Remote Indicating Magnetic Compass — Page 220, Lesson 261

Let's start with the remote indicating magnetic compass. This is the system that gives you your heading readout in the cockpit, and it's built around a gyro-magnetic principle. I want to walk you through how the pilot interacts with it, how it fails, and how you can keep it working when the magnetic input goes away. First, the pilot's controls. You have a heading selector control, which you rotate to select a desired heading. That selected heading is shown by a marker on the instrument, usually called a "bug." So the bug sits on the dial at whatever heading you've dialled in. Now, if the magnetic input from the flux valve fails, the system 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. Now, let's talk about operating as a DGI. That stands for Directional Gyro Indicator. If the magnetic input from the flux valve fails, or if it becomes unreliable because the aircraft is too close to one of the earth's magnetic poles, you can still operate the gyro-magnetic compass in gyro mode only. In that mode, it acts as a DGI, and here's the key limitation: it will need to be re-set periodically to a directional reference, such as a standby compass or another source of aircraft heading. That's because a free gyro drifts over time; it doesn't hold its heading reference on its own. Now, the terminology. When the compass operates as a DGI, that's called FREE mode. Its normal, magnetically monitored operation is called SLAVED mode. So you have two distinct modes: SLAVED, where the gyro is continuously corrected by the magnetic sensor, and FREE, where it's just a free gyro. Let me show you the controller. Figure 17.12 shows a typical modern compass controller. There's a FREE/SLAVE switch on it. When that switch is at SLAVE, the compass operates as I described earlier, with the gyro slaved, in the long term, to the input from the flux valve. When you move the switch to FREE, the magnetic signal from the flux valve is disconnected. The rotor/stator comparison ceases, and the gyro is no longer tied in azimuth. It acts as a free gyro, a DGI. Now, when the Heading Indicator is in FREE, the pilot adjusts the indicated heading to correct it to an external datum heading. You do this using the CCW/CW control switch, which stands for counter-clockwise/clockwise. That switch is spring-loaded to the central position, so it returns to centre when you release it. You hold it one way to slew the heading one direction, the other way to slew it the other. Now, let's talk about the annunciator. During normal flight in SLAVE mode, there's usually continuous slight motion. This comes from oscillations of heading and from vibration. What that means is that the rotor/stator comparison of the magnetic flux valve signal against the gyro shaft position continuously generates very small error signals. So the error signal, and therefore the precession amplifier, are continuously "hunting." That's normal. That's how the system is designed to work. It's constantly making tiny corrections. Here's the key point: these error signals pass through an indicator on their way from the amplifier to the precession motor. That indicator is called an annunciator. You can see an example in Figure 17.12, and Figure 17.13 shows the annunciator in circuit. So the annunciator is essentially showing you that the system is alive and hunting, making those small corrections. Let me trace the signal path for you. You have the Detector Unit, which is the flux valve, sensing the earth's magnetic field. That produces an error signal, which is AC, alternating current. That error signal goes to the amplifier, which amplifies it. The amplified error signal then passes through the annunciator on its way to the precession motor. The precession motor then torques the horizontal gyro to correct its heading. And the gyro drives the heading indicator through a direct drive shaft. So the whole loop is: flux valve senses, error signal generated, amplified, shown on the annunciator, sent to the precession motor, which corrects the gyro, which drives the heading indicator. So, to summarise the key points: you have SLAVED mode for normal operation, FREE mode for when magnetic input fails or is unreliable near the poles. In FREE mode, you use the CCW/CW switch to re-set the heading against an external reference. And the annunciator tells you the system is hunting, which is normal in SLAVED mode. The heading warning flag tells you the magnetic input has failed.