Ferris, try and picture a square with points marked a & b across the bottom, and c & d across the top...
c . . . . d
a . . . . b
Now aircraft No 1 is flying from a to d, and aircraft No 2 is flying from b to a. No 1 is at FL 380, No 2 at 400 descending 360...
Still with me?
If I tell No1 and No2 to "continue present heading" then I can prove my separation because both aircraft were under my direct control and, assuming the headings were providing the required distance (5miles), would not hit each other...
If I leave both aircraft on own nav then the possibility exists that either aircraft may have an intermediate point on the route that would erode the separation and I would therefore not have PROVED it.
I hope that makes sense, next week intermediate Nuclear physics for the retired hostie!!!
