Deviation

A magnetic compass mounted in an airplane is not only affected by the earth's magnetic field, but in addition by any magnetic fields created by the airplane itself. These include permanent and induced magnetism in any iron and steel airframe components and field produced by nearby DC electrical circuits. (Older Mooneys can have problems from structural steel tubing getting magnetized, and the original Grumman AA5A had a panel mounted compass that was strongly affected by the turn coordinator.)

Much of this deviation error can be cancelled by adjusting the two compensating magnets mounted on the compass. After the errors have been minimized to the extent possible, the remaining deviation is documented on the compass correction card mounted under the compass.

This is usually in the form of a table of MH vs. CH. The best way to make use of this in flight is to refer to it when setting the DG (assuming you have one!). The procedure is:

• Stabilize the airplane wings level, constant airspeed.
• Read the magnetic compass - averaging the oscillations if any.
• Using the compass heading, refer to the deviation card to obtain the magnetic heading, interpolating as required.
• Set the DG to the magnetic heading obtained.

The DG is then corrected for deviation. If this is not done, apart from the inaccuracy, the DG will appear to precess when turning to new heading on which the deviation differs from that on which the DG was set! More than one perfectly good DG has been replaced for this!