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Aviation History
1976
1976 - 0008.PDF
AIR TRANSPORT The real world of Cat 3a JOHN BELSON reports on the latest developments in the Trident Autoland programme. B i EALINE 234 is cleared to Heathrow—standard routing for landing 28L—runway visual range two zero zeroi metres." Clearances of this type have resulted in, over 70 Category 3a landings by British Airways Tri dents at London Heathrow this winter. During the long and often tedious development of the Smiths Industries Autoland automatic-landing system for British Airways' Tridents the advantages claimed for guaranteed regularity have been obvious. Not so obvious, perhaps, is the way in which Autoland appears tailor-made to the State carrier's London-Glasgow Shuttle service. The Trident fleet is fitted with a common standard of equipment and ail aircrew are qualified on the type for Category 3a approaches. In conditions worse than Cate gory 2 other airlines find themselves grounded at Heath row or diverted, leaving the field to British Airways' Tri dents. "When that happens," says Capt F. Ormonroyd, European Division's all-weather operations controller, "you know the cost is worth it" The inch-by-inch pace of Autoland development reflects the fact that Trident manufacturer Hawker Siddeley, Smiths Industries, which makes the system, and British Airways, the customer, lead a field in which the pitfalls are many and the means to overcome them largely un- tried. The UK certification process is more demanding than its US or French equivalents; the spectacular results achieved in those countries owe much to the relatively undemanding requirements imposed by their airworthi ness authorities. The Trident 1 flight trials, which finished in January 1967, provided data for triplex operation down to Cate gory 3a weather conditions. A full technical description appeared in Flight for November 12, 1970. In February 1969 the airline was authorised to operate to mid-Category 2 limits (150ft decision height and 500m runway visual range), with full Category 2 clearance depending on a history of successful operations at the interim limits and improvements to ground facilities at a number of airfields. Initially only automatic landings were allowed but manual landings in Category 2 conditions—100ft decision height and 400m runway visual range—are now permitted. Formal Civil Aviation Authority approval clearing the system for use in Category 3a conditions was granted on May 22, 1972, but modification of the fleet, training of aircrew and provision of ground facilities have resulted in a rather slow introduction of Autoland into regular service. Refinement of the landing ground-roll phase has now been completed (see Flight for June 8, 1972) and the system is now ready for Category 3b operations (under which the pilot does not have to see the runway lights for any great distance ahead until the landing roll is almost complete). In practice, zero visibility means anything up to 25m, the first figure beyond zero on the RVR scale. Capt Ormonroyd stresses that the programme has been lengthy because "we didn't know much about it when we started, but it has done a lot for the future and all new air craft have benefited from our lead in the field." The pro gramme has been a "national effort and might have faltered but for the patient guidance of the CAA's All-Weather Operations Group," which ultimately approves regular airline operations in low visibility. Several other Government agencies have been involved and most recently it has been the airfield operators and air traffic control services which have contributed to the success of the latest series of Category 3 landings. The development programme has been dominated by the need to demonstrate that the autopilot system will never contribute to an accident rate more often than once per ten million landings (1X10-7)—ten times better than the historical landing^accidenit rate. This represents the average target for the equipment design and caters for a double failure. By using a triplex autopilot a faulty sub channel can be identified and discarded by majority vote of the two good sub-channels, leaving the aircraft still under autopilot (duplex) control but at a reduced safety margin. If individual sub-channel reliability is high enough to ensure that the autopilot does not go duplex too often, the chances of losing two sub-channels on the same approach (resulting in autopilot disengagement) are low and it can also be shown that no single fault will cause autopilot disengagement, the system is then deemed suit able for Category 3 operation. The accompanying Trident Category 3 safety analysis diagram shows that the achieved design risk, at 0-7X10—7, handsomely improves on the requirement. The risk of an accident as a result of faults within the system (including ground installations) is now of a very low order, about one in 100 million approaches. Accident risk during a missed approach, with its low decision height, is three in 100 million. By 1973, with the system fully approved and aircrew training well under way, the British Airports Authority found itself under increasing pressure from British Air ways to provide the necessary ground installations. Airfield clearance involves not only the quality of the ILS equip ment but also lighting standards, topographical layout in the final approach zone and such support facilities as RVR measurement. An illustration of the way in which the various agencies work together is shown by British Airways' use of Heath row's 28R for automatic landings. The Smiths Industries Autoland equipment fitted to the Trident does not require glidepath guidance below 133ft; a Category 2 ILS glide- path is therefore sufficient, although a Category 3 localises is of course required. When it was realised that the 28R glidepath would not be upgraded to Category 3 this winter, steps were taken to amend the flight-manual clearance wording to allow for use of a Category 2 glidepath and
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