The Earth's Magnetic Field - part 3

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.

In previous Newsletters we have explored the concept of permeability, or μ and one of the errors, hard iron, created when the permeability changes while passing through an aircraft structure. This edition will cover the other error that we must recognize and correct in our compass systems – soft iron errors.

Ferrous metals tend to be iron, steel, nickel and cobalt and when they have no inherent magnetization, distort the Earth’s magnetic fi eld in the vicinity of the material. The bending and concentration of the magnetic lines of fl ux are different from the hard iron errors discussed previously and therefore the two errors tend to add to the overall distortion. Some of the characteristics of soft iron errors are as follows:

• The amount of distortion depends upon the compass orientation.
  
• They infl uence the fi eld values measured by the X and Y sensors but do not
  disturb the origin.
  
• They affect a relatively small area; a rule of thumb is a volume 1.5 times the
  diameter of the offending material.
  
• If plotted in a circle, the errors appear as an ellipse with the origin unchanged
  and are commonly called two-cycle errors.
  

To compensate for soft iron errors, one must rotate the readings by 45°, scale the major axis to change the ellipse to a circle, then rotate the reading back by 45°. Electronic systems can then store the errors for a given heading and correct accordingly. For fl ux valve based systems, this effect cannot easily be removed because the available adjustments, E-W, N-S and circumferential rotation, simply add the error in one direction or another. The best solution is to remove the offending soft iron in the vicinity of the sensor and deal with the hard iron effects directly. Degaussing the local area is a good universal corrective action.

This concludes our study of compass systems and their inherent errors. We will
explore other maintenance issues in subsequent editions of The READOUT.