The Direct Indicating Compass — Page 491, Lesson 495

I want to walk you through the direct indicating compass, starting with the magnetic compass itself. This is the instrument that tells the pilot which way the aircraft is pointing, and it's the foundation for everything else in navigation. First, the definition. 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 needle aligns with the Earth's magnetic field, and that's 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 of longitude running through the geographic poles — that's the most desirable datum from which to measure direction, because it's fixed and doesn't move. But the magnetic field points to the magnetic poles, which are offset from the geographic ones. The angular difference between the true meridian and the magnetic meridian is called magnetic variation, which was discussed back in chapter 3. So variation is the angle between true north and magnetic north. 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 of the aircraft is aimed. Now, there's a problem. Magnetic influences — things like iron and steel components in the aircraft, 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 we have two separate corrections: variation is the difference between true and magnetic meridians, and deviation is the error caused by the aircraft's own magnetic influences pulling the compass away from the magnetic meridian. The rules for applying variation and deviation to the compass heading indication in order to determine true heading are detailed in the Navigation notes. So the chain is: compass heading, then apply deviation to get magnetic heading, then apply variation to get true heading. That's the sequence you'll need to remember. 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. So the magnet is mounted on a pivot, and the pilot reads the heading directly off the card attached to it. There are two basic types of direct reading magnetic compasses used in aircraft. The first is the vertical card compass — that's the common one. The second, less commonly used, is the grid ring compass. Let me show you what a vertical card compass looks like. So to summarize what we've covered: a compass indicates direction relative to a datum, the magnetic compass uses the horizontal component of the Earth's field, variation is the angle between true and magnetic meridians, deviation is the error from the aircraft's own magnetic influences, and the direct indicating compass lets the pilot read heading directly off the pivoted magnet assembly. The two types are the vertical card and the grid ring.