What is a common source of magnetic interference that can affect a magnetic compass?

Magnetic compasses are sensitive instruments that provide a means of navigation based on the Earth's magnetic field. One significant source of magnetic interference that can affect a magnetic compass is the electrical systems within the aircraft. These systems generate magnetic fields that can distort the local magnetic environment around the compass, leading to erroneous readings.

The proximity of various electrical equipment, such as radios, autopilot systems, and other avionics, can produce electromagnetic fields that interfere with the compass's ability to accurately align with the Earth's magnetic field. As a result, pilots must be aware of these potential sources of interference, especially during critical phases of flight, such as takeoff and landing.

In contrast, GPS signals, changes in air pressure, and radio waves from VOR stations do not create direct magnetic interference with a compass. GPS relies on satellite signals and does not influence magnetic direction, while air pressure changes affect flight dynamics but not magnetic orientation. Similarly, VOR stations communicate navigation information using radio waves that do not interact with the magnetic field utilized by the compass. Therefore, the correct answer highlights the importance of understanding the impact of electrical systems on magnetic compass accuracy during flight operations.