Which factor contributes to magnetic compass deviation due to onboard equipment and structures?

• A. Certain metals and electrical systems within the aircraft
• B. Earth's magnetic field
• C. Altitude changes
• D. Fuel weight

Answer: (A)

Magnetic compass deviation comes from the aircraft’s own magnetic environment. The compass needle normally lines up with the Earth's magnetic field, but ferrous metals and electrical systems inside the aircraft generate their own magnetic fields. These local fields distort the needle’s reading in different ways depending on the aircraft’s heading and which equipment is active. That interference is what pilots correct for with a deviation card or calibration. In contrast, the Earth’s magnetic field itself sets the baseline and isn’t caused by onboard gear, and changes in altitude or fuel weight don’t create the same kind of magnetic disturbance the compass senses.