The Direct Indicating Compass — Page 121, Lesson 139

Let’s start with what a compass actually is, because the definition sets up everything else. A compass is an instrument designed to indicate direction on the surface of the earth, relative to some known datum. A datum is just a reference line or point you measure from. The magnetic compass uses the horizontal component of the earth’s field as its directional datum. So the compass aligns itself with the earth’s magnetic field, and that field is your reference for direction. Now here’s the catch. The earth’s field is normally not aligned with the true meridian. The true meridian is the line running through the geographic north and south poles — that’s the most desirable datum from which to measure direction, because it’s fixed. But the magnetic field doesn’t line up with it. The angular difference between the true meridian and the magnetic meridian is called magnetic variation, which was covered in the previous chapter. So variation is the natural, geographic offset between true north and magnetic north. The purpose of a magnetic steering compass in an aircraft is to indicate heading — that is, the direction in which the aircraft is pointing. So when you read the compass, you’re reading the aircraft’s heading relative to magnetic north. But there’s a problem. Magnetic influences — things like iron and steel components in the aircraft, and electric currents — distort the earth’s field. When that happens, the compass magnet assembly deviates from the magnetic meridian. That’s called compass deviation. So we have two separate offsets: variation is the natural difference between true and magnetic north, and deviation is the error caused by the aircraft’s own magnetic influences pulling the compass away from the magnetic meridian. Keep those two distinct. Now, this chapter deals specifically with the direct indicating, or direct reading, magnetic compass. That’s the type where the pilot directly reads his heading in relation to the pivoted magnet assembly. In other words, you look at the instrument and read the heading straight off it — no remote sensing, no electronics in between. The basic type of direct reading magnetic compass used in aircraft is the vertical card design. And that’s what we’re focusing on. Let me show you what that looks like. That’s Figure 10.1, a vertical card compass. So let’s break down the vertical card compass. It’s also known as the B-type or E-type compass. It’s the direct reading compass in general use. In light aircraft, it’s usually the main magnetic heading reference. In larger aircraft, it serves as the standby compass — the backup if the primary heading system fails. Now, how is it built? It consists of a circular compass card attached directly to the magnet assembly. So the card and the magnet move together as one unit. That combined unit is suspended in liquid within the compass bowl. The liquid dampens the movement so the card doesn’t swing wildly. Then there’s a vertical lubber line on the glass window of the bowl. The lubber line is the fixed reference mark on the instrument. You read the heading off the compass card where it aligns with that vertical lubber line. So to summarise the whole picture: the magnet aligns with the horizontal component of the earth’s field, the card is fixed to the magnet so it rotates with it, the whole assembly floats in liquid inside the bowl, and you read your heading where the card meets the lubber line on the glass. That’s the vertical card compass — the B-type or E-type — the direct reading magnetic compass.