M apart (or such — Page 303, Lesson 398

Let's pick up right where the material leaves us — we're inside the longitudinal separation rules, and I want to walk you through the Mach number technique, which is a more precise way of keeping turbojets apart in time. First, the name itself. This is called the Mach number technique, and it's based on time, not distance. The whole idea is that we use the aircraft's Mach number — that's the ratio of its true airspeed to the local speed of sound — as the controlling parameter. So instead of measuring miles between aircraft, we measure minutes. Here's the operating principle. The technique requires turbojet aircraft to fly at the Mach number approved by ATC. That's a hard requirement — the pilot doesn't get to pick a convenient Mach number; ATC approves one, and that becomes the assigned value. And because that Mach number is the basis of the separation, the pilot must request approval before making any speed changes. You don't just nudge the throttles; you ask first. Now, there's a practical escape clause. If it's essential to make immediate temporary changes to speed — the classic example is turbulence — then you may act first, but you must inform ATC as soon as possible. So the rule is: act in an emergency, but tell them immediately afterwards. The separation is only valid if ATC knows what's happening. There's another situation that needs handling. If it's not feasible due to aircraft performance to maintain the last assigned Mach number during en route climbs and descents, then the pilot must advise ATC at the time clearance to climb or descent is requested. Notice the timing — you tell them when you ask for the climb or descent clearance, not after you've started. That way ATC can factor the speed change into the separation plan before they clear you. Now, the heart of it — when does separation actually exist? The rule says separation will be deemed to exist when the required time interval exists, and that's satisfied under one of two conditions. The first condition: The aircraft concerned have reported over the same reporting point and follow the same track or continuously diverging tracks until some other form of separation is provided. So both aircraft have passed the same reporting point — a defined fix they report over — and from there they either stay on the same track, or their tracks continuously diverge, meaning they keep spreading apart and never converge again. That divergence holds until some other form of separation takes over. The second condition covers the case where they haven't yet reported over the same point. Then it must be possible to ensure, by radar or other means, that the appropriate time interval will exist at the common point from which they will either follow the same track or continuously diverge. So radar or some other reliable method lets ATC predict that when both aircraft reach that common point, the required time gap will be there. Now let's get to the actual numbers — the time intervals. When the Mach number technique is applied, the minimum longitudinal separation between turbojet aircraft on the same track, whether they're in level, climbing, or descending flight, is 10 minutes — but there's a critical condition attached. That 10-minute minimum applies providing the preceding aircraft maintains a Mach speed equal to or greater than that maintained by the following aircraft. Think about why. If the lead aircraft is flying at a Mach number equal to or higher than the trailing aircraft, then the gap between them can't shrink — the lead is at least as fast, so it pulls away or holds the gap. That's what makes 10 minutes safe. If the trailing aircraft were faster, the gap would close, and 10 minutes wouldn't be enough. So the logic chain is: ATC approves the Mach numbers, the pilot flies that number and asks before changing it, and the 10-minute minimum only holds when the lead is at least as fast as the follower. That's the Mach number technique in its core form. I want to show you the geometry of this, because the "same track" versus "continuously diverging" distinction is much clearer visually. Let me bring up the longitudinal separation figure. There you can see how the time interval is measured along the track between two aircraft on the same route. And for the crossing tracks case, where the 15-minute standard applies — that's a different figure, but it shows the same principle of measuring time to a common point. So to tie it together: the Mach number technique is a time-based method for turbojets, it demands ATC-approved Mach numbers and prior approval for speed changes, it has clear rules for turbulence and for climb/descent performance limits, and it gives you a 10-minute minimum on the same track — but only when the lead aircraft is at least as fast as the one behind.