Separation — Page 298, Lesson 393

Let’s start with the big picture. We’re now in the Separation chapter, and I want to walk you through longitudinal separation — this is the tool we use to keep aircraft apart along the same path, in time, rather than in space. The core idea is in paragraph 16.21. Longitudinal separation is applied so that the spacing between the estimated positions of the aircraft being separated is never less than a prescribed minimum. So we’re not just eyeballing it — we’re working with estimated positions, and we’re guaranteeing that the gap between those positions never shrinks below a set value. That prescribed minimum is the heart of the whole system. Now, how do we actually establish that spacing? Longitudinal separation is established by requiring aircraft to do one of four things: depart at a specific time, lose time, arrive over a geographical location at a specified time, or hold over a geographical location until a specified time. So we’re using time as the currency — we either push the departure, we make them slow down, we make them hit a fix at a certain time, or we make them hold until a certain time. All four are just ways of controlling when they pass a point. There’s also a special technique mentioned here — the Mach number technique. Longitudinal separation between aircraft following the same or diverging tracks may be maintained by applying this technique, but only when it’s prescribed on the basis of RAN agreement. RAN stands for Regional Air Navigation — that’s the ICAO regional planning body. So this isn’t something you just decide to use; it has to be prescribed regionally. Now, before we can apply any of this, we need to define the track cases, because the separation standard depends on which case we’re in. There are three. First, the same track case. This applies when the tracks of two aircraft that require separation converge or diverge by an angular difference of less than 45°, or more than 315°, and whose protection areas overlap. So if the angle between their tracks is small — less than 45 degrees — or nearly the same — more than 315 degrees — they’re essentially on the same path, and their protection areas overlap. Second, the reciprocal track case. This applies when the tracks converge or diverge by an angular difference of more than 135° but less than 225°, and whose protection areas overlap. That’s the head-on situation — the aircraft are coming at each other from opposite directions. Third, the crossing tracks case. This is when the angular difference between tracks is 45° or more but not exceeding 315°. In other words, 045 to 135 degrees, or 225 to 315 degrees. So anything that isn’t same track or reciprocal falls into crossing. Now, there’s a figure here that shows this visually — Figure 16.5, Longitudinal Separation. It shows the position accuracy ‘area’, the safety ‘buffer’ area, and the minimum longitudinal separation. The point is that the separation isn’t just the distance between the aircraft — it’s measured between their estimated positions, and there’s a buffer built in for position uncertainty. Now let’s move to the actual standards. Paragraph 16.22 tells us that the separation standards applied depend upon whether the aircraft concerned are maintaining the same level, or are climbing or descending. So we split it into two situations. Paragraph 16.23 covers the same level case. In this situation, the separation is dependent upon the track case. So for the same track case, the basic standard is that aircraft should be at least 15 minutes apart. That’s the default — 15 minutes. But there are reductions. If the navigation aids for the route being flown permit frequent determination of position and speed, the basic standard may be reduced to 10 minutes. So better navigation capability buys you a tighter standard. And it may be further reduced to 5 minutes, providing the aircraft have departed from the same aerodrome. So that’s the tightest — 5 minutes — but only when they’ve come from the same departure point. So let me summarise the ladder: 15 minutes is the baseline for same track at the same level. 10 minutes if navigation aids allow frequent position and speed determination. 5 minutes if they’ve also departed from the same aerodrome. Each reduction is conditional — you have to earn it with better navigation or a common origin. That’s the same level, same track case. We’ll continue with the other track cases and the climbing/descending situation next.