Aircraft Magnetism — Page 211, Lesson 245

We're starting a brand-new chapter now: the Remote Indicating Magnetic Compass. Before we get into the hardware, I want to set the stage with the occasions when we have to swing the compass — that's the process of calibrating it against known headings to correct for magnetic deviation. These are the exact conditions that demand a swing, and you need to know them cold. We swing the compass when compass components are installed or replaced. We swing it whenever the accuracy of the compass is in doubt. We swing it after a maintenance inspection, if required by the schedule. We swing it after a significant aircraft modification, repair, or replacement involving magnetic material. We swing it when carrying unusual ferromagnetic payloads — ferromagnetic meaning materials strongly attracted to a magnet, like iron or steel. We swing it when the compass has been subjected to significant shock. We swing it if the aircraft has been struck by lightning. We swing it after significant modification to the aircraft's radio or electrical systems. We swing it after the aircraft has been given a new theatre of operations, if the move involves a large change of magnetic latitude. And finally, we swing it if the aircraft has been in long-term storage standing on one heading. Now, why does this chapter exist at all? It's because the direct reading compass — the simple magnetic compass you see on the panel — has limitations, and so does the directional gyro indicator, the DGI. The direct reading compass is accurate in steady flight but it's subject to errors during turns and acceleration. The directional gyro is stable in a turn, but it drifts over time and has no reference to magnetic north. So the requirement for the gyro-magnetic compass is to combine the best of both: the magnetic reference of the compass with the stability of the gyro. Let me walk you through the basic system description. The gyro-magnetic compass has a detector unit, which senses the Earth's magnetic field, and a gyro that maintains a stable heading reference. The system compares the two and corrects the gyro's drift using the magnetic detector. In steady heading operation, the gyro drift is continuously corrected by the magnetic reference. In a turn, the gyro drift is small over the period of the turn, so the system doesn't need to correct it aggressively — that's why there's a rapid synchronization feature to bring the system back into agreement quickly when needed. The detector unit is called a flux valve. It's positioned in a part of the aircraft least affected by on-board electrical interference. The flux valve senses the direction of the Earth's magnetic field and produces an electrical signal proportional to the heading. The error signal comparison happens when the gyro's heading differs from the magnetic heading — the system generates an error signal and uses it to correct the gyro. The heading indicator displays the corrected heading to the pilot. When operating as a DGI, the system uses only the gyro, without magnetic correction. There's an annunciator that tells you when the system is in which mode — whether it's using magnetic correction or running purely on gyro. We also have to keep the gyro axis horizontal, because a gyro that's tilted will precess and give false readings. Finally, the system transmits heading output to other instruments, like the autopilot or the horizontal situation indicator. Let me show you the flux valve components. The circular plate is screwed to the underside of the wing. The black hemisphere protrudes out — that's the part that senses the magnetic field. And there's a direct drive shaft that connects the detector to the system. So the whole point of this chapter is: the direct reading compass has errors, the DGI drifts, and the gyro-magnetic compass solves both problems by using the flux valve to keep the gyro honest. That's the foundation. Now let's dig into each component in detail.