Separation — Page 303, Lesson 397

Let’s pick this up right where the separation standards get practical. We’ve already covered the basic time-based longitudinal separation, where aircraft on the same track are kept apart by a set number of minutes. Now I want to walk you through a different, more precise method: longitudinal separation based on DME, which is paragraph 16.25. First, the acronym. DME stands for Distance Measuring Equipment. It’s a radio navigation aid that tells the pilot the slant distance from the aircraft to a ground station, in nautical miles. So instead of separating aircraft by time, we can separate them by actual distance. The rule here is simple in principle: where DME information is available, separation can be established by maintaining not less than the specified distances between aircraft positions. So the controller sets a minimum distance in nautical miles, and as long as the aircraft keep at least that distance apart, they’re separated. But there’s a hard requirement attached to this method. It is a requirement that direct pilot-controller communication is maintained. That means the pilot and the controller must be talking to each other directly, in real time, because the controller needs to get those DME readings and act on them immediately. Now, there’s an important note here, and it’s a classic exam trap. In the NAT region — that’s the North Atlantic — communication is via HF, high frequency radio. And on HF, the pilot does not talk directly to the controller. Instead, communication goes through a radio operator. So the note says: in the NAT region using HF, communication is via a radio operator, not direct to the controller. That means the direct pilot-controller requirement I just mentioned does not apply in the NAT region when using HF, because the link is through that radio operator. Keep that contrast clear: normally you need direct pilot-controller communication for DME separation, but not in NAT on HF. Now let’s move to paragraph 16.26, which gives us the specific distance standards for aircraft at the same level. And here the previously defined same and crossing track situations apply. So we’re going to go through four cases: same track, crossing tracks, climbing or descending, and reciprocal tracks. First, same track. The normal standard is 20 nautical miles, provided each aircraft uses on-track DME stations. On-track means the DME station is on the aircraft’s route, ahead or behind along the track, not off to the side. And separation is checked by obtaining simultaneous DME readings from the aircraft at frequent intervals. So the controller asks both aircraft for their DME distance at the same moment, frequently, and confirms they’re at least 20 NM apart. Now, that 20 NM standard can be reduced to 10 NM, but only under a specific condition: the leading aircraft maintains a TAS — that’s True Airspeed — of 20 knots or more, faster than the succeeding aircraft. So if the aircraft in front is at least 20 knots faster than the one behind, the gap can be halved to 10 NM, because the faster lead aircraft is pulling away. Second case, crossing tracks. The same track standards apply to crossing traffic, but with two extra conditions. First, each aircraft reports distance from the station located at the crossing point. So the DME station must be right at the point where the tracks cross. And second, the relative angle of the tracks must be less than 90 degrees. So if the tracks cross at a shallow angle, less than a right angle, you can use the same 20 NM or 10 NM standards. Third case, climbing or descending. Here the standard separation is 10 NM whilst vertical separation does not exist. Vertical separation means the aircraft are at different altitudes. So while they’re still at the same level, before one has climbed or descended clear, you need 10 NM. And the conditions are: each aircraft uses on-track DME stations, one aircraft maintains a level whilst vertical separation does not exist, and separation is established by simultaneous DME readings from the aircraft. So one aircraft holds its altitude, the other is climbing or descending through, and you keep them 10 NM apart by simultaneous DME checks. Fourth case, reciprocal tracks. Reciprocal means opposite directions, heading toward each other. Here, aircraft using on-track DME may be cleared to climb or descend to or through levels occupied by other aircraft using on-track DME. So one aircraft can be cleared to change level through the level of another, provided it has been positively established that the aircraft have passed each other and are at least 10 NM apart — or such other value as the authority specifies. So the key is: they must have already passed each other, and they must be at least 10 NM apart, or whatever distance the local authority sets. That’s the positive establishment — the controller must be sure they’ve crossed, not just assume it. Now, paragraph 16.27 starts a new technique: longitudinal separation with Mach number technique based on time. And that’s where the excerpt cuts off, so we’ll pick that up next. But before we do, let me make sure the picture is clear. The whole idea of this DME-based separation is that we’re using distance in nautical miles instead of time in minutes, and the controller verifies it with simultaneous DME readings. The standards are 20 NM normally, 10 NM when the lead aircraft is at least 20 knots faster, and 10 NM for climbing or descending while vertical separation doesn’t exist. And remember the NAT HF exception — no direct pilot-controller link there. Let me show you the geometry of the same-track case, because it’s the foundation for all of these.