The Direct Indicating Compass — Page 121, Lesson 139

Let’s start with the big picture. 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 earth’s magnetic field has a direction, and the compass uses the horizontal part of that field as its reference. 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 — the most desirable datum from which to measure direction. The angular difference between the true meridian and the magnetic meridian is called magnetic variation. That’s the term you’ll see on charts, and it’s covered in the previous chapter, so we just need to know it exists. The purpose of a magnetic steering compass in an aircraft is to indicate heading — the direction in which the aircraft is pointing. So it’s not just a general direction finder; it’s specifically telling you which way the nose is aimed. Now, a key problem. Magnetic influences — iron and steel components, and electric currents — distort the earth’s field. That distortion makes the compass magnet assembly deviate from the magnetic meridian. This is called compass deviation. So variation is the angular difference between true and magnetic meridians, a property of the earth’s field itself. Deviation is the error caused by the aircraft’s own magnetic influences. Two different things, and you’ll need to keep them straight. This chapter deals with the direct indicating, or direct reading, magnetic compass. That means the pilot directly reads his heading in relation to the pivoted magnet assembly. There’s no remote sensing or transmission — you look at the compass and read the heading straight off it. The basic type of direct reading magnetic compass used in aircraft is the vertical card design. Let me walk you through how it’s built. The vertical card compass — also known as the B-type or E-type — is the direct reading compass in general use. It’s usually the main magnetic heading reference in light aircraft, and the standby compass in larger aircraft. So in a big airliner it’s the backup; in a small plane it’s the primary. Here’s the construction. It consists of a circular compass card attached directly to the magnet assembly. So the card and the magnet move together as one unit. This combined unit is suspended in liquid within the compass bowl. The liquid damps 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, and you read the heading off the compass card against that line. So the whole picture: the magnet aligns with the earth’s magnetic field, the card is attached to the magnet, and you read the heading where the card meets the lubber line. That’s the vertical card compass in a nutshell.