Instrument Landing System (ILS) — Page 163, Lesson 154

Let's pick up with the ILS categories and the accuracy limitations. We've already covered Category I and II, so now I want to walk you through Category IIIC, which is the most demanding of all. Category IIIC operations have no DH and no RVR limitations. Let me unpack that. DH is the decision height — the altitude at which you must either see the runway or execute a missed approach. RVR is the runway visual range — how far along the runway you can actually see. In Category IIIC, neither of those applies. There is no decision height, and there is no runway visual range limitation. That means the approach is flown all the way to touchdown and through rollout with essentially zero visibility, relying entirely on the autopilot and the ILS guidance. That's the extreme end of the scale. Now, the acceptance of Category II or III operations — and this is a critical point for you as a professional — depends on four criteria being met. First, the aeroplane must have suitable flight characteristics. Second, the aeroplane will be operated by a qualified crew in conformity with laid down procedures. Third, the aerodrome is suitably equipped and maintained. And fourth, it can be shown that the required safety level can be maintained. So it's not just about the equipment on the ground or in the aircraft — it's a combined approval covering the aircraft, the crew, the airport, and the overall safety case. Now let's move to errors and accuracy, because the ILS has several limitations you must be alert to. The indications can be affected by three specific things. The first is beam bends caused by atmospheric conditions. The second is scalloping caused by reflections, which results in rapid fluctuations of the needles on the CDI or HSI — and those fluctuations are impossible to follow. CDI is the course deviation indicator, and HSI is the horizontal situation indicator — they're the instruments that show you the localizer needle. Scalloping makes that needle flicker rapidly, and you simply cannot chase it. The third is beam noise generated by the transmitter or due to interference. Because of all this, the pilot must be alert to the existence of potential problems and constantly cross-check the information being received. You never trust a single source blindly. There's also an operational consequence. To minimize interference to the ILS transmissions, the rate of landings has to be kept relatively low, and vehicle and aircraft movement must be restricted on the ground, especially during low visibility procedures. So the airport slows down operations to protect the signal. Now, how do you check the serviceability of the localizer and glide path? There are two pilot checks. First, ensure the warning flags are not visible — those flags indicate a failed or unreliable signal. Second, the pilot monitors the identification signals. This is crucial: cessation of the Ident means the ILS is unserviceable and the procedure must be discontinued immediately. The Ident is the Morse code identifier transmitted with the ILS signal. If it stops, you stop the approach. No exceptions. Finally, let's look at factors affecting range and accuracy, specifically multipath interference due to large reflecting objects. Multipath interference to ILS signals is dependent upon antenna characteristics plus any large reflecting objects, vehicles, and fixed structures within the radiated signal coverage. Moving objects can degrade the directional signals to an unacceptable extent. To protect the ILS signals from this interference, protected areas are defined. The first is the ILS Critical Area. This is an area of defined dimensions about the localizer and glide path antennae where vehicles and aircraft are excluded during all ILS operations. It is protected because the presence of vehicles or aircraft inside its boundaries will cause unacceptable disturbance to the ILS signal-in-space. So during any ILS operation — not just low visibility — that area stays clear. That's the full picture on Category IIIC and the accuracy limitations. The key takeaways: IIIC has no DH and no RVR limits, the four approval criteria, the three error sources — beam bends, scalloping, beam noise — the Ident check with immediate discontinuation, and the ILS Critical Area protection.