ATC deals in absolutes. When you have two aircraft pointed right at each other, you'd better be sure they're not going to smack. It's not "they probably won't hit" or "I think they should have separation."
A word that tends to fall right in front of the word separation in ATC texts is "ensure". Sometimes its preceded by the words "the controller failed to". Its our job to be absolutely, 100% positive that two aircraft will not come closer than is permitted.
Let's say you're sitting there working a sector and you see N123 and N456 coming together on a hazy morning. They're C172's and they're 10 miles apart at 120 knots, but they're both at 2000 feet on headings that will bring them together. When your instructor looks over your shoulder and asks you "Are those guys going to hit?" your answer cannot be "They probably won't." o
That's where the might and the will come into play. If they might have separation, but you're not sure they will have it, then you have to do something.
Now, what will that something be? Sure... you can wait until they're two miles apart and issue traffic in hopes of getting visual separation. If N123 does see N456 and reports him in sight, you can just tell him "Maintain visual separation from that traffic." But, once again, you'll be using the word might. You're placing a bet, taking a risk, praying that N123 can actually see N456.
But what if he doesn't? What if they're head-on and that other white Cessna is a white dot blending into the white haze? You'll be left with nothing but a tightening in your stomach and "panic vectors" that usually involve the word "immediately".
How could you have avoided this? Any number of ways:
- "N123, climb and maintain 2500." There, now you have 500 feet of vertical separation.
- Or... "N456, turn left 20 degrees for traffic." Great, now you've got divergence.
- Or - legal but stupid - "N123, maintain at or below 80 knots" Now you've got a confused pilot slowing to approach speed at cruising altitude, but the speed differential will make N456 beat the other aircraft.
Let's say you issued one of the above instructions, such as the vertical separation. You could literally unplug, walk away, and be certain that they'll never hit. You could have been handed off those planes when they were 30 miles away from each other and never have to look at them again until it's time to hand them off to the next sector. Instead of focusing on them for all 30 miles, wondering if they'll hit, now you know for certain that they won't (as long as the pilots adhere to your instructions).
Remember Yoda in Empire Strikes Back? "Do or do not - there is no try"? In ATC the saying would be reworded as "They're going to hit or they aren't - there is no maybe."