What factor primarily affects the difference between the selected course and the magnetic heading when tracking a VOR radial?

The primary factor affecting the difference between the selected course and the magnetic heading when tracking a VOR (VHF Omnidirectional Range) radial is wind. When flying, wind can have a significant impact on an aircraft's actual path over the ground compared to its intended course.

When tracking a VOR radial, the desired course is determined by the radial selected on the VOR. However, if there is wind, it can push the aircraft off this intended path. Pilots need to compensate for this wind drift by adjusting their heading accordingly. For example, if there is a crosswind, the pilot might bank into the wind to maintain the intended track over the ground.

Other factors like temperature, aircraft weight, and fuel load can affect aircraft performance, such as climb rates or fuel efficiency, but they do not significantly alter the relationship between the selected course and magnetic heading in the context of VOR tracking. Therefore, wind is the key factor in ensuring that the aircraft remains on the correct radial despite any lateral drift caused by wind conditions.