Visual Passage

When two aircraft are travelling in opposite directions along the same track and one wants to climb or descend the altitude of the other, ATC has to find some way of proving that they have passed before he can issue such a clearance. Most often, radar is used, since it's normally the most efficient. In a non-radar environment, ATC has to use other methods. The typical standard is DME. One reports 20DME west of a facility westbound and the other reports 15DME west of it eastbound and separation is proven. The 5DME difference was chosen intentionally, since that's the minimum useable.

When the two aircraft involved see each other, visual passage can be used to prove "tail to tail". Many pilots who fly in non-radar environments regularly are already familiar with this trick, as many offer such a report before it is asked for by ATC. The kicker is that both aircraft must report visual passage before ATC can discontinue vertical separation. If only one reports the passage, ATC has to wait for something else. This has actually been used quite successfully between two sharp pilots in a radar environment, too. Typically, one pilot reports passage before they have passed, giving the other pilot the hint that the visual passage report may help, and he reports in quickly, too. Do I feel this is dangerours? Not at all. You can't tell me that if a pilot reports passage before his traffic has passed and I clear him down that he's going to dive, knowing full well where is traffic is (remember that he saw him already) and run into him. Mind you, both pilots have to know that the visual report would be helpful. The one wanting to descend, for example, is often the one who is quickest with his report, while the other guy doesn't always realize that he is holding someone else up.