Separation — Page 303, Lesson 399

Let's pick up right where the Mach number separation table leaves off, because that table is the key to understanding the RNAV separation that follows. First, let me anchor you on that table, because it's the foundation. We're talking about longitudinal separation — that's the distance or time gap along the same track between two aircraft. When we use the Mach number technique, we're using the difference in true airspeed, expressed as a Mach number difference, to establish that gap. Look at the table. If the preceding and following aircraft have the same Mach number — a difference of zero — the longitudinal separation standard is 10 minutes. Now, as the Mach number difference increases, the required separation time decreases. A difference of 0.01 still requires 10 minutes. But at 0.02, it drops to 9 minutes. At 0.03, it's 8 minutes. At 0.04, 7 minutes. At 0.05, 6 minutes. And at 0.06, the minimum, it's 5 minutes. So the principle is: the faster the preceding aircraft is relative to the following one, the less time separation you need, because the gap is opening up on its own. That's the Mach number separation technique in a nutshell. Now, here's where it gets interesting. We move to a new concept: longitudinal separation based on RNAV. RNAV stands for Area Navigation — it's a navigation system that lets an aircraft fly along any desired flight path, not just directly over ground-based beacons. This separation method applies to RNAV aircraft operating along RNAV routes, or along ATS routes — that's Air Traffic Service routes — that are defined by VOR, the VHF Omni-directional Range beacons. Here's the core idea. Instead of separating by time — those 10 minutes or 5 minutes we just talked about — we separate by distance. Separation is established by maintaining a specified distance between aircraft positions, and those positions are reported by reference to the RNAV equipment. So the aircraft tells the controller where it is based on its RNAV system, and the controller maintains a distance gap between them. There's a hard requirement here: direct controller/pilot communications must be maintained. That means the controller and the pilot are talking directly to each other, with no relay through another station. That's essential for this technique to work. Now, what defines the positions? RNAV positions are defined as standard waypoints common to both aircraft that are subject to separation. A waypoint is a defined geographical position used for navigation. So both aircraft must be reporting their distance from the same waypoint — that's what makes the separation check valid. Here's the big number. The minimum distance-based separation is 150 kilometers, which is 80 nautical miles. That replaces the normally required 10 minutes. So instead of a 10-minute time gap, we use an 80-nautical-mile distance gap. But there's a critical condition: it is essential that the Mach number technique is applied. So even though we're using distance, we still need that speed difference between the aircraft to be managed. And there's a failure condition you must know. In the event of equipment failure that reduces the navigation capability to less than the RNAV requirement, the normal longitudinal separation will be applied. That means we fall back to the standard time-based separation — the 10 minutes, or whatever the Mach number table gives us. Now let me walk you through the specific separation requirements, because there are three distinct cases. First, same cruising level. The standard is 150 kilometers, or 80 nautical miles, providing each aircraft reports its position from the same point. And the separation must be checked by obtaining simultaneous RNAV distance readings from the aircraft at frequent intervals. So the controller asks both aircraft for their distance from the same waypoint at the same time, and verifies the gap is at least 80 nautical miles. Second, climbing or descending on the same track. Again, 150 kilometers, or 80 nautical miles, whilst vertical separation does not exist. Vertical separation means being at different altitudes. So while they're at the same level — while vertical separation is not in effect — we need that distance gap. The conditions are: each aircraft reports distance from the same waypoint, one aircraft maintains level flight whilst vertical separation does not exist, and separation is established by obtaining simultaneous RNAV distance readings from the aircraft. Third, reciprocal tracks. This is when aircraft are flying in opposite directions on the same track. Here's the rule: aircraft may be permitted to climb or descend through levels occupied by other aircraft, providing it has been positively established by simultaneous RNAV distance readings to or from the same on-track waypoint that the aircraft have passed each other by at least 150 kilometers, or 80 nautical miles. So before one aircraft is allowed to climb or descend through the other's level, we must confirm they've already passed each other — and the distance between them is at least 80 nautical miles. Let me show you what this looks like visually. This figure shows the longitudinal separation concept. And here's the crossing tracks case — the basic standard is 15 minutes, but if the frequent... well, let me not get ahead of myself. That's a different scenario we'll cover separately. So to tie it all together: RNAV separation replaces a time-based gap with a distance-based gap of 150 kilometers or 80 nautical miles, but it only works if both aircraft report from the same waypoint, if direct controller-pilot communications are maintained, if the Mach number technique is applied, and if we fall back to normal time-based separation the moment navigation capability degrades below the RNAV requirement. That's the complete picture.