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Say you have an eastbound flight at FL210 that wants to climb and a westbound flight on a converging track at FL220 that wants to descend. There's the minimum required 1000 ft vertical separation. Once the planes have passed each other can ATC clear them to climb/descend right away or do they have to wait until they are 5nm apart before they can break the vertical separation? And what about TCAS? Would it consider that a conflict?

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  • $\begingroup$ TCAS is based on closure rate, so it shouldn't trigger. I have no idea what is legal for the ATC though. $\endgroup$ – Jan Hudec May 14 '16 at 5:51
  • $\begingroup$ "Traffic in sight. Will maintain visual separation" is also good enough to break separation minima $\endgroup$ – rbp Jul 9 '16 at 20:50
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Yes, this is allowed, subject to certain conditions. See FAA Order 7110.65 (the ATC "bible"), section 5-5-7, for the full details.

...all other approved separation may be discontinued and passing or diverging separation applied when ... Aircraft are on opposite/reciprocal courses and you have observed that they have passed each other; or aircraft are on same or crossing courses/assigned radar vectors and one aircraft has crossed the projected course of the other, and the angular difference between their courses/assigned radar vectors is at least 15 degrees.

That's in the terminal environment; it's still possible in the enroute environment but a bit more restrictive due to the greater aircraft speeds and poorer radar accuracy.

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It depends wether you get procedural or radar service. In case of procedural service if both pilots report clear of traffic (beeing previously informed of the traffic) the longitudinal sepparation criteria is met. In case of radar service longitudinal sepparation minimas can vary from 5 to 10 NM or more.

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You will have to comply with the minimum horizontal separation required by the regulations before crossing their levels.

TCAS might consider them clear of conflict before they have the minimum horizontal separation required (3, 5, 8Nm, whatever that may be), but remember TCAS is not a way of providing ATC separation.

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In my experience it's moot. Usually when you observe that the tracks have crossed, by the time you issue the clearance to climb/descend the acft are 5 miles apart anyway due to speeds at those altitudes.

Cavver - Interesting that I've never heard it referred to as "procedural" separation before! Is that a non-us reference? We always called it manual separation or non-radar. It was always a challenge to try and set off the immediate alert buzzer at the desk and have a manual rule to invoke. Made management grumble :-D

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  • $\begingroup$ I've heard it referred to as procedural control also, but when there was no radar available, like over oceans or in countries that don't have much radar control. $\endgroup$ – TomMcW Jul 9 '16 at 20:44
  • $\begingroup$ I've always called this "the bubble" $\endgroup$ – rbp Jul 9 '16 at 20:48
  • $\begingroup$ Also, asking a question from another answerer should go in the comments below their question. You should have that feature available now, it only takes 50 rep. But unless it's under their question you need to put an @ in front of their username to get their attention. I.e.: @Iain $\endgroup$ – TomMcW Jul 9 '16 at 20:50
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    $\begingroup$ It's great to have some more knowledge from someone with ATC experience here! $\endgroup$ – Zach Lipton Jul 10 '16 at 0:02

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