The Direct Indicating Compass — Page 500, Lesson 496

I want to walk you through the direct indicating compass, and we're starting with the two main types you'll meet: the vertical card compass and the grid ring compass. Let's begin with the vertical card compass. It's also known as the B-type or E-type, and it's the direct reading compass in general use. That means it's the one you'll see most often. It's usually the main magnetic heading reference in light aircraft, and in larger aircraft it serves as the standby compass. So in a big jet, this is your backup heading source; in a small plane, it's your primary one. Here's how it's 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 damps the movement. Then there's a vertical lubber line on the glass window of the bowl, and that line is your fixed reference — you read the heading off the compass card against that lubber line. Now the second type: the grid ring compass, also called the P-type. You'll find this on older aircraft. It's more accurate than the vertical card compass and it's more stable. But there are trade-offs. It's heavier, bulkier, and more expensive. And there's a significant operational limitation: 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. So you can't just glance at it during a turn — you have to set it up first. The grid ring compass also differs from the vertical card compass in how it achieves greater periodicity. Periodicity here refers to the oscillation behaviour of the compass — how it swings and settles. The grid ring compass achieves greater periodicity by the addition of damping wires, and those wires also rotate through the compass liquid. So the damping wires both stabilise the oscillation and move through the liquid, which adds to the damping effect. Now, let's step back and think about what makes any direct reading magnetic compass work. The direct reading magnetic compass contains a pivoted magnet, and that magnet must be able to align itself — and remain aligned — with the horizontal component of the Earth's magnetic field. That's the key principle. The Earth's field has both a horizontal and a vertical component; the compass magnet responds to the horizontal component, and it must stay aligned with it. For the compass to succeed, certain requirements must be satisfied, and the most important are these three. The magnet system must be horizontal. It must be sensitive. And it must be aperiodic. Let me unpack each one. Horizontal means the magnet assembly must be able to sit level, so it can respond to the horizontal component of the field. Sensitive means it must respond readily to small changes in heading — it can't be sluggish. And aperiodic means it must settle to its final position without oscillating back and forth — it should come to rest directly, not swing around the correct heading. That's why the damping matters, and that's why the liquid and the damping wires are so important. So to tie it together: the vertical card compass is your everyday direct reading compass, simple and compact. The grid ring compass is the older, more accurate, more stable — but heavier and more limited — option. And regardless of type, the magnet system must be horizontal, sensitive, and aperiodic to do its job properly.