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 at a really important point — the annunciator, and then how the gyro is kept horizontal, and finally how heading gets sent to other instruments. First, the annunciator. That's the little indicator light or flag on the compass control panel. It's useful to the pilot 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." Second, on systems where the pilot has to synchronize manually, it indicates which way to turn the compass. So it's both a status light and a direction cue. Now, the big topic: keeping the gyro axis horizontal. Gyro wander takes two forms — drift and topple. Drift is the tendency of the gyro to move in azimuth, and that's overcome by slaving the gyro to the flux valve output, which makes it a tied gyro in azimuth. But even with drift handled, the gyro would still topple over a period of time unless we prevent it. So it 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 here's the key point — both systems use a levelling switch and a torque motor. 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. To tie the gyro to the vertical, mercury gravity switches are used. 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 at a slow precession rate, so it doesn't react wildly to temporary departures from the horizontal, like turns, accelerations, climbs, and descents. Finally, transmitting heading output to other instruments. One of the advantages of the gyro-magnetic compass over the simple direct reading compass is that it can 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. So let me tie that together. The gyro is slaved to the flux valve to stop drift, and tied to either the yaw axis or gravity to stop topple. The levelling switch senses the error, the torque motor applies the correcting force, and precession slowly brings the gyro back to horizontal. Then the heading is picked off the drive shaft and sent out electrically through the Selsyn unit to feed other instruments. That's the complete picture of how the remote indicating magnetic compass keeps itself accurate and shares that heading with the rest of the aircraft.