Let’s pick this up right where the MLS coverage story gets interesting — Page 170, Lesson 167

Let’s pick this up right where the MLS coverage story gets interesting. I want to walk you through the rest of the Microwave Landing System chapter, and we’re starting with the coverage limits and the operational advantages that come from them. First, the range. The MLS coverage extends out to 20 to 30 nautical miles from the MLS site, and in the UK the standard is 20 nautical miles. So when you’re flying an MLS approach, you can expect the guidance signals to be reliable out to that distance. Now, here’s a big one for you: the back course. With a conventional ILS, the back course is a problem — you get false guidance and you have to be careful. With MLS, there is no problem with back course transmissions. Instead, a secondary system is provided to give overshoot and departure guidance. That secondary system covers ±20° of runway direction, up to 15° in elevation, to a range of 10 nautical miles and a height of 10,000 feet. So if you go around or you’re departing, you still have guidance in that wedge behind the runway. Next, the frequency band. MLS operates in the SHF band — that’s Super High Frequency — specifically from 5031 to 5090.7 MHz. That high frequency is what lets it be sited in hilly areas without having to level the site. And here’s the key advantage: course deviation errors — what we call bending — of the localizer and glide path, which are caused by aircraft, vehicles, and buildings, are no longer a problem. Why? Because the MLS scanning beam can be interrupted and therefore avoids the reflections. So the beam is scanned, and if something reflects it, the system can ignore that interruption. That’s a fundamental improvement over ILS. Because of its increased azimuth and elevation coverage, aircraft can choose their own approaches. That means you’re not locked onto a single straight-in path. This increases runway utilization, and it’s particularly beneficial to helicopters and STOL aircraft — Short Take-Off and Landing aircraft — because they can fly steeper or more curved approaches. Now let’s move to the system integration and the cockpit side. The MLS has a built-in DME — Distance Measuring Equipment — so you get slant range distance as part of the system. It’s compatible with conventional localizer and glide path instruments, with EFIS — that’s Electronic Flight Instrument System — with autopilot systems, and with area navigation equipment. For indications, MLS gives you positive automatic landing indications, plus definite and continuous on/off flag indications for the localizer and glide slope needles. So you always know whether the guidance is valid — the flag tells you. The identification prefix for MLS is an ‘M’ followed by two letters. So when you identify the station, you’ll hear or see that M plus two letters. And the aim is for all MLS-equipped aircraft to operate to CAT III criteria — that’s the lowest visibility landing category, meaning the system is designed to support autoland in very poor weather. Let me point you to the figures that show this. shows the approach coverage volume — you can see the azimuth coverage of ±40° and the elevation coverage up to 15°, with the range out to 20-30 NM and the height up to 20,000 feet. shows the LMS coverage in more detail. And shows a typical MLS flight deck control panel. Let me walk you through that control panel, because you’ll need to know it. There’s a MODE SELECTOR with two positions. In AUTO, the glide slope and azimuth are dictated according to the selected channel — so the system picks the right angles for you. In MAN, you can make preferred glide slope and azimuth selections on a given channel manually. There’s a DISPLAY SELECT PUSHBUTTON. It calls up the AZ, G/S, or CHAN legend — that’s azimuth, glide slope, or channel — and the values of whichever one you’ve selected are then set on the ANGLE/CHANNEL SELECTOR. The ANGLE/CHANNEL SELECTOR itself has two concentric selectors — two knobs, one inside the other — used for AZ, G/S, and CHAN selection, depending on the mode you’ve set on the DISPLAY SELECT PUSHBUTTON. So on the display you’ll see: approach azimuth, which is the direction relative to the runway centre line; the required glide slope; and the channel number, which is selectable from 500 to 699. That’s the full picture of the MLS system — the coverage, the frequency advantages, the integration, and the cockpit controls. You’ve got the range limits, the back course solution, the SHF band, the CAT III aim, and the panel layout all in one place.