Instrument Landing System (ILS) — Page 163, Lesson 160

Right, let's get into the Instrument Landing System — the ILS. This is the precision approach workhorse, and I want to build it up properly from the ground. First, the big picture. The ILS is a ground-based system that gives you both lateral and vertical guidance to the runway. It's made of several distinct components, each on its own frequency band, and I want you to hold the whole family in your head before we look at how they work together. Let's start with the components and frequencies, because these are the hard numbers you'll be quizzed on. The Localizer gives you lateral guidance — left and right. It operates on VHF, from 108 to 111.975 MHz, and there are 40 channels available. The aerial is sited at the upwind end of the runway — that's the far end, the threshold you're landing towards. The Glide path — sometimes called the glide slope — gives you vertical guidance. It operates on UHF, and here's the key relationship: its frequency is paired with the localizer frequency. You don't tune it separately; selecting the localizer channel automatically selects the glide path frequency. The aerial sits abeam the touchdown point — that is, level with where your wheels are meant to touch. Then we have the Markers. These are on VHF, 75 MHz, and they radiate a fan-shaped vertical pattern. There are three of them: the OM, the MM, and the IM — outer, middle, and inner markers. I'll come back to their individual characteristics in a moment. There's also the Back beam — this is radiation from the localizer that points in the opposite direction, away from the approach. It's only usable for a non-precision approach, and I'll explain why when we get to the principle of operation. The Locator is a low-power NDB — a non-directional beacon — sited at the outer marker. It helps you find the approach path and gives you a position fix. And the DME — distance measuring equipment — is frequency paired with the ILS as well. It can be used in place of markers on some installations, and crucially, it's zero-referenced to the threshold. That means when the DME reads zero, you're at the runway threshold, not at the antenna. Finally, the Ident — the identification signal. It's 2 or 3 letters, sent at 7 groups per minute. This is your integrity check: if the ILS is unserviceable, the ident is suppressed — you lose it entirely. And during maintenance, you get a continuous tone instead of the normal code. So the ident tells you the system is alive and healthy. Now, let me give you the marker characteristics in detail, because each one has its own colour, code, and frequency. The Outer Marker — the OM — is blue. It transmits 2 dashes per second at 400 Hz. It sits between 6.5 and 11.1 km from the threshold. The Middle Marker — the MM — is orange. It transmits 3 characters per second, and those characters are alternate dots and dashes, at 1300 Hz. It's at 1050 m from the threshold. The Inner Marker — the IM — is white. It transmits 6 dots per second at 3000 Hz, and it's at 75 to 450 m from the threshold. So you can see the pattern: as you get closer to the runway, the markers get closer together, the tone gets higher, and the code gets faster. That's how you know where you are on the approach without looking at a chart. Now, ground monitoring. The ILS has to prove it's working within tight tolerances, or it has to shut down or downgrade. The localizer must be accurate to within 35 feet at the reference datum for Category I operations. The glide path must be within 0.075 times the glide path angle. And the power must be within 50% of nominal. If any of these are exceeded, the system must either cease radiation, remove the ident, or lower the category of the approach. That's the safety net. Coverage is also defined. The localizer provides coverage out to 25 NM within ±10° of the centreline, and 17 NM within ±35°. The glide path covers 10 NM within ±8°, and vertically from 0.45 to 1.75 times the glide path angle. Now let's get to the principle of operation — this is where it all clicks together. The localizer transmits two lobes. The left-hand lobe is modulated at 90 Hz, and the right-hand lobe at 150 Hz. The receiver compares the depth of modulation of these two signals. The difference in depth of modulation — the DDM — is what drives your course deviation indicator. As you move away from the centreline, the DDM increases. When you're exactly on the centreline — on the "localizer course", which we write as the symbol ℄ — the DDM is zero. So the needle centres when the two modulations are equal. The back course is the same radiation seen from behind. If it's approved for use, it's a non-precision approach — no glide path. The key trap is that the CDI readings are reversed on the back course. However, if you're flying an HSI — a horizontal situation indicator — you can get correct sense by setting the front course QDM into the course selector. I'll come back to that. The glide path works the same way, but vertically. The upper lobe is 90 Hz, the lower lobe is 150 Hz. The DDM increases as you move away from the glide path centre line, and the DDM is zero on the centre line itself. So the glide slope needle centres when you're on the correct descent angle. One thing to be aware of: there are false glide paths at multiples of the glide path angle. If the true glide path is 3°, there can be false ones at 6°, 9°, and so on. You must be aware of these and not lock onto one. The reference datum is the height of the glide path over the threshold — that's the point the whole system is aligned to. Now, the indicators. On a basic CDI — course deviation indicator — the localizer sensitivity is 0.5° per dot, with a maximum deflection of 2.5°. And remember, on the back course, that indication is reversed. On an HSI, you set the course selector to the front course QDM to get correct indications — that's the trick I mentioned. For the glide path indicator, sensitivity is 0.14° per dot, with a maximum of 0.7°. And there's a safety limit: the maximum safe deviation is 2.5 dots fly up, which corresponds to 0.35° above the glide path. Beyond that, you're too high to recover safely. Now, the guidance limits — this is the EASA categorisation, and it's critical for your operations. There are two axes here: the facility category and the operational category. The facility category — what the ground equipment is certified for — is I, II, or III. Category I supports approaches down to 200 ft decision height. Category II down to 50 ft. Category III down to 0 ft. The operational category — what you, the crew, are certified and equipped for — is I, II, IIIA, IIIB, and IIIC. The decision height for each: Category I is ≥ 200 ft. Category II is ≥ 100 ft. Category IIIA is < 100 ft or 0 ft. Category IIIB is < 50 ft or 0 ft. And IIIC is 0 ft — no decision height at all. And the RVR — runway visual range — minimums: Category I needs ≥ 550 m. Category II needs ≥ 300 m. IIIA needs ≥ 200 m. IIIB needs ≥ 75 m. And IIIC needs 0 m — zero visibility. So the facility category tells you what the ground can do, and the operational category tells you what you can do. The approach you fly is limited by the lower of the two. Finally, the errors — the things that can degrade the signal and that you must be alert to. There's beam bends — the signal path curving due to terrain or obstacles. There's scalloping — a rapid oscillation of the needle. There's beam noise — general signal degradation. There are restricted vehicle movements during low visibility operations — ground traffic can distort the signal, so it's controlled. And you must always check the failure flags on your instruments, and monitor the ident — because if the ident disappears, the system is telling you it's not safe to use. So that's the ILS in full — the components, the frequencies, the modulation principle, the categories, and the errors. Let me know if you want to dig into any part of it.