When may separation between aircraft be discontinued during passing/diverging?

Separation between aircraft may be discontinued when aircraft on reciprocal courses have passed each other because once they have crossed paths, the risk of collision significantly decreases. This situation is typically monitored and controlled by air traffic control to ensure safety, as the aircraft are moving away from each other on their respective flight paths. The principle behind this is rooted in established separation standards that require the aircraft to maintain a defined distance until they are determined to be safely apart.

In this context, the other scenarios present varying degrees of potential safety risks. For instance, being on the same flight plan does not guarantee sufficient separation as both aircraft could still be on a converging path. Likewise, simply being at least 10 miles apart can sometimes be misleading since, depending on the vectors and altitudes, the aircraft may still have a high probability of conflict. Finally, both aircraft beginning their descent does not ensure that they are clear of potential encounters, as altitude changes can still result in conflicts if proper separation isn't upheld. Therefore, the valid rationale for discontinuing separation is clearly present in the case of aircraft having passed each other on reciprocal courses.