From Wikipedia, the free encyclopedia

In aviation, autoland describes a system that fully automates the landing phase of an aircraft's flight, with the human crew merely supervising the process.

Contents 1 Description 2 History and evolution 3 References 4 See also

Autoland systems are intended to make landing possible in visibility too poor to permit any form of visual landing, although they can be used at any level of visibility. They are usually used when visibility is less than 600 metres RVR and/or in adverse weather conditions, although limitations do apply for most aircraft—for example, for a B747-400 the limitations are a maximum headwind of 25 kts, a maximum tailwind of 10 kts, a maximum crosswind component of 25 kts, and a maximum crosswind with one engine inoperative of five knots. They may also include automatic braking to a full stop once the aircraft is on the ground, in conjunction with the autobrake system, and sometimes deployment of spoilers and thrust reversers.

Autoland may be used for any Category III Instrument Landing System approach, and is sometimes used for to maintain currency of the aircraft and crew, as well as for its main purpose of assisting an aircraft landing in low visibility and/or bad weather. Only certain ILS approaches are certified as Category III, and both aircraft and crews must be certified for Category III approaches and autoland.

Autoland requires the use of a radio altimeter to determine the aircraft's height above the ground very precisely so as to initiate the landing flare at the correct height (usually about 50 feet). The localizer signal of the ILS is used to give lateral steering control even after touchdown until the pilot disengages the autopilot. For safety reasons, once autoland is engaged and the ILS signals have been acquired by the autoland system, it will proceed to landing without further intervention, and can be disengaged only by completely disconnecting the autopilot (this prevents accidental disengagement of the autoland system at a critical moment). At least two and often three independent autopilot systems work in concert to carry out autoland, thus providing redundant protection against failures. Most autoland systems can operate with a single autopilot in an emergency, but they are only certified when multiple autopilots are available.

Because autoland systems are fully automated and extremely precise, landings made with them can be smoother than landings made by human pilots, which means that a landing at an airport completely hidden in fog may be smoother than a landing made on a clear day. Some autoland systems include small randomizing factors in order to avoid constantly stressing the same points on a Category III runway (if all aircraft touch down at exactly the same spot, that spot on the runway will wear much more quickly than the rest).

The first aircraft to be certified to CAT III standards was the Sud-Aviation Caravelle, followed by the Hawker-Siddeley HS.121 Trident.

Autoland capability has seen the most rapid adoption in areas and on aircraft that must frequently operate in very poor visibility. Airports troubled by fog on a regular basis are prime candidates for Category III approaches, and including autoland capability on jet airliners helps reduce the likelihood that they will be forced to divert by bad weather.

Traditionally autoland systems have been very expensive, and have been rare on small aircraft. However as display technology has developed the addition of a Head Up Display allows for a trained pilot to manually fly the aircraft using guidance cues from the flight guidance system. This siginficantly reduces the cost of the system and also allows aircraft which could not otherwise perform fully coupled autolandings to do so. In the future the limitation may not be system cost in dollars, but the inability to get the hardware into the aircraft while still allowing sufficient safe headroom for the pilot. Alaska Airlines was the first airline in the world to manually land a passenger-carrying jet (B727) in FAA Category III weather (dense fog) made possible with the Head-Up Guidance System ^[1]

Likewise, Category III ILS approaches are expensive to develop and maintain, and are usually used only at airports where they are likely to be used often enough to justify the cost.

• Instrument Landing System