When does ADF Coastal Refraction Error occur most severely?

The option stating that ADF Coastal Refraction Error occurs most severely when the signal crosses a shoreline at an angle of less than 30 degrees is correct because coastal refraction is predominantly influenced by the change in medium from water to land. When an ADF (Automatic Direction Finder) signal travels from water to land, it can bend or refract due to differences in the propagation speed of the radio waves in these different media.

When the shoreline is approached at a shallow angle (less than 30 degrees), the transition from water to land is more pronounced, leading to greater refraction effects. This can cause the indicated bearing on the ADF to lead the aircraft off course since the signal may appear to come from a different direction than it actually does.

This phenomenon is exacerbated under specific conditions where the characteristics of the terrain and the atmosphere can further influence the signal. The impact of coastal refraction is minimized when the angle of approach is steeper, which reduces the likelihood of misinterpretation by the ADF system. Thus, understanding the specific angle at which the shoreline is crossed is crucial for pilots relying on ADF navigation in coastal areas.