The Earth’s Magnetic Field - pt. 1

NOTE: During the design and construction of this company’s CA-320 digital compass, we were surprised at how little we knew about the Earth’s magnetic field. This turns out to be important from an aircraft maintenance standpoint and therefore will be the subject of a multipart series, starting with this edition.

We begin with the concept of permeability, or µ. We have all taken a bar magnet and sprinkled metal filings on it to see the lines of force expressed as continuous clumps from the north pole to the south pole of the magnet. The Earth’s magnetic field is no different. Its lines of force (or flux) flow from one pole to another but are too weak to see in this way. We can, however, detect these lines of force and determine aircraft heading by measuring the magnetic field with a wet compass or flux valve based sensor. In the case of the wet compass, a floating bar magnet aligns parallel to the lines of force, its north pole pointing to the Earth’s south pole. A 360° cylindrical index is attached to this magnet and as it continually points south, the pilot knows his magnetic heading. In the case of flux valve based sensors, the magnetic field is systematically allowed to energize a high permeability core surrounded by triaxial coils, then it is abruptly expelled from it. This induces voltages into the coils, the magnitude of which represent the magnetic heading of the aircraft.

So where does permeability come into this? Aircraft contain many different types of metals and self-generated magnetic fields which alter the Earth’s magnetic field as it passes through the airframe, inducing errors that need compensation. It is these errors that we must understand and permeability is the key. It is defined as:

The ratio of flux density produced by a given material compared to the flux density produced by an air core.
or
where B is the flux density in gauss and H is the magnetic field
strength in oersteds.

The magnetic field entering the aircraft from free space (with a permeability of one) has parallel lines of force. As these lines of force strike other magnetic fields and ferrous materials with permeabilities other than one, they no longer travel in parallel lines. If a compass or flux valve is in one of these areas, their heading measurement will contain errors. So as aircraft technicians, it is the change in permeability that we must concern ourselves with.