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

Let's pick up with the Instrument Landing System, and I want to walk you through the final category of operation, then the accuracy limits, and finally the protected areas that keep the system trustworthy. First, Category IIIC. This is the most demanding ILS category. In Category IIIC there is no DH — that's decision height, the altitude at which you must either see the runway or go around — and there are no RVR limitations. RVR is runway visual range, the distance a pilot can see along the runway. So in Cat IIIC, you can land with zero visibility and no decision height at all. That's the extreme end of the scale. Now, before any operator can accept Category II or III operations, four criteria must be met. I want you to hold these as a checklist. First, the aeroplane has suitable flight characteristics — the aircraft itself must be capable. Second, the aeroplane will be operated by a qualified crew in conformity with laid down procedures — the pilots must be trained and must follow the published procedures exactly. Third, the aerodrome is suitably equipped and maintained — the airport's ground equipment must meet the standard. And fourth, it can be shown that the required safety level can be maintained — there must be evidence that the overall operation stays safe. Now let's talk about errors and accuracy, because the ILS has real limitations. The indications you see can be affected by three things. The first is beam bends caused by atmospheric conditions — the signal path can be distorted by the air it travels through. The second is scalloping caused by reflections, which produces rapid fluctuations of the needles on the CDI/HSI — that's the course deviation indicator or horizontal situation indicator — and these fluctuations are impossible to follow. The needle dances so fast you can't chase it. The third is beam noise generated by the transmitter or due to interference — electrical noise in the signal itself. Because of all this, the pilot must be alert to potential problems and constantly cross-check the information being received. You never trust one source blindly. There's also a practical operational point. To minimize interference to 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. That's why airports slow down operations in fog. Now, the pilot's serviceability checks. You can check the localizer and glide path two ways. First, ensuring the warning flags are not visible — those flags on your instruments indicate a failed signal, so if they're not showing, the signal is healthy. Second, the pilot monitoring the identification signals. This is critical: cessation of the Ident means the ILS is unserviceable and the procedure must be discontinued immediately. If the Morse code identification stops, you abort the approach right away. Finally, let's look at factors affecting range and accuracy, specifically multipath interference from large reflecting objects. Multipath interference depends on 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 — a truck driving through the signal path can ruin it. To protect the signals, protected areas are defined. The key one 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's 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, that ground area stays clear. That's the full picture: Cat IIIC with no DH and no RVR limits, the four acceptance criteria, the three sources of error, the two serviceability checks with the Ident rule, and the critical area protection.