Separation — Page 298, Lesson 391

Let's pick up with lateral separation using different navigation aids, because that's where the text takes us next. When two aircraft are using different navigation aids — say, one is tracking a VOR radial and the other is on an NDB bearing — or when one aircraft is using RNAV equipment, we can't just apply a simple distance. The rule is that lateral separation is established by ensuring that the derived protected airspaces — the buffer areas around each aircraft's intended path — do not overlap. So the controller doesn't measure from aircraft to aircraft; they measure from protected airspace to protected airspace. If those buffers touch or overlap, separation is lost. Now, RNAV operations. Within designated airspace, or on parallel routes where RNP is specified, lateral separation between RNAV aircraft may be obtained by requiring aircraft to be established on the centre lines of parallel tracks or ATS routes. Those tracks or routes must be spaced at a distance which ensures that the protected airspaces do not overlap. This is the policy that determines the spacing of the NAT tracks — the North Atlantic Tracks — employed in the NAT MNPSA, the Minimum Navigation Performance Specifications Area. So when you hear about the North Atlantic tracks being spaced a certain way, this is the principle behind it: the protected airspaces must not overlap. Now we come to longitudinal separation, and I want to be clear: this is the most complex application of separation standards. Here's why. In procedural ATC — that is, not radar control — the position of the aircraft is that which is reported by the pilot. The positions reported are usually specified reporting points, radio navigation facilities, or geographic points for routes not specified by navigation aids. In any event, the position known to the ATCO — the air traffic control officer — is only as good as that reported by the pilot. So the controller is working from the pilot's word, not from a radar blip. Because of that, in applying procedural separation, all possible errors must be allowed for, and then a safety margin — a buffer — is applied on top. Now, there is one situation where those safety margins may be relaxed: where the pilots of the aircraft to which separation should be applied have, and have confirmed, that they have visual contact with each other, and that they can maintain visual contact during the necessary manoeuvre to which separation would otherwise be applied. So the pilots must both see each other and confirm it, and they must be able to keep that visual contact through the whole manoeuvre. Clearly, the separation standards assume operations in IMC — instrument meteorological conditions. That's the baseline assumption. The accuracy of pilot position reporting relies on the accuracy of the navigation system in use. And here's a key point: in remote areas — over the oceans and desert regions — where RNAV procedures may be used, greater protection needs to be applied. Why? Because there are fewer reporting points, less ground-based navigation aid coverage, and the position information is less certain. So the buffers get bigger. Finally, longitudinal separation applied is either in terms of time or distance. That's the two ways you'll see it expressed — a time interval, like ten minutes, or a distance interval, like a certain number of nautical miles along track. So the whole picture: lateral separation is about protected airspaces not overlapping, whether the aids are the same or different; and longitudinal separation is the complex one, built on pilot-reported position, with buffers for error, relaxed only when pilots have confirmed visual contact, assuming IMC, and expressed in time or distance.