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

We're starting a brand-new chapter: the Microwave Landing System, or MLS. Let's dive right in. The MLS was designed to replace the Instrument Landing System, ILS, with an advanced precision approach system. The whole point was to overcome the disadvantages of ILS and give its users greater flexibility. Now, an important 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 this isn't something you'll see everywhere, but it's a system you need to understand. Let me define what MLS actually is. It's a precision approach and landing system that provides position information and various ground-to-air data. The position information is provided in a wide coverage sector, and it's determined by three measurements: an azimuth angle measurement, an elevation measurement, and a range measurement. So you're getting a three-dimensional fix on your position relative to the runway. Now, to really understand why MLS was built, we need to look at the disadvantages of ILS. There are five key ones I want you to remember. 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 aircraft converging on the same narrow beam too closely. Third, there are no special procedures available for slower aircraft, helicopters, and Short Take-off and Landing aircraft — that's STOL. The fixed, narrow ILS approach just doesn't accommodate their different performance. 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. The terrain has to be essentially clear of obstructions. 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. Anything that reflects or blocks the signal distorts the course. Now let's look at how the MLS system answers each of those problems. It has these features. First, there are 200 channels available worldwide. That's five times more than ILS. Second, the azimuth coverage is at least ±40° of the runway on-course line, which is the QDM. And glide slopes from 0.9° to 20° can be selected. That's a huge range compared to ILS's fixed glide slope. The usable range is 20 to 30 nautical miles from the MLS site, with 20 nautical miles being the figure in the UK. And third, there is no problem with back course transmissions. A secondary system is provided to... — and that's where the excerpt cuts off. But the key point is that the back course issue that plagues ILS is simply not a problem for MLS. So to tie it together: MLS gives you more channels, a much wider coverage sector, selectable glide slopes, and no back course concerns. That's the fundamental advantage over ILS.