferromagnetic parts, and understanding which materials are which is the first step in managing that error.