Which scenario would require Terminal controllers to apply radar separation?

Radar separation is a method used by air traffic controllers to ensure safe distances between aircraft, primarily through the use of radar technology. In the context of the provided scenarios, terminal controllers typically apply radar separation when aircraft are in close proximity to one another, generally during phases of flight that require precise maneuvers, such as descending for landing or climbing after departure.

When considering the scenario where aircraft are at least 40 miles from the radar antenna, this generally indicates that the aircraft are in a region where radar coverage may be less reliable or effective, and thus, would not benefit from radar separation techniques. In contrast, aircraft that are approaching for landing, departing from an airport, or cruising within 20 miles of the radar are generally in areas where radar coverage is strong and where applying radar separation is crucial for maintaining safe distances as they approach congested airspace or airports.

Therefore, the correct choice emphasizes a situation where the need for radar separation becomes less applicable, as aircraft at least 40 miles from radar might be outside the typical range where radar is effectively used to manage separation. It highlights the operational context in which radar separation is vital and clarifies that as distance increases beyond effective radar coverage, the procedural requirements shift.