What causes magnetic compass deviation?

• A. Variation in magnetic declination
• B. Magnetic compass error caused by local magnetic fields within the aircraft
• C. GPS interference
• D. Electromagnetic interference from radio waves

Answer: (B)

Deviation of the magnetic compass comes from local magnetic fields inside the aircraft. The compass responds to Earth's magnetic field, but nearby ferrous metal, engines, electrical systems, and wiring create their own magnetic fields that distort the needle. This internal interference varies from one airplane to another and can change with power changes, maneuvers, and fuel load, so each aircraft has a deviation correction chart to dial in the exact adjustments for every heading. Variation, on the other hand, is the difference between true north and magnetic north at a location and is a separate concept used for converting true headings to magnetic headings. GPS interference and radio-frequency EMI aren’t the primary causes of compass deviation.