Remote Indicating Magnetic Compass — Page 226, Lesson 264

I want to walk you through the remote indicating magnetic compass, and we're going to pick it up right at the point where we're looking at the annunciator and then how the gyro stays horizontal. First, the annunciator. This is the little indicator on the compass control panel that tells the pilot what the compass is doing. It's useful for two main reasons. First, it's an indication that magnetic monitoring of the gyro is actually taking place — it shows that the compass is "synchronized." In other words, the gyro's heading is matched to the magnetic heading from the flux valve. Second, on systems where the pilot has to synchronize manually, the annunciator indicates which way to turn the compass to bring it back into sync. So it's both a status light and a direction guide. Now, the big problem we're solving here is keeping the gyro axis horizontal. Gyro wander takes two forms — drift and topple. Drift is the tendency of the gyro to move away from its heading reference over time. We overcome drift by slaving the gyro to the flux valve output, which makes it a "tied gyro" in azimuth — meaning its heading is locked to the magnetic reference. But that's only half the problem. The gyro would still topple — that is, tilt away from the horizontal — over a period of time, unless we prevent it. So the gyro needs to be tied either to the aircraft yaw axis, or to gravity, in order to keep it erect. Both the yaw axis and the vertical as defined by gravity have been used as the datum in various models of compass. And both systems use the same two components: a levelling switch and a torque motor. Let me explain how each works. To tie the gyro to the yaw axis, the inner and outer gimbals are maintained at 90° to each other by a system of commutators, insulating strips, and brushes. That's the mechanical arrangement that senses when the gyro is no longer level. To tie the gyro to the vertical instead, mercury gravity switches are used — these sense the direction of gravity. Either way, the correcting signals are passed to a torque motor, which applies a rotational force to the gyro in the yaw axis. The resulting precession causes the gyro to return to the horizontal. But — and this is important — it happens at a slow precession rate. That slow rate is deliberate, so the gyro does not react wildly to temporary departures from the horizontal, such as turns, accelerations, climbs, and descents. If it corrected too fast, every manoeuvre would cause a violent swing in the heading indication. Now let's look at how the heading output gets transmitted to other instruments. One of the advantages of the gyro-magnetic compass over the simple direct reading compass is the facility to electrically transmit heading information, to use as an input into other instruments. The information is picked off from the drive shaft between the gyro and the compass card. The transmitting and receiving device is called a Selsyn Unit. That's the component that takes the mechanical rotation of the drive shaft and converts it into an electrical signal that can be sent to other instruments in the aircraft. So to summarise the whole picture: the flux valve gives the magnetic reference, the gyro holds the heading, the levelling switch and torque motor keep the gyro horizontal, and the Selsyn unit distributes that heading information electrically to the rest of the aircraft's systems.