The Direct Indicating Compass — Page 500, Lesson 499

Let’s pick up with the direct indicating compass, and I want to start with the heart of it — the magnet assembly and what makes it sensitive. The compass works because a magnet aligns with the Earth’s magnetic field. But for it to be a good instrument, it has to be sensitive — it has to respond to small changes in heading. The magnetic moment of the magnet can be increased by increasing the magnet’s length and/or its pole strength. Now, the magnetic moment is essentially a measure of how strongly the magnet will align with an external field. But here’s the catch: it’s undesirable to increase the magnet length, because a longer magnet would make the assembly bulky and harder to damp. So instead, the pole strength is increased by using two, four, or six short magnets, or a circular magnet, made of an alloy which will accept and retain the high degree of magnetism required. So the material matters — it has to be able to take on a strong magnetisation and hold it. Sensitivity is further increased by reducing friction. And there are three ways this is done. First, by using an iridium-tipped pivot in a jewelled cup — so the pivot point, the bearing, is iridium, and it sits in a jewelled cup, which gives a very low-friction bearing. Second, by lubricating the pivot with the liquid which fills the compass bowl — so the same liquid that fills the bowl also lubricates the pivot. And third, by reducing the effective weight of the magnet assembly acting down through the pivot, because the liquid that the magnet assembly is displacing is denser than air. That’s a buoyancy effect — the assembly is supported by the liquid, so it doesn’t press down as hard on the pivot, which reduces friction. Now, there’s a second requirement, and that’s aperiodicity. The magnetic assembly is required to be aperiodic, or ‘dead beat’. That means it should settle down quickly on a steady indication after being displaced by turbulence or manoeuvres. Any tendency to oscillate must be quickly ‘damped out’. So we don’t want the needle swinging back and forth — we want it to come to rest promptly. How is that achieved? First, several short magnets are used instead of one longer one. This keeps the mass of the assembly near the centre, so reducing the moment of inertia and consequently making any oscillations easier to damp out. Light alloy is utilized wherever possible in order to minimize the weight of the assembly framework. So the lighter and more compact the assembly, the easier it is to stop it oscillating. Second — and this is important — the primary purpose of the liquid in the compass bowl is to act as a damping liquid on the compass assembly. So the liquid isn’t just there for lubrication and buoyancy; its main job is damping. And there’s a distinction between two types of compass: the grid ring compass dampens oscillations more rapidly than the vertical card compass, due to the addition of damping wires. These wires are attached to the magnet assembly and also pass through the damping liquid. So the wires move through the liquid and that drag creates the damping effect. Now, serviceability checks. These are the things you’d check on the ground before flight. First, the compass body — check that there is no obvious damage such as dents or cracks. Any lighting system should be checked, as should the efficacy of the luminous paint. So you need to be able to read it at night, and the luminous paint has to be working. Second, the compass liquid. It should be checked and be free from three things. Sediment and discolouration — either of which would indicate corrosion which would result in increased pivot friction. So if you see sediment or the liquid has changed colour, that’s a sign of corrosion, and corrosion means more friction at the pivot, which reduces sensitivity. Bubbles — which would probably indicate a leaking seal. Turbulence and manoeuvres would cause any bubbles to move about, creating eddies which could disturb the magnet system. So bubbles are a sign of a leak, and they can physically disturb the magnet. And finally, the accuracy limit. Under CS-OPS1, the accuracy limit is ±10°. That’s the tolerance — the compass indication must be within 10 degrees of the true heading, under the relevant operating conditions. So to tie it together: sensitivity comes from strong magnets and low friction; aperiodicity comes from a compact, light assembly and the damping liquid, with damping wires on the grid ring compass; and serviceability is about checking the body, the lighting, and the liquid for sediment, discolouration, and bubbles. And the whole thing must be accurate to within ±10° under CS-OPS1.