In illustration 4, which will occur if the airplane's heading is maintained?

The correct choice is based on the principles of navigation and the relationship between an aircraft's heading, its course, and the radials associated with navigational aids such as VORs (VHF Omnidirectional Range stations).

If the airplane's heading is maintained, it implies that the aircraft will be following a constant directional path in relation to ground navigation references. As it approaches a specific radial, the angle at which it crosses that radial is significant because it provides insight into the aircraft's positional relationship to that radial.

In this scenario, if the aircraft crosses the 180 radial at a 75° angle, it suggests that the aircraft is coming in from a direction that is slightly offset from the radial itself. This angle indicates a specific navigational understanding where the aircraft maintains its heading and crosses the radial at the described angle.

By understanding how radials work and how they relate to the aircraft’s course and heading, one can predict the resulting intercept angles with precision. This understanding is crucial in practical flight navigation, ensuring that pilots can adjust their headings as necessary to accurately intercept the desired radial. The context here heavily relies on the application of navigation principles, demonstrating the significance of maintaining a steady heading while approaching a specific radial.