The Direct Indicating Compass — Page 121, Lesson 143

Let’s start with the liquid inside the compass, because it’s the heart of the whole design. The liquid is essential — it damps the movement of the compass card so it doesn’t swing wildly. But two problems can arise with it. First, the liquid expands and contracts with temperature changes. To absorb that expansion, the compass has an expansion chamber, and the specific device used is called a Sylphon tube. Think of it as a flexible bellows that takes up the volume change so the case doesn’t burst or create pressure problems. Second, there’s a phenomenon called liquid swirl. This happens during sustained turns — the liquid, because of its viscosity, keeps swirling after the compass card has settled, dragging the card with it and causing errors. The key point is that the liquid’s viscosity is the cause, so the liquid chosen must have low viscosity to minimize liquid swirl. We’ll come back to liquid swirl later in the chapter. Now, what liquids are used? Alcohol has been used historically. But the main properties required of a compass liquid are six: low coefficient of expansion, low viscosity, transparency, low freezing point, high boiling point, and non-corrosiveness. Each one matters — low expansion so the Sylphon tube doesn’t have to work too hard, low viscosity to reduce swirl, transparency so you can read the card, low freezing point so it doesn’t solidify in cold conditions, high boiling point so it doesn’t vaporize in heat, and non-corrosiveness so it doesn’t attack the internal components. Now let’s move to deviation. Deviation is produced by the iron and steel components in the aircraft. It’s defined as the angle between the local magnetic meridian and the direction in which the compass magnets are lying. So, the magnetic meridian is the reference — the true direction of magnetic north at that location — and deviation is how far the compass magnets are pulled away from that by the aircraft’s own iron and steel. Deviation is named easterly, or plus, if the north-seeking — that’s the red — ends of the magnets point to the east of magnetic north. It’s named westerly, or minus, if the north-seeking ends point to the west of magnetic north. So the sign convention is tied to which side of magnetic north the red ends are deflected. Here’s the critical thing: deviation varies with heading. It’s not a single fixed value — it changes as the aircraft turns. So it has to be measured on a series of different headings. This is done by conducting a compass swing, which is covered fully in the chapter on aircraft magnetism. During the swing, you reduce deviation as far as possible, and the remaining amount — the residual deviation — is recorded on a compass deviation card, which is located in the aircraft. Now, how should the swing be conducted? Normal flying conditions should be simulated as far as possible: engines running, electrical and radio services switched on, and the aircraft in a level flight attitude. That way the magnetic environment matches what you’ll actually have in flight. And it’s absolutely vital that no ferromagnetic objects — like tools or watches — are placed near the compass, because they’d introduce unknown amounts of deviation. Ferromagnetic payloads should be stowed as far away from the compass as permissible within the loading limits. And if you have exceptionally large ferromagnetic loads, a compass swing may have to be carried out before flight with the load aboard — because the load itself changes the magnetic field around the compass. Finally, accuracy. The EASA Part-25 requirement for the direct indicating compass is ±10°. That’s the tolerance the compass must meet. So to tie it together: the liquid with its Sylphon tube and low-viscosity choice keeps the compass stable and readable; deviation from the aircraft’s iron and steel is measured, minimized, and recorded on the deviation card; and the whole system must hold within ±10° under Part-25.