Autoland — Page 381, Lesson 465

I want to walk you through the autoland system, and I want to start by setting the scene, because the whole reason this system exists is the difficulty of the approach and landing manoeuvre. This is the most difficult thing a pilot is asked to do, and the reason is that it demands control of the aircraft in all three axes simultaneously — pitch, roll, and yaw — at the same time as controlling airspeed through engine power changes. So you're not just steering; you're managing a three-dimensional problem plus energy. Let me break down exactly what the pilot has to do, because these are the tasks the autoland system must replicate. First, align the aircraft with the runway centreline. Second, achieve a sink rate of about 2 feet per second before touchdown — that's the vertical speed you want at the moment of landing. Third, reduce the airspeed from 1.3 VS to about 1.15 VS by progressive reduction of engine power. Here, VS is the stall speed, so 1.3 VS is the typical approach speed, and you're bleeding that down to about 1.15 VS as you flare. And fourth, level the wings before actual landing, and yaw the aircraft to remove any drift angle — that's the drift "kick off" or de-crabbing, where you straighten the nose to align with the runway despite a crosswind. Now, the key point: an automatic landing system that takes over from a pilot must be able to provide guidance and control better than that required of the pilot. It's not enough to be as good as a human — it has to be better, because it's being trusted with the full landing. Let me talk about how this fits into the broader picture. Autopilots have for a long time been able to fly most of the approach, which lets the pilot concentrate on navigating the approach correctly. The pilot would then take over at decision height and continue to land manually. But there's a distinction I want you to hold onto: an aircraft fitted with all the equipment for a fully automatic landing may, due to lack of required ground equipment or simply for pilot experience requirements, only be allowed to carry out an auto-approach. Essentially, all the procedures are carried out as for an autoland, but when decision height is reached, the pilot takes over manually. So auto-approach is the same procedure, but the pilot completes the landing by hand. Now let's get into the autoland system itself, starting with its objective. For the operation of an automatic flight control system to achieve automatic landing, it must be of such a nature that it will do four things. First, not disturb the flight path as a result of an active malfunction — that means if a component fails in a way that actively drives the controls, the system must not cause a dangerous deviation. Second, have adequate authority for sufficiently accurate control along the required flight path — the system must have enough control power to fly accurately. Third, warn of a passive failure — a passive failure is one that doesn't drive the controls but degrades the system, and the crew must be alerted. And fourth, allow the intended flight manoeuvre to be completed following an active or a passive failure — so even after a failure, the landing must be able to be finished. Then we have the requirements — what the autoland system needs to complete an automatic landing. There are three. A minimum of two independent autopilots capable of following ILS signals — ILS is the Instrument Landing System, the radio guidance that defines the approach path. Two independent radio altimeters to give accurate height-from-ground information — note these are radio altimeters, which measure actual height above the terrain, not barometric altitude. And a Category 3 ILS ground installation at the airport — that's the ground equipment classification that permits very low visibility approaches. Finally, there's the concept of autoland status. The number of autopilots required depends upon the autoland status of the aircraft, and these fall into two main categories. Now, the excerpt cuts off right there, but I want you to understand the structure: the autoland status determines how many autopilots you need, and that's the next thing we'll look at when we continue.