Remote Indicating Magnetic Compass — Page 220, Lesson 261

Let's start with the control panel, because that's where you, as the pilot, actually interact with this system. On a modern compass controller, you have a heading selector control. You rotate that to select a desired heading, and a marker on the instrument — we call it the "bug" — physically indicates the heading you've selected. So the bug is your reference point on the dial. Now, the system is normally fed by a magnetic input from the flux valve. If that magnetic input fails, the instrument gives you a warning in the form of a heading warning flag. That flag is your cue that something is wrong with the magnetic sensing. Here's the key concept I want you to grasp: this gyro-magnetic compass can operate in two distinct modes. The normal mode is called SLAVED mode. In SLAVED mode, the gyro is slaved — in the long term — to the input from the flux valve. That means the magnetic signal continuously corrects the gyro, keeping it aligned to magnetic north over time. But there's a second mode called FREE mode. This is when the compass operates as a DGI — a Directional Gyro Indicator. When would you use FREE mode? Two situations: if the magnetic input from the flux valve fails, or if the magnetic input becomes unreliable because you're flying close to one of the earth's magnetic poles. Near the poles, the earth's magnetic field lines converge and dip, so the flux valve signal becomes unreliable. In FREE mode, the gyro is no longer tied to the magnetic input, so it acts as a free gyro — a DGI. And because a free gyro drifts over time, you'll need to re-set it periodically to a directional reference, like a standby compass or another source of aircraft heading. Now let's look at the physical switching. On the controller, there's a FREE/SLAVE switch. When it's at SLAVE, the compass operates as I described — the gyro is slaved to the flux valve input. When you move the switch to FREE, here's what happens mechanically: the magnetic signal from the flux valve is disconnected, the rotor/stator comparison ceases, and the gyro is no longer tied in azimuth. It becomes a free gyro. When you're in FREE mode, you correct the indicated heading to an external datum heading using a CCW/CW control switch — that's counter-clockwise/clockwise. This switch is spring-loaded to the central position, so it returns to center when you release it. You hold it one way to slew the heading counter-clockwise, the other way to slew it clockwise, and you adjust until the indicated heading matches your external reference. Now, here's something interesting about normal SLAVE operation. During normal flight, there's continuous slight motion — oscillations of heading and vibration. This means the rotor/stator comparison of the magnetic flux valve signal against the gyro shaft position continuously generates very small error signals. The error signal, and therefore the precession amplifier, are continuously "hunting." That's not a fault — that's how the system is designed to work. It's constantly making tiny corrections. These error signals pass through an indicator on their way from the amplifier to the precession motor. That indicator is called an annunciator. So the annunciator is essentially a monitor that shows you the error signal activity — it's the visual representation of that hunting behavior. You'll see it on the compass control panel. Let me trace the full circuit for you, because it's important to see how the components connect. The Detector Unit — that's the flux valve — produces an error signal, which is an AC signal. That error signal goes to the precession amplifier, where it's amplified. The amplified error signal then passes through the annunciator, and from there it drives the precession motor. The precession motor acts on the horizontal gyro through a direct drive shaft. So the chain is: flux valve → error signal (AC) → amplifier → annunciator → precession motor → gyro. So to summarize the whole picture: the flux valve senses the earth's magnetic field, compares it against the gyro's shaft position, generates small error signals, amplifies them, monitors them through the annunciator, and uses them to precess the gyro to keep it aligned. In FREE mode, you break that magnetic link and manually correct the gyro using the CCW/CW switch. And if the magnetic input fails entirely, the heading warning flag tells you, and you can fall back to FREE mode as a DGI.