FAA Ground School Practice Test

What causes deviation in a magnetic compass?

1. Changes in atmospheric pressure

2. Magnetic fields within the aircraft

3. User error in reading the compass

4. Magnetic storms in the vicinity

The correct answer is: Magnetic fields within the aircraft

A magnetic compass deviates from true magnetic north due to various magnetic fields present within an aircraft. As aircraft contain many electronic devices, metal structures, and even the magnetic properties of the aircraft materials themselves, these factors can create localized magnetic fields that interfere with the compass's ability to accurately point towards magnetic north. For instance, if an aircraft has metallic components or electronics that generate a magnetic field, those could deflect the compass needle from its true alignment. This phenomenon is known as "magnetic deviation," and it varies based on the aircraft's heading and the specific distribution of magnetic materials within the aircraft. Understanding this concept is crucial for pilots because they must account for deviation when navigating, ensuring they can make accurate heading adjustments and maintain safe flight operations.