Aircraft Magnetism — Page 211, Lesson 245

We're starting a brand-new chapter today: the Remote Indicating Magnetic Compass. Before we get into the hardware, I want to set the stage with the list of occasions when we have to swing the compass. Swinging the compass is the process of calibrating it to correct for magnetic deviation caused by the aircraft's own magnetic fields. This isn't a casual checklist—it's a strict maintenance and operational requirement. So, when do we swing it? First, whenever compass components are installed or replaced. Second, whenever the accuracy of the compass is in doubt. Third, after a maintenance inspection, if required by the schedule. Fourth, after a significant aircraft modification, repair, or replacement involving magnetic material. Fifth, when carrying unusual ferromagnetic payloads—ferromagnetic means materials strongly attracted to magnets, like iron or steel. Sixth, when the compass has been subjected to significant shock. Seventh, if the aircraft has been struck by lightning. Eighth, after significant modification to the aircraft's radio or electrical systems. Ninth, after the aircraft has been given a new theatre of operations, if the move involves a large change of magnetic latitude. And tenth, if the aircraft has been in long-term storage standing on one heading. Now, why does this chapter exist? The direct reading compass—the simple magnetic compass you see in the cockpit—has limitations. And the Directional Gyro Indicator, or DGI, also has limitations. The DGI is a gyroscopic instrument that holds a heading reference, but it drifts over time. So the requirement here is for a gyro-magnetic compass—a system that combines the long-term accuracy of the magnetic compass with the short-term stability of the gyro. Let me walk you through the basic system description. The system has a detector unit, which is the flux valve—that's the magnetic sensing element. It measures the direction of the Earth's magnetic field. Then there's the heading indicator, which is the gyro. And there's an error signal comparison stage that compares the magnetic heading from the flux valve with the gyro heading. When they disagree, the system corrects the gyro. Here's how it works in steady flight. On a steady heading, the gyro drifts slowly. The flux valve provides a continuous, accurate magnetic reference. The error signal comparison detects the small difference between the gyro's heading and the magnetic heading, and it applies a correction to eliminate the gyro drift. So the gyro is constantly being slaved to the magnetic reference. Now, in a turn, the situation is different. During a turn, the gyro drift is small over the period of the turn. So the system doesn't need to make large corrections. But there's a problem: the flux valve can be unreliable during turns because of acceleration errors and banking. So the system uses a feature called rapid synchronization. This allows the gyro to be quickly re-aligned with the magnetic reference when conditions are suitable, typically after the turn is completed. Let me now focus on the detector unit, the flux valve. This is the heart of the magnetic sensing. The detector unit is positioned in a part of the aircraft least affected by on-board electrical interference. That's crucial—electrical currents in the aircraft create their own magnetic fields, which would corrupt the reading. So the flux valve is mounted remotely, away from those sources. Here's the figure of the magnetic detector unit. You can see it's a compact device. And here's another figure showing the flux valve components. Notice the circular plate—it's screwed to the underside of the wing. And the black hemisphere protrudes out. That hemisphere contains the sensing elements. The flux valve works on the principle of saturable core magnetometry. Inside, there are three legs or arms, typically arranged in a triangle, each wound with a primary and secondary coil. The Earth's magnetic field induces voltages in these coils, and the relative magnitudes of these voltages tell the system the direction of the magnetic field relative to the aircraft. Now, the error signal comparison. This is where the flux valve output and the gyro output are compared. The difference between them is the error signal. This error signal is used to drive a torque motor that precesses the gyro to align it with the magnetic heading. So the gyro is continuously corrected. The heading indicator itself—this is the instrument that displays the heading to the pilot. It operates as a DGI, a Directional Gyro Indicator, when the magnetic correction is not available or not desired. In that mode, it's just a free gyro holding a heading reference. There's also an annunciator. This is a flag or indicator that tells the pilot the system status. If the magnetic sensing is working and the gyro is slaved, the annunciator shows normal. If there's a failure or the system is in free gyro mode, the annunciator warns the pilot. Now, one more critical detail: keeping the gyro axis horizontal. The gyro in a directional indicator must have its spin axis horizontal to accurately measure heading. If the aircraft pitches or rolls, the gyro axis can tilt. So the system includes an erection mechanism—typically a pendulum or level sensor—that applies a torque to keep the gyro axis horizontal. This is separate from the magnetic slaving. Finally, transmitting heading output to other instruments. The heading information from this system isn't just for the pilot's display. It's transmitted electrically to other instruments and systems—autopilot, radar, navigation computers, flight management systems. So the heading indicator acts as a source of heading data for the entire aircraft. Let me summarize what we've covered. The remote indicating magnetic compass combines a flux valve for magnetic sensing with a gyro for stability. On steady heading, it corrects gyro drift. In turns, it relies on the gyro's short-term stability and uses rapid synchronization to re-align afterward. The detector unit is mounted away from electrical interference. The error signal comparison drives the correction. The heading indicator can operate as a DGI. The annunciator shows system status. The erection mechanism keeps the gyro axis horizontal. And the system transmits heading to other instruments. That's the foundation of this chapter. We'll build on each of these components in detail as we go through the sections.