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

Let’s start with the big picture. The Instrument Landing System, or ILS, is the precision approach aid that gets you from the en-route phase all the way down to the runway threshold, and in the highest categories, all the way to touchdown with zero visibility. I want you to think of it as three separate radio beams working together: the localizer for left-right steering, the glide path for up-down steering, and the markers for distance checks along the way. First, the components and their frequencies. The localizer operates on VHF, from 108 to 111.975 MHz, and it has 40 channels. Its aerial is at the upwind end of the runway. The glide path is on UHF, and its frequency is paired with the localizer — you tune one, the other comes along automatically. Its aerial is abeam the touchdown point. The markers are on VHF at 75 MHz, and they radiate a fan-shaped vertical pattern. You have the outer marker, OM, the middle marker, MM, and the inner marker, IM. Then there’s the back beam, which comes from the localizer and gives you a non-precision approach. The locator is a low-power NDB sitting at the outer marker. And DME, if fitted, is frequency-paired and zero-referenced to the threshold — meaning its distance reads zero at the threshold, not at the antenna. DME may replace the markers entirely. Now the identification. The ILS ident is two or three letters, sent at seven groups per minute. If the ILS is unserviceable, the ident is suppressed — you lose it. During maintenance, you get a continuous tone instead. That ident is your life check that the system is actually radiating a usable signal. Let’s go through the markers in detail, because each has its own colour, its own modulation, and its own distance from the threshold. The outer marker is blue, it sends two dashes per second at 400 Hz, and it sits between 6.5 and 11.1 kilometres from the threshold. The middle marker is orange — note, the book says orange — it sends three characters per second, alternating dots and dashes, at 1300 Hz, and it sits at 1050 metres. The inner marker is white, six dots per second at 3000 Hz, and it sits between 75 and 450 metres. So the closer you get, the higher the tone frequency and the faster the repetition. That’s your auditory cue for progress down the approach. Ground monitoring is what keeps the system honest. The localizer must stay within 35 feet at the reference datum for Category I. The glide path must stay within 0.075 times the glide path angle. And the power must stay within 50 percent. If any of those limits are exceeded, the system must cease radiation, remove the ident, or lower the category. That’s the fail-safe. Coverage is defined in two parts. The localizer covers 25 nautical miles within ±10 degrees, and 17 nautical miles within ±35 degrees. The glide path covers 10 nautical miles within ±8 degrees, and from 0.45 to 1.75 times the glide path angle vertically. Now the principle of operation. The localizer has a left-hand lobe modulated at 90 Hz and a right-hand lobe at 150 Hz. The depth of modulation, DoM, increases as you move away from the centre line, and the difference in depth of modulation, DDM, is zero exactly on the centre line. That DDM is the electrical signal that drives your course deviation indicator. On the back course, if it’s approved for a non-precision approach, the CDI readings reverse. But the HSI can operate in the correct sense if you set the front course QDM. The glide path works the same way, but vertically. The upper lobe is 90 Hz, the lower lobe is 150 Hz. DoM increases away from the glide path centre line, and DDM is zero on the centre line. Be aware of the false glide path at multiples of the glide path angle — that’s a real trap, and you must be aware of it. The reference datum is the height of the glide path over the threshold. Let’s talk indicators. On the CDI, one dot is 0.5 degrees, and the maximum deflection is 2.5 degrees. On the back course, the indication reverses. On the HSI, you set the course selector to the front course QDM to get correct indications. For the glide path, one dot is 0.14 degrees, maximum deflection is 0.7 degrees, and the maximum safe deviation is 2.5 dots fly up, which is 0.35 degrees. Now the guidance limits, and this is the EASA table you must know cold. The facility categories are I, II, and III, with decision heights of ≤200 feet, ≤50 feet, and 0 feet respectively. The operational categories are I, II, IIIA, IIIB, and IIIC. Category I has a DH of ≥200 feet and RVR of ≥550 metres. Category II has a DH of ≥100 feet and RVR of ≥300 metres. Category IIIA has a DH of <100 feet or 0 feet, and RVR of ≥200 metres. Category IIIB has a DH of <50 feet or 0 feet, and RVR of ≥75 metres. And Category IIIC has a DH of 0 feet and RVR of 0 metres — that’s full autoland to touchdown with no visual reference. Finally, the errors. You have beam bends, scalloping, and beam noise. During low visibility operations, restricted vehicle movements are in force to protect the signal. You must check the failure flags on your instruments, and you must monitor the ident. That ident is your continuous confirmation that the system is radiating correctly. That’s the complete ILS picture. You now have the frequencies, the markers, the monitoring limits, the coverage, the principle of operation, the indicator scales, the EASA categories, and the error sources.