Autoland — Page 388, Lesson 479

Let’s start with the big picture. We’re talking about Autoland — the system that lets an aircraft land itself in low visibility. Before we get to the landing itself, I want to walk you through the foundation, which is the Instrument Landing System, or ILS. An ILS is a short-range navigational aid. It provides two kinds of guidance during the approach to a runway: azimuth guidance, which is left and right, and vertical guidance, which is up and down. So it tells the aircraft where the runway is horizontally and where the glide path is vertically. The system is split into two halves. There are ground-based transmitting elements, and there are receiving elements carried on board the aircraft. On the ground, we have three things. First, a localizer transmitter. That sends runway azimuth approach information — in other words, it tells the aircraft whether it’s left or right of the runway centreline. Second, a glide slope transmitter, which provides the vertical approach information — whether the aircraft is above or below the correct descent path. Third, marker beacons. These 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 instead of a marker beacon telling you distance, you get DME. On the aircraft side, the airborne elements, we have several. There’s a localizer signal receiving antenna — and usually this is the same antenna 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, for redundancy. There’s an indicator that shows whether the aircraft is on the correct approach path — this shows localizer and glide slope deviation, so the pilot can see left/right and up/down errors. There’s a marker beacon antenna and receiver, or DME. And finally, marker lights on the main instrument panel. Now, here’s the key point about why we need Autoland in the first place. It comes down to weather minima. In low visibility operations, the weather limits for landing are given in two specific terms. The first is Runway Visual Range, or RVR. This is an instrumentally derived value. It represents the range at which high-intensity lights can be seen in the direction of landing along the runway. So it’s not just visibility in general — it’s specifically how far along the runway you can see the high-intensity lights. The measurements are transmitted to the air traffic controller, who can then inform the pilot of the very latest visibility conditions. The second term is Decision Height, or 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 height, measured from the wheels to the runway, and it’s the point of no return: either you have the visual reference and you’re confident, or you go around. The minimum values of DH and RVR together are known as ‘weather minima’. These are specified by the national licensing authorities for various types of aircraft and airports. So the authority sets, for each aircraft type and each airport, what the minimum RVR and minimum DH are. 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 — that is, go around — and 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 a factor called ‘slant visual range’. The pilot must take this into account to avoid misinterpretation of visual cues. So the visual range you actually see is slanted down the glide path, not horizontal along the runway — and that difference matters. 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. We’ll look at how these categories define what an aircraft can do in low visibility, and how Autoland fits into that picture.