The Direct Indicating Compass — Page 500, Lesson 496

Let’s start with the vertical card compass, because that’s the one you’ll actually fly with most of the time. It’s also called the B-type or E-type, and it’s the direct reading compass in general use. In a light aircraft it’s usually the main magnetic heading reference, and in a larger aircraft it’s the standby compass. So you need to know it cold. The construction is simple. You have a circular compass card attached directly to the magnet assembly. That combined unit — card plus magnets — is suspended in liquid inside the compass bowl. The liquid damps the movement. On the glass window of the bowl there’s a vertical lubber line, and that lubber line is your fixed reference. As the aircraft turns, the card rotates, and the heading is read off the card against that lubber line. Now the grid ring compass, or P-type. This is the older design, found on older aircraft. It’s more accurate than the vertical card compass and it’s more stable. But there’s a price. It’s heavier, bulkier, and more expensive. And here’s the operational limitation you must remember: it can only be read in straight and level flight. Why? Because the grid ring has to be unclamped and aligned with the north reference before you can take a reading against the lubber line. You can’t do that while manoeuvring. There’s another difference. The grid ring compass achieves a greater periodicity — that’s the time it takes for the compass to settle back to its reading after being disturbed — and it does that by adding damping wires, which also rotate through the compass liquid. So the damping wires both stabilise it and give it that longer period. Now, before we go further, I want you to understand what the whole system is trying to do. The direct reading magnetic compass contains a pivoted magnet. That magnet must be able to align itself, and remain aligned, with the horizontal component of the Earth’s magnetic field. Note the word horizontal — it’s not the total field, it’s the horizontal component that does the steering. For the compass to succeed, three requirements must be satisfied. The magnet system must be horizontal, it must be sensitive, and it must be aperiodic. Let me unpack those. Horizontal means the magnet must be free to sit level, so it can sense the horizontal component properly. Sensitive means it must respond to small changes in heading — it has to be light and free enough to move. And aperiodic means it must settle to its final reading without oscillating back and forth — it should come to rest cleanly, not swing around the correct heading. That’s the whole game: a magnet that lies flat, reacts quickly, and stops without overshooting. That figure shows the equilibrium of the system in the northern hemisphere, viewed from the west. Keep it in mind as we go on — it’s the foundation for everything that follows about how this compass behaves in turns and accelerations.