What external factor impacts a magnetic compass?

A magnetic compass is primarily influenced by external magnetic fields, with the most significant impact coming from the magnetic field of the aircraft itself. When the aircraft's magnetic field is affected by its electrical systems, metallic structure, or magnetic materials, the readings of the compass can be distorted, causing navigational errors. This phenomenon is commonly referred to as "magnetic deviation," which is a direct result of how the magnetic compass interacts with the aircraft's magnetic field.

The other factors, such as weather conditions, flight altitude, and aircraft weight, do not have the same direct influence on the magnetic compass. Weather can affect visibility and might impact overall flight safety, but it does not change the magnetic properties that affect compass readings. Similarly, the altitude and weight of the aircraft do not produce magnetic fields strong enough to influence the compass. Therefore, the correct answer relates specifically to the effect that the aircraft's magnetic field has on the operation of the magnetic compass.