The Direct Indicating Compass — Page 491, Lesson 495

I want to walk you through the direct indicating compass, and we're starting right at the heart of it: the magnetic compass itself. First, let's pin down what a compass actually is. 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 point or line you measure from. Now, the magnetic compass uses the horizontal component of the Earth's field as its directional datum. So it's not the whole Earth's magnetic field — it's specifically the horizontal part of it that the compass uses as its reference for direction. Here's the catch, and it's an important one. 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 and doesn't move. But the magnetic field doesn't line up with that. The angular difference between the true meridian and the magnetic meridian is called magnetic variation. That's a term you'll have met in chapter 3, and it's the fundamental reason a compass reading isn't automatically your true direction. Now, what's the actual job of a magnetic steering compass in an aircraft? Its purpose is to indicate heading — that is, the direction in which the aircraft is pointing. So when you look at the compass, you're reading the direction the nose is aimed, not the direction you're travelling over the ground, just the direction the aircraft is pointing. But there's a complication. Magnetic influences — and these include 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. In other words, the magnet inside the compass no longer lines up with the magnetic north line it should be following. This is called compass deviation. So we have two separate corrections: variation, which is the angular difference between true and magnetic meridians, and deviation, which is the error caused by the aircraft's own magnetic influences distorting the field. The rules for applying both variation and deviation to the compass heading indication, in order to determine true heading, are detailed in the Navigation notes — that's the full chain: compass heading, apply deviation to get magnetic heading, apply variation to get true heading. Now, this chapter deals specifically with the direct indicating, or direct reading, magnetic compass. The name tells you exactly what it does — the pilot directly reads his heading in relation to the pivoted magnet assembly. So you're looking straight at the magnet and reading the heading off it, with no intermediate processing. There are two basic types of direct reading magnetic compasses used in aircraft. The first, and the more common one, is the vertical card compass. The second, which is less commonly used, is the grid ring compass. We'll be looking at the vertical card compass in detail, and you can see one in Figure 29.1. So, to summarise where we are: a compass indicates direction relative to a datum, the magnetic compass uses the horizontal component of the Earth's field, variation is the difference between true and magnetic meridians, deviation is the distortion caused by the aircraft's own magnetic influences, and the direct indicating compass is the type where you read the heading straight off the pivoted magnet assembly — either as a vertical card or a grid ring compass.