Separation — Page 298, Lesson 388

Let’s pick this up right where the vertical separation rules leave off, because the next few paragraphs are all about how we actually apply those levels in practice. First, 16.10 Same Destination. If it’s practicable — meaning if it can reasonably be done — aircraft that are all bound for the same destination will be assigned cruising levels that help set up the correct approach sequence at that destination. So the controller isn’t just thinking about keeping you apart; they’re thinking ahead, stacking you so that when you all arrive, the approach order works out cleanly. Next, 16.11 Priority. This is a simple but important rule. An aircraft that is already at a cruising level has priority over any aircraft that is requesting that level. And if two or more aircraft are already at the same level, then the preceding aircraft — the one ahead — has priority. So the level belongs to whoever is there first, and among those at the same level, the leader keeps the right of way. Now 16.12 Allocation Separation. This tells you when a level can be reused. An aircraft may be assigned a level that was previously occupied by another aircraft — but only after that other aircraft has reported vacating it. There’s a catch though. If there is severe turbulence, or if the aircraft is in a cruise climb, then the assignment is withheld until the other aircraft has reported at another level that is separated by the required minimum. So in normal cases, a simple "vacated" report is enough; in turbulence or cruise climb, you need proof that the other aircraft is safely at a different, properly separated level before you hand that level out. Then 16.13 Vertical Separation During Ascent or Descent. Here’s a case where the pilots themselves can take over the vertical separation. If pilots are in direct communication with each other, and they both concur — meaning they both agree — they may be cleared to maintain a specified vertical separation between their aircraft during ascent or descent. So instead of ATC assigning each aircraft its own level, the two crews agree to hold a set vertical gap between them while climbing or descending. Finally, 16.14 Clearance to Maintain Own Separation in VMC. This is a very specific authorization. If the pilot of an aircraft requests it, and the pilot of the other aircraft agrees, and ATC authorizes it, then a flight operating in Class D or Class E airspace, in VMC — visual meteorological conditions — and during daylight, may be cleared to climb or descend maintaining its own separation from one other aircraft. But there are three conditions attached. First, the clearance is specifically for the portion of flight below 10,000 feet. Second, alternative instructions must be passed to the pilot to cover the case of loss of VMC — so if the weather closes in, the pilot has a fallback plan from ATC. And third, if conditions deteriorate to the limits of VMC, the pilot must inform ATC that they are complying with those alternative instructions. So the pilot keeps their own separation only while the weather holds and only below 10,000 feet, and the moment VMC is at risk, they switch back to ATC’s instructions and say so. Now we move into Horizontal Separation, and this is a new category. 16.15 Definition. Horizontal separation relates to the distance between aircraft in the horizontal plane. And it comes in two forms. The first is longitudinal — that’s when aircraft are following the same route, and the separation standard is based on time (or distance) along track between the aircraft. The second is lateral — and that’s the other form, which we’ll get into next. So think of it this way: longitudinal separation is the gap along the path between two aircraft on the same route, measured in time or distance; lateral separation is the gap sideways between them. That figure shows the ICAO level occupancy standard, which ties back to how we assign those cruising levels we just talked about. And the next figure, Figure 16.2, illustrates the horizontal separation concept — the longitudinal versus lateral distinction. So to summarize where we are: vertical separation handles the up-and-down spacing, and now horizontal separation handles the side-to-side and along-track spacing. Longitudinal is time or distance along the same route; lateral is the sideways gap. We’ll build on that lateral definition next.