Deviation in a magnetic compass is caused by which of the following?',

• A. Magnetic fields within the aircraft distorting the lines of magnetic force
• B. Earth's rotation
• C. Variation of the Earth's magnetic field with latitude
• D. The altitude of the aircraft

Answer: (A)

Deviation happens because the compass senses the horizontal component of Earth’s magnetic field, but the aircraft itself creates magnetic fields. The metal in the fuselage, ballast, engines, instrumentation, and wiring can become magnetized or generate their own magnetic fields. These local fields combine with Earth's field and distort the patterns of magnetic force that the compass needle aligns with. As a result, the needle no longer points to the true magnetic heading and shifts by a value that depends on the aircraft’s heading and configuration. That’s why pilots use a deviation correction (a deviation card) to adjust compass readings.

The other factors don’t cause this local compass error. Variation with latitude is about how magnetic north differs from true north at a location, not about distortions inside the cockpit. The Earth’s rotation isn’t something the compass reacts to in this context. Altitude changes the Earth’s magnetic field only slightly and isn’t the primary cause of the deviation you observe inside the airplane.