Determining Magnetic Deviation
The compass is not only affected by Earth's magnetic field, but also by those of onboard avionics and electrical components. These components may cause the compass to indicate slightly right or left from what it should show. This difference is magnetic deviation.
To allow pilots to compensate for deviation, each airplane is marked with a compass correction card. The card shows deviation at the cardinal headings. Deviation is normally less than a few degrees.
Add or subtract the deviation on the flight log.
