Remote Indicating Magnetic Compass — Page 211, Lesson 250

Let’s start with the problem that this whole system is built to solve. The Direct Reading Compass — the classic magnetic compass you see on the panel — has three major limitations, and I want you to hold all three in your head because the gyro-magnetic compass is essentially a machine built to fix them. First, turning and acceleration errors. The Direct Reading Compass cannot be read accurately during a turn. That’s a hard physical limitation of the magnetic card swinging under gravity and magnetic forces. Second, the magnetic sensing element — the magnets on the vertical card — is contained inside the instrument itself. That means the sensing element has to sit right in front of the pilot so the card can be seen. And the cockpit is a terrible place for a magnetic sensor, because it’s close to sources of deviation: electric lights, electric motors, and ferrous metal. Ferrous metal means iron-containing metal, which disturbs the local magnetic field. So the sensor is forced to live in the worst possible magnetic environment. Third, the instrument is self-contained. There’s no way to take the magnetic heading it senses and feed it into other equipment. It just sits there showing you a card. Now, the Directional Gyro Indicator tries to solve some of these problems. It uses an air-driven or an electromechanical gyro. Turning and acceleration errors are eliminated, and you can take an output from it to drive other equipment. But here’s the catch: there is no magnetic input. A gyro drifts with time — it wanders off its reference. And with no magnetic input, there’s no automatic correction. The only fix is the pilot manually synchronizing the gyro to the direct reading compass at regular intervals. So you’ve traded one set of problems for another. What we actually want is a system that combines the best of both. The gyro gives short-term rigidity — it holds a heading steady through turns and accelerations, so it overcomes those turning and acceleration errors. But we need the longer-term monitoring of the earth’s magnetic field to catch the gyro when it starts to drift. So a servo system slaves the gyro to alignment with a magnetic input. That combination — gyro rigidity for the short term, magnetic field for the long term — is the gyro-magnetic compass. Now, this system goes by several names, and they all mean exactly the same thing: the Gyro-magnetic Compass, the Remote Indicating Compass, and the Slaved Gyro Compass. Don’t let the different labels confuse you — same system. Let me walk you through the basic system description. At its simplest, the system comprises five elements. First, the Magnetic Detector Unit. This is also often known as a flux valve or a flux detector. This is the sensor that senses the earth’s magnetic field. It’s the remote sensing element — it gets positioned away from the cockpit, in a part of the aircraft least affected by on-board electrical interference. That’s the whole point of “remote indicating” — the sensing element is no longer trapped next to the pilot. Second, the Heading Indicator. This is what most people refer to as “the compass.” It’s the display you actually read. Third, the Precession Amplifier. This may also be known as a slaving amplifier. It takes the magnetic signal and amplifies it. Fourth, the Precession Motor. This may also be known as a slaving or synchronizing motor. This is the servo that physically drives the gyro to align with the magnetic input. And fifth, the Horizontal Gyro. This is the gyro that provides that short-term rigidity. Now, here’s an important simplification for how these fit together. In simple systems, the horizontal gyro is directly connected to the compass card of the heading indicator via a bevel gear and a drive shaft. So the gyro physically turns the compass card through that gear and shaft. And that’s the assumption we carry through the description that follows — the gyro is mechanically linked straight to the card. So the picture is this: the magnetic detector unit senses the earth’s field remotely, away from cockpit interference. The precession amplifier processes that signal, and the precession motor drives the horizontal gyro to align with it. The gyro, in turn, drives the compass card through the bevel gear and drive shaft. The gyro gives you the stable short-term heading, and the magnetic detector keeps it honest over the long term, correcting drift automatically — no more manual synchronizing. That’s the foundation. The next step is to look at how the magnetic detector unit actually senses the field — that flux valve — and how the slaving loop closes. But first, make sure you’ve got the five elements and the logic of why each one exists.