What causes an aircraft to experience magnetic deviation?

Magnetic deviation occurs due to the presence of ferromagnetic materials and electronic equipment within the aircraft. These factors can distort the Earth's magnetic field in the area surrounding the aircraft's magnetic compass or magnetic sensing devices. As the compass relies on the Earth's magnetic field to indicate direction, any interference from these materials or equipment will lead to inaccuracies in the displayed heading.

In practical terms, components such as metal structures, instruments, or even the aircraft's electrical systems can create local magnetic fields that interact with the compass, resulting in deviation from the true magnetic heading. Pilots must be aware of this phenomenon and often need to compensate for it in their navigation calculations to ensure precise navigation and aircraft orientation.

Other choices do not directly cause magnetic deviation in a significant manner: stereo equipment typically does not have enough magnetic influence to affect the compass, altitude itself does not alter magnetic fields, and although weather can affect visibility and flight conditions, it does not induce magnetic deviation.