What causes the deviation error in a magnetic compass?

The deviation error in a magnetic compass is primarily caused by electric and magnetic fields present in the aircraft. Each aircraft contains various systems and equipment, such as electronic devices, radios, and magnetic materials, which can interfere with the magnetic field of the compass. This interference distorts the magnetic reading, leading to deviation.

While options like changes in air pressure, magnetic variations in the earth, and instrument fatigue might affect flight operations or navigation to some extent, they do not specifically address the causes of deviation error in a compass. Changes in air pressure primarily affect altimeters or performance calculations rather than magnetic instruments. Magnetic variations refer to the difference between true north and magnetic north, impacting navigation over larger distances but not directly causing deviation in an aircraft's compass. Instrument fatigue is more relevant to wear and tear or mechanical failure and does not explain the influence of local magnetic fields that create deviation. Understanding these influences can help pilots correct for deviation using a compass deviation card during flight.