Separation — Page 298, Lesson 390

Let’s start with the big picture, because separation is the heart of air traffic control. When two aircraft are at the same level — that is, the same altitude — the only thing keeping them apart horizontally is lateral separation. So I want to walk you through what that means precisely, because the wording matters for your exams and for real operations. Lateral separation is applied so that the distance between those portions of the intended routes for which the aircraft are to be laterally separated is never less than an established distance. That established distance is made up of two parts: it accounts for navigational inaccuracies, plus a specified buffer. The buffer is determined by the appropriate authority — that’s the national or regional aviation authority — and it’s included in the lateral separation minima. So the minimum isn’t just a raw number; it’s the navigational error allowance plus a safety margin on top. Now, how do we actually achieve lateral separation at the same level? We require aircraft to operate on different routes, or in different geographical locations. And that determination is made in one of three ways: by visual observation, by use of navigation aids, or by use of area navigation equipment — that’s RNAV. And here’s a key point: the buffer concept applies to all methods of separation, not just some. Every method builds in that safety margin. Let me show you the geometry. Here we have the position accuracy area — that’s where the aircraft could actually be, given navigational error. Around it is the safety buffer area. And the minimum lateral separation is measured between those buffer areas, not between the nominal route lines. That’s the whole point of the buffer: it absorbs the uncertainty so that even with navigational error, the aircraft never come closer than the minimum. Now let’s get into the specific criteria and minima — this is where the numbers come in. There are three means by which lateral separation may be achieved. First, geographical separation. This is separation positively indicated by position reports over different geographical locations, as determined visually or by reference to a navigation aid. So the aircraft are at different places on the ground, and we know that from their reported positions. Second, track separation. This is achieved between aircraft using the same navigation aid or method. We require the aircraft to fly on specified tracks that are separated by a minimum amount — both an angle and a distance — appropriate to the navigation aid or method employed. And here are the three cases, and you need to know these cold. For all three cases, the distance required is 15 nautical miles from the common position. The common position is the point where the tracks diverge — where they split apart. And the track divergence — the angle between the tracks — depends on the aid: - With VOR — that’s VHF omnidirectional range — the track divergence required is 15 degrees. - With NDB — non-directional beacon — the track divergence required is 30 degrees. - With DR fix — that’s dead reckoning fix — the track divergence required is 45 degrees. Notice the pattern. The less accurate the navigation aid, the greater the divergence angle required. VOR is the most accurate, so 15 degrees is enough. NDB is less accurate, so we need 30 degrees. Dead reckoning is the least accurate, so we need a full 45 degrees of divergence. And in every case, the 15 nautical mile distance from the common position applies. So to tie it together: lateral separation is about keeping aircraft at the same level apart horizontally, using the buffer concept to cover navigational error, and achieving it through geographical separation, track separation, or RNAV. And when we use track separation with a common navigation aid, the minima are 15 NM from the common position, with divergence angles of 15, 30, or 45 degrees depending on whether it’s VOR, NDB, or DR fix.