by Jonathan M. Stern
Principles of Operation
The magnetic compass works on the basis of the magnetic field that surrounds the Earth with lines of force that run to the North and South magnetic poles. The location of the magnetic poles differs from the actual location of the geographic poles. This is reflected on navigation charts as isogonic lines, or lines of equal magnetic variation (see Figure 2.29).
Figure 2.29. This map shows lines of equal magnetic variation in the United States. These lines change with time.
Figure 2.30 is an excerpt of a low altitude enroute chart that includes an isogonic line.
Figure 2.30. The lines of equal magnetic variation are shown in green on the enroute charts. Minneapolis is on the 3°E line.
The variation is shown in degrees East or West.
Conversions between true courses and magnetic courses require the addition or subtraction of the magnetic variation at a particular point. For example, if the variation is 13 degrees West and the true course is 120 degrees, the magnetic course is 133°.
You can remember this by the saying, "East is least and West is best," To convert true to magnetic, add Westerly variation and subtract Easterly variation. Winds aloft are reported relative to true North and must be converted if they are to be applied to a magnetic course.
Flying is generally performed with reference to the directional gyro rather than the magnetic compass. There are two main reasons for this:
- The directional gyro is more stable and easier to read than most magnetic compasses.
- The magnetic compass has inherent errors that require additional interpretation for use.
Because the directional gyro is reset by reference to the magnetic compass, and because a pilot must be prepared to fly with reference to the magnetic compass should the directional gyro fail, the inherent errors of the magnetic compass must be understood.