Now, one of the classic ILS problems is the back course — Page 170, Lesson 167

I want to walk you through the Microwave Landing System — the MLS — and I want to start by clearing up a common point of confusion. The MLS is the modern replacement for the older ILS, the Instrument Landing System, and it fixes several of the ILS's weaknesses. Let's look at the coverage first, because that's where the big differences show up. The MLS approach coverage extends out to 20 to 30 nautical miles from the MLS site, and in the UK specifically, that's 20 nautical miles. Now, the ILS localizer only gives you a narrow fan of coverage, but the MLS gives you a wide azimuth sector — 40 degrees either side of the runway centre line — and an elevation sector of 15 degrees up to a height of 20,000 feet. That's the key: the MLS scans a beam across that whole volume, so you get guidance over a much wider area. Now, one of the classic ILS problems is the back course. With a conventional localizer, flying the back course is awkward and often not usable. The MLS has no such problem. It provides a secondary system specifically for overshoot and departure guidance, covering plus or minus 20 degrees of runway direction, up to 15 degrees in elevation, out to a range of 10 nautical miles and a height of 10,000 feet. So if you go around, you still have full guidance. The 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 levelling the site, because the beam is narrow and the antenna is small. And here's the big one: course deviation errors — what we call bending — of the localizer and glide path, caused by aircraft, vehicles, and buildings, are no longer a problem. Why? Because the MLS scanning beam can be interrupted, and therefore it avoids the reflections. The ILS beam gets bent by reflections off hangars and moving vehicles; the MLS scans, so it can ignore those reflected signals. Because of that increased azimuth and elevation coverage, aircraft can choose their own approaches. They don't have to fly a single fixed path. That 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 curved approaches that suit their performance. Now let's look at the system integration. The MLS has a built-in DME — Distance Measuring Equipment — so you get continuous range information as part of the same system. It's compatible with conventional localizer and glide path instruments, with EFIS — that's the Electronic Flight Instrument System — with autopilot systems, and with area navigation equipment. So it slots into the existing cockpit. It gives positive automatic landing indications, plus definite and continuous on/off flag indications for the localizer and glide slope needles. Those flags are the little indicators that pop up to tell you the signal is unreliable — with MLS, that flag indication is definite and continuous, so you always know the state of your guidance. The identification prefix for the MLS is an 'M' followed by two letters. So when you hear the Morse identifier, it starts with M, then two letters — that's how you confirm you're tuned to the right station. And the aim is for all MLS-equipped aircraft to operate to CAT III criteria — that's Category III, the lowest visibility landing conditions, essentially automatic landing capability. Now, let me show you the flight deck control panel, because that's where you'll actually interact with this system. There's a MODE SELECTOR with two positions. In AUTO, the glide slope and azimuth are dictated according to the selected channel — the system decides which glide slope and azimuth go together. 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 AZ, G/S, or CHAN legends — 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 is two concentric selectors — an inner and an outer knob — used for AZ, G/S, and CHAN selection, according to the mode you've set on the DISPLAY SELECT pushbutton. So you can dial in the 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. So to tie it together: the MLS gives you wide coverage — 40 degrees azimuth, 15 degrees elevation, out to 20-30 nautical miles — it's immune to the reflection problems that plague the ILS, it has a built-in DME, it's compatible with your existing instruments and autopilot, and it's designed to get you to CAT III. The control panel lets you select your channel, your azimuth, and your glide slope, either automatically or manually. That's the MLS in a nutshell.