Autoland — Page 388, Lesson 479

I want to walk you through the Autoland chapter now, and we're starting with the foundation of it all — the Instrument Landing System, or ILS. Think of the ILS as a short-range navigational aid. Its whole job is to give the pilot both azimuth and vertical guidance during the approach to an airport runway. Azimuth is your left-right position relative to the runway centreline; vertical guidance is your height relative to the correct descent path. The system is split into two halves: ground-based transmitting elements, and receiving elements carried on board the aircraft. Let me name the ground-based elements, because there are three of them. First, a localizer transmitter, which sends runway azimuth approach information — that's your left-right steering. Second, a glide slope transmitter, which provides the vertical approach information — your descent path. Third, marker beacons, which transmit information about the distance to the runway threshold. Now, an important modern note: marker beacons have been replaced with Distance Measuring Equipment, or DME, in most installations. So in many places you won't see the old marker beacons anymore; DME does that distance job. Now the airborne elements — what the aircraft carries. There's a localizer signal receiving antenna, and here's a nice detail: it's usually the same antenna as the one used for the VOR. There's a glide slope signal receiving antenna. There's a dual ILS receiver installation — note the word "dual," meaning two receivers, which is part of the redundancy that makes autoland safe. There's an indicator which shows whether the aircraft is on the correct approach path, showing both localizer and glide slope deviation — that's your "Loc and GS deviation." There's a marker beacon antenna and receiver, or the DME instead. And finally, marker lights on the main instrument panel. That figure shows the autoland sequence, and it's worth keeping in mind as we go. Now let's move to the next big idea: weather minima. In low visibility operations, the weather limits for landing are given in two specific terms, and I want you to know both precisely. First, Runway Visual Range, abbreviated RVR. This is an instrumentally derived value that represents the range at which high-intensity lights can be seen in the direction of landing along the runway. So it's not a human guess — it's measured by instruments. The measurements are transmitted to the air traffic controller, who can then inform the pilot of the very latest visibility conditions. Second, Decision Height, abbreviated DH. This is the wheel height above the runway elevation at which a go-around must be initiated by the pilot — unless adequate visual reference has been established, and the position and approach path of the aircraft have been visually assessed as satisfactory to safely continue the approach or landing. So DH is a hard decision point: you either have the visual reference and the confidence to continue, or you go around. Now, the minimum values of DH and RVR together are known as "weather minima," and they're specified by the national licensing authorities for various types of aircraft and airports. Here's how it works in practice: when the traffic controller advises that the RVR is above the specified minimum, the pilot may descend to the specified decision height. If, by then, he has sighted a sufficiently large segment of the ground to enable him to be confident of his judgement, he may carry on and land. He must otherwise overshoot — and then either enter the holding pattern pending another approach, or divert to an alternative airport. There's one more subtle factor here. During the approach, the pilot's line of sight is down the glide path, not along the runway. That gives rise to something called "slant visual range," which a pilot must take into account to avoid misinterpreting visual cues. So even when the RVR looks acceptable, the slant view down the glide path can be different, and you have to be aware of that. Finally, we come to the ICAO categorization of low visibility landing capabilities, under the heading "All Weather Operations." The term "all weather operations" is frequently used in connection with automatic landing systems — and that's exactly where we're heading next, into how those categories define what an aircraft can do in low visibility.