The magnetic compass has been used in navigation for more than 1,000 years, and in aviation since at least 1909. This simple device continues to be standard equipment in today’s aircraft, in part because of its reliable nature and minimal need for maintenance.
As one of the most basic instruments, the magnetic compass consists only of a compass card that swings on a pivot assembly. Attached to that assembly are two magnets and a float. These pieces are submerged in mineral spirits or kerosene, which stabilizes the compass card and makes it easier to read. The glass face includes a reference point called the lubber line, where the direction is referenced.
The biggest drawback to the compass is its six inherent errors. These range from a small nuisance to major navigational implications. They are variation, deviation, dip error, turning error, acceleration error, and oscillation error.
But the biggest challenge when reading the airplane compass is that it appears backward. When flying north, for example, a turn to the right toward the east will cause the compass to spin left. The vertical card compass overcomes this confusing feature (see “Going Vertical”).
Despite these limitations, the compass remains an essential piece of equipment that doesn’t require power or any outside infrastructure to continue operating.
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