Remote Indicating Magnetic Compass — Page 220, Lesson 255

Let’s start with the big picture, because this is the heart of the whole system. The remote indicating magnetic compass is a way of sensing the Earth’s magnetic field from a part of the aircraft that is magnetically clean, and then displaying that heading somewhere else — usually on the pilot’s instrument panel. The key component that does the sensing is called the detector unit, and the sensing element inside it is the flux valve. I want to walk you through the detector unit first. Its job is to sense the direction of the Earth’s magnetic field. Now, the critical thing here is where it’s mounted. The detector unit is positioned in a part of the aircraft least affected by on-board electrical fields — usually the wing tip or the tail fin, where any aircraft-generated magnetic disturbances are at a minimum. That placement is deliberate: if you put a magnetic sensor near the generators, the bus bars, or the heavy electrical wiring, the aircraft’s own fields would swamp the Earth’s field and the reading would be useless. So we go to the wing tip or tail fin, where the magnetic environment is cleanest. Inside that detector unit is a pendulous magnetic detecting element. Let me unpack that. “Pendulous” means it hangs, like a pendulum, so it can swing. It’s mounted on a Hooke’s Joint — that’s a universal joint, the same kind of mechanical coupling that lets a shaft transmit rotation while allowing angular movement. The purpose of that joint is to let the detector swing within limits of 25° about the pitch and roll axes, but it allows no rotation in azimuth. So the element can tilt forward and back, and side to side, up to 25 degrees each way, but it cannot rotate around the vertical axis. That’s crucial, because the whole point is to measure heading — if the element could spin in azimuth, it would lose its reference to the aircraft’s direction. Now, the unit itself is contained in a sealed case partially filled with oil. The oil is there to dampen any oscillations created during flight. Think about it — the aircraft is vibrating, turning, bumping through turbulence. Without the oil, the pendulous element would swing and oscillate wildly and the reading would be unstable. The oil provides viscous damping to settle it down. Let me show you the physical arrangement. — that’s the magnetic detector unit. And shows the flux valve components. Look at that figure: the circular plate is screwed to the underside of the wing. The black hemisphere protrudes out into the airflow — and that hemisphere is simply a protective cover for the flux valve inside. It’s not a sensor itself; it’s just shielding the delicate mechanism from the slipstream. The cable carrying the signals passes along inside the structure of the wing, so the wiring is protected and routed internally. Now, the primary component inside that cover is the flux valve itself. It’s a 3-spoked device, fixed in azimuth but with some freedom in the vertical, to allow alignment with the plane of the Earth’s magnetic field. So again, the same idea as the detector element: it can tilt vertically to align with the field, but it’s locked in azimuth so it always measures the same directional reference. Let’s look at the legs of the flux valve. shows all three legs together, as they are in the actual unit. Each leg ends in a curved shape called a ‘rams’ horn’ — and I want to be clear about what that does. The curved rams’ horns at the end of each of the three legs are simply to improve magnetic flux gathering efficiency, but they do not affect the principle. The flux valve would detect even without them. So they’re an efficiency improvement, not a functional necessity. To understand how the flux valve actually works, we start by considering just a single leg, without the rams’ horns. shows the flux fields. Here’s the operating principle: alternating current is fed to the coil wound around the centre post. That AC current, in turn, produces fields of opposite sign in the top and bottom legs of the flux valve. So the centre post is wound with a coil, AC is applied, and that induces magnetic fields in the two legs — one of opposite polarity to the other. Now, here’s where the sensing happens. The Earth’s magnetic field is also present, threading through the legs. The interaction between the AC-induced fields and the Earth’s field produces a signal that depends on the orientation of the valve relative to the Earth’s field. That signal is what gets sent down the cable, through the wing structure, to the instrument that displays the heading. The rams’ horns just gather more of the Earth’s flux to make that signal stronger. So let me tie it all together. The detector unit sits in a magnetically clean location, its pendulous element is free to tilt up to 25 degrees in pitch and roll but locked in azimuth, and it’s oil-damped to stop oscillation. Inside, the flux valve — a three-legged device — uses AC excitation on the centre post to create opposing fields in the legs, and the Earth’s field modifies that to produce a heading signal. The rams’ horns improve flux gathering but aren’t essential to the principle. That signal travels through the wing to give you your remote magnetic heading. That’s the detector unit and the flux valve. When you’re ready, we’ll move on to how that signal is processed and displayed.