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 instrument. The liquid is essential to the design — it damps the movement of the magnet system so the card settles quickly instead of oscillating. But the liquid brings two difficulties with it. First, the liquid expands and contracts with changes in temperature. To absorb that expansion, the compass has an expansion chamber, also called a Sylphon tube. Think of it as a flexible bellows that takes up the volume change so the case doesn’t crack or distort. Second, there’s a problem called liquid swirl. This happens in sustained turns. Because the liquid has viscosity — internal friction — it gets dragged around by the rotating case, and that swirling liquid carries the compass card with it, producing errors during the turn. The way to minimise liquid swirl is to choose a liquid with low viscosity, so it doesn’t drag the card as much. We’ll come back to liquid swirl later in the chapter. Now, what liquids have been used? Alcohol has been used, among others. But the main properties required of a compass liquid are these: a low coefficient of expansion, so it doesn’t change volume much with temperature; low viscosity, to minimise swirl; transparency, so you can read the card; a low freezing point, so it doesn’t solidify in cold conditions; a high boiling point, so it doesn’t vaporise in heat; and non-corrosiveness, so it doesn’t attack the metal parts inside. Now let’s move to deviation. This is a key concept. Deviation is produced by the iron and steel components in the aircraft itself. It is defined as the angle between the local magnetic meridian and the direction in which the compass magnets are lying. In plain terms: the aircraft’s own metal bends the magnetic field, so the compass magnets don’t align with the true magnetic north line — they align with the distorted field. The angle between those two is deviation. Deviation is named easterly, or plus, if the north-seeking ends of the magnets — the red ends — 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 tells you which side of magnetic north the compass is pulled toward. Here’s the important part: deviation varies with heading. It’s not a single fixed value — it changes as the aircraft turns, because the aircraft’s iron is oriented differently relative to the Earth’s field on each heading. So deviation has to be measured on a series of different headings. This is done by conducting a compass swing — that procedure is fully covered in the chapter on aircraft magnetism, so I won’t go deep into it here. After the swing, once deviation has been reduced as far as possible, the remaining error — the residual deviation — is recorded on a compass deviation card, which is located in the aircraft. Now, during the swing, you must simulate normal flying conditions as far as possible. That means engines running, electrical and radio services switched on, and the aircraft in a level flight attitude. Why? Because all those systems draw current and generate magnetic fields, and the aircraft’s attitude changes the field geometry — so you want the compass calibrated in the conditions it will actually be used in. It’s also critical that no ferromagnetic objects — things like tools or watches — are placed near the compass, because they would introduce unknown amounts of deviation. Ferromagnetic payloads should be stowed as far away from the compass as the loading limits permit. And if you have exceptionally large ferromagnetic loads, a compass swing may have to be carried out before flight, with the load aboard, so the compass is calibrated with that load’s field included. Finally, the accuracy requirement. Under EASA, Part-25, the requirement is ±10 degrees. That’s the tolerance the direct indicating compass must meet. So to tie it together: the liquid damps and stabilises the card, but brings expansion and swirl problems; deviation is the aircraft’s own magnetic influence, corrected by a compass swing and recorded on a deviation card; and the whole system must hold within ±10 degrees under EASA Part-25.