Right, let's pick this up — Page 286, Lesson 370

Right, let's pick this up. We've just dealt with the emergency situations where a forced landing is likely or has actually happened. Now I want to walk you through two very different procedural tools that an Air Traffic Controller uses, and they're both in this passage. First, Speed Control, which is item 15.41. This is a fundamental ATC tool. The controller can prescribe a specific speed for your aircraft, and the sole purpose of that is to maintain separation between aircraft. It's a spacing tool, pure and simple. Now, there are strict rules about how it's applied. The controller must give you adequate notice so that you, the flight crew, can actually comply with the instruction. And here's a key operational point: prolonged use of speed control is not recommended, because it directly affects your fuel usage. So it's a short-term measure, not a cruise management technique. When speed control is applied, the changes in speed should be limited to only what's necessary to maintain the separation minima or the spacing. There's a very specific set of numbers here, and you need to know these. At or above FL250 — that's Flight Level 250, or 25,000 feet on the standard altimeter setting — speed adjustments should be in multiples of M 0.01. That's Mach number, so you're adjusting in increments of one-hundredth of a Mach. Below FL250, the adjustments are in multiples of 10 kt IAS — that's 10 knots of Indicated Airspeed. So the higher you are, the more you're thinking in Mach; lower down, you're thinking in knots. Then we get to the approach phase, and the rules tighten up. On intermediate and final approach, speed changes are to be limited to minor adjustments of 20 kt IAS. And there's a hard stop: speed control is not to be applied to an aircraft after it has passed a point 4 NM — that's 4 nautical miles — from the threshold of the landing runway. Once you're inside that 4-mile point, the controller cannot touch your speed; you're committed to your approach configuration. Now, the second item, 15.42, is a completely different system: Automatic Dependent Surveillance, or ADS. Let me unpack that name, because it tells you everything. It's automatic because the aircraft does it without the pilot doing anything. It's dependent because it depends on the aircraft's own navigation systems to know where it is. And it's surveillance because it's a way for ATC to watch you. The specific variant we're talking about is ADS-B, the Broadcast version. Here's how it works: your aeroplane constantly broadcasts its current position and altitude, its category of aircraft, its airspeed, its identification, and whether the aircraft is turning, climbing or descending. All of that goes out over a dedicated radio data link to the Controller. This system was developed in the 1990s, and it relies on data from the Global Positioning System — the GPS — or any navigation system that provides an equivalent or better service. So the position it broadcasts is only as good as the navigation source feeding it. The maximum range of the system is line-of-sight, which is typically less than 200 nautical miles, and that's 370 kilometres. Because it's radio, if the aircraft is over the horizon, the signal doesn't reach. Now, who receives these transmissions? Two groups. First, the Air Traffic Service unit serving the airspace in which the aircraft is operating — that's your controller. And second, all other ADS-equipped aircraft within reception range. So it's not just a ground-to-air system; it's air-to-air as well, which gives pilots situational awareness of the traffic around them. So to tie it together: speed control is a procedural, voice-command tool for spacing, with very precise limits. ADS-B is a technological, automatic broadcast system that gives the controller a continuous picture of where you are and what you're doing. Both are about maintaining that safe separation, but they achieve it in completely different ways.