In aviation, autoland describes a system that fully automates the landing procedure of an aircraft's flight, with the flight crew supervising the process. Such systems enable airliners to land in weather conditions that would otherwise be dangerous or impossible to operate in.
A few general aviation aircraft have begun to be fitted with "emergency autoland" systems that can be activated by passengers, or by automated crew monitoring systems. The emergency autoland systems are designed to complete an emergency landing at the nearest suitable airport, without any further human intervention, in the event that the flight crew is incapacitated.
Description Autoland systems were designed 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 meters runway visual range and/or in adverse weather conditions, although limitations do apply for most aircraft—for example, for a Boeing 747-400 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 auto deployment of spoilers and thrust reversers.
Autoland may be used for any suitably approved instrument landing system (ILS) or microwave landing system (MLS) approach, and is sometimes used 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.
Autoland requires the use of a radar 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 ). The localizer signal of the ILS may be used for lateral control even after touchdown until the autopilot is disengaged. 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.
It can be disengaged only by completely disconnecting the autopilot (this prevents accidental disengagement of the autoland system at a critical moment) or by initiating an automatic go-around. 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.
The autoland system's response rate to external stimuli work very well in conditions of reduced visibility and relatively calm or steady winds, but the purposefully limited response rate means they are not generally smooth in their responses to varying wind shear or gusting wind conditions – i.e., not able to compensate in all dimensions rapidly enough – to safely permit their use.
The first aircraft to be certified to CAT III standards, on 28 December 1968,Slat Retraction During Reverse Thrust? 747-400 Tech Ops Forum |Airliners.net was the Sud Aviation Caravelle, followed by the Hawker-Siddeley HS.121 Trident in May 1972 (CAT IIIA) and to CAT IIIB during 1975. The Trident had been certified to CAT II on 7 February 1968. Besides providing automatic landing, automatic ground roll and extensive en route facilities, the Trident's AFCS (Automatic Flight Control System) also provided automatic overshoot (go-round) which was essential for Cat II operation, PVD (paravisual display) ground roll guidance for take-off in 100 metres runway visual range (RVR) and as back up to the ‘fail-soft’ automatic rudder control system during Cat. IIIB landings, and a Ground Run Monitor (GRM) for measuring ground speed and distance travelled as an aid for estimating runway turn-off points and taxying.Interavia 1976 ‘Autoland Starts to Payoff for British Airways’ Don K. Craig
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.
Autoland is highly accurate. In his 1959 paper,W. J. Charnley (1959). Blind Landing. Journal of Navigation, Vol. 12, No.2, April 1959, p 128 http://journals.cambridge.org/abstract_S037346330001794X John Charnley, then Superintendent of the UK Royal Aircraft Establishment's (RAE) Blind Landing Experimental Unit (BLEU), concluded a discussion of statistical results by saying that "It is fair to claim, therefore, that not only will the automatic system land the aircraft when the weather prevents the human pilot, it also performs the operation much more precisely".
Previously, autoland systems have been so expensive that they were rarely used on small aircraft. However, as display technology has developed, the addition of a head up display (HUD) allows for a trained pilot to manually fly the aircraft using guidance cues from the flight guidance system. This significantly reduces the cost of operating in very low visibility, and allows aircraft that are not equipped for automatic landings to make a manual landing safely at lower levels of look ahead visibility or runway visual range (RVR). In 1989, Alaska Airlines was the first airline in the world to manually land a passenger-carrying jet (Boeing B727) in FAA Category III weather (dense fog) made possible with the head-up guidance system.
History
Background Commercial aviation autoland was initially developed in the United Kingdom, as a result of the frequent occurrence of very low visibility conditions in winter in Northwest Europe. These occur particularly when anticyclones are in place over Central Europe in November/December/January when temperatures are low, and radiation fog easily forms in relatively stable air. The severity of this type of fog was exacerbated in the late 1940s and 1950s by the prevalence of carbon and other smoke particles in the air from coal burning heating and power generation.
Cities particularly affected included the main UK centers, and their airports such as London Heathrow, London Gatwick, Manchester, Birmingham and Glasgow, as well as European cities such as Amsterdam, Brussels, Paris, Zurich and Milan. Visibility at these times could become as low as a few feet (hence the London fogs of movie fame) and, when combined with the soot, created lethal long-persistence smog. These conditions led to the passing of the UK's "Clean Air Act," which banned the burning of smoke-producing fuel.
During the immediate post-war period, British European Airways (BEA) suffered a number of accidents during approach and landing in poor visibility, which caused it to focus on the problems of how pilots could land safely in such conditions. A major breakthrough came with the recognition that in such low visibility the very limited visual information available (lights and so on) was extraordinarily easy to misinterpret, especially when the requirement to assess it was combined with a requirement to simultaneously fly the aircraft on instruments. This led to the development of what is now widely understood as the "monitored approach" procedure.
One pilot is assigned the task of accurate instrument flying while the other assesses the visual cues available at decision height, taking control to execute the landing once satisfied that the aircraft is in fact in the correct place and on a safe trajectory for a landing. The result was a major improvement in the safety of operations in low visibility, and as the concept clearly incorporates vast elements of what is now known as crew resource management (although predating this phrase by some three decades) it was expanded to encompass a far broader spectrum of operations than just low visibility.
However, associated with this "human factors" approach was a recognition that improved autopilots could play a major part in low-visibility landings. The components of all landings are the same, involving navigation from a point at altitude en route to a point where the wheels are on the desired runway. This navigation is accomplished using information from either external, physical, visual cues, or from synthetic cues such as flight instruments. At all times, there must be sufficient total information to ensure that the aircraft's position and trajectory (vertical and horizontal) are correct.
The problem with low visibility operations is that the visual cues may be reduced to effectively zero, and hence there is an increased reliance on "synthetic" information. The dilemma faced by BEA was to find a way to operate without cues, because this situation occurred on its network with far greater frequency than on that of any other airline. It was particularly prevalent at its home base, London Heathrow, which could effectively be closed for days at a time.
Development of autoland The United Kingdom government's aviation research facilities including the Blind Landing Experimental Unit (BLEU) set up during 1945/46 at RAF Martlesham Heath and RAF Woodbridge to research all the relevant factors. BEA's flight technical personnel were heavily involved in BLEU's activities in the development of Autoland for its Trident fleet from the late 1950s. The work included analysis of fog structures, human perception, instrument design, and lighting cues amongst many others. After further accidents, this work also led to the development of aircraft operating minima in the form we know them today. In particular, it led to the requirement that a minimum visibility must be reported as available before the aircraft may commence an approach – a concept that had not existed previously. The basic concept of a "target level of safety" (10^-7) and of the analysis of "fault trees" to determine probability of failure events stemmed from about this period.
The basic concept of autoland flows from the fact that an autopilot could be set up to track an artificial signal such as an Instrument Landing System (ILS) beam more accurately than a human pilot could – not least because of the inadequacies of the electro-mechanical flight instruments of the time. If the ILS beam could be tracked to a lower height then clearly the aircraft would be nearer to the runway when it reached the limit of ILS usability, and nearer to the runway less visibility would be required to see sufficient cues to confirm the aircraft position and trajectory. With an angular signal system such as ILS, as altitude decreases all tolerances must be decreased – in both the aircraft system and the input signal – to maintain the required degree of safety.
This is because certain other factors – physical and physiological laws which govern for example the pilot's ability to make the aircraft respond – remain constant. For example, at 300 feet above the runway on a standard 3 degree approach the aircraft will be 6000 feet from the touchdown point, and at 100 feet it will be 2000 feet out. If a small course correction needs 10 seconds to be effected at 180 kts it will take 3000 ft. It will be possible if initiated at 300 feet of height, but not at 100 feet. Consequently, only a smaller course correction can be tolerated at the lower height, and the system needs to be more accurate.