We’re starting a brand-new chapter today: the Microwave Landing System, or MLS — Page 170, Lesson 165

We’re starting a brand-new chapter today: the Microwave Landing System, or MLS. And I want to set the stage properly, because MLS only makes sense if you first understand what it was built to fix. So let’s begin with the introduction. MLS was designed to replace ILS—the Instrument Landing System—with an advanced precision approach system. The whole point was to overcome the disadvantages of ILS and give its users greater flexibility. But here’s the reality check: there are very few MLS installations in use at present, and they are likely to co-exist with ILS for a long time. So you won’t see MLS everywhere, but you must understand it as a professional. Now, what exactly is MLS? It is a precision approach and landing system that provides position information and various ground-to-air data. Let me unpack that. The position information is provided in a wide coverage sector, and it is determined by three measurements: an azimuth angle measurement, an elevation measurement, and a range measurement. So think of it as giving you a full three-dimensional position in space relative to the runway—not just a left-right and up-down needle like ILS, but an actual angle and distance picture. Before we go further into how MLS works, we have to look at why ILS needed replacing. The chapter lists five specific disadvantages of ILS, and I want you to know each one precisely. First, there are only 40 channels available worldwide. That’s a hard limit on how many ILS installations can operate without interfering with each other. Second, the azimuth and glide slope beams are fixed and narrow. Because they’re so narrow, aircraft have to be sequenced and adequately separated, and that causes landing delays. You can’t have multiple aircraft on the approach path close together because the beam is so tight. Third, there are no special procedures available for slower aircraft, helicopters, and Short Take-off and Landing aircraft—that’s STOL for short. So ILS simply doesn’t cater to those types. Fourth, ILS cannot be sited in hilly areas. It requires large expanses of flat, cleared land to minimize interference with the localizer and glide slope beams. That’s a big siting constraint. And fifth, vehicles, taxiing aircraft, low-flying aircraft, and buildings all have to be kept well away from the transmission sites. Why? To minimize localizer and glide slope course deviations—what we call bending of the beams. Any large object near the site can distort the beam and give you false guidance. Now, here’s where MLS answers each of those problems. The chapter gives us the MLS features, and I want you to contrast them directly with the ILS disadvantages. First, MLS has 200 channels available worldwide. That’s five times the capacity of ILS. Second, the azimuth coverage is at least ±40° of the runway on-course line—that’s the QDM, the magnetic bearing to the runway. And glide slopes from 0.9° to 20° can be selected. The usable range is 20 to 30 nautical miles from the MLS site, with 20 nautical miles being the figure in the UK. So you have a much wider sector to work with, which directly addresses the narrow-beam problem. Third, there is no problem with back course transmissions. And here’s where the excerpt cuts off mid-sentence: it says a secondary system is provided to g—and that’s where we stop. So I’ll pause here, because the next part of the chapter will explain what that secondary system is and how it handles the back course issue. Let me just make sure you’ve got the core picture before we move on. MLS is a precision approach and landing system giving you position via azimuth angle, elevation angle, and range, over a wide coverage sector. It was built to fix ILS’s five weaknesses: channel scarcity, narrow fixed beams, no special procedures for slow or STOL aircraft, difficult siting in hilly terrain, and vulnerability to beam bending from nearby objects. And it does that with 200 channels, wide azimuth and glide slope coverage, and no back course problem—which we’ll continue with next.