Besides the magnetic fields generated by the Earth, metal and electrical accessories within the airplane produce other magnetic fields. These magnetic fields distort the Earth's magnetic force, and cause the compass to swing away from the correct heading.
This error is called deviation. Manufacturers install compensating magnets within the compass housing to reduce the effects of deviation. The magnets are usually adjusted while the engine is running and all electrical equipment is operating. However, it is not possible to completely eliminate deviation error; therefore, a compass correction card is mounted near the compass. This card corrects for deviation that occurs from one heading to the next as the lines of force interact at different angles.
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