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Aviation History
1969
1969 - 0694.PDF
,~(\ - fLIGHT International. 17 April 1969 TRIDENT AUTOLANDING —reviewed in depth by BEA's chief engineer in RAeS de Havilland Memorial Lecture PART ONE Last evening, April 16, the 2nd Geoffrey de Havilland Memorial Lecture was being given at the Hatfield Branch of the Royal Aeronautical Society. Its author is Mr K. G. Wilkinson, chief engineer of BEA, and his paper—the full title of which is "Automatic Landing in BEA's Trident Operations—a Review of Effort and Achievement"—is a masterly exposition of an intricate subject of the greatest significance in commercial aircraft operation. The first section, only slightly abbreviated, appears below. J UST OVER TWO YEARS AGO, on the occasion of the inaugural de Havilland lecture. Richard Clarkson gave one of the finest accounts of an aviation pioneer and his work that I have ever heard. When I was asked to give this second lecture, I felt more than a little conscious of the standard set by my predecessor and by the quality of the man these lectures commemorate. T accepted the task because I felt that it was appropriate for an airline man to speak, since so much of Sir Geoffrey de Havilland's work had to do with the develop ment of air transport. My company is, moreover, the major user of the airliners currently being produced by the firm that he founded. Richard Clarkson commented in his lecture on de Havilland's belief that the designer should have the closest possible contact and understanding with the prospective user. This is even truer today, when feedback from operations is vital to the production of aircraft that are good enough to sell in a world market. What could be more appropriate, then, than a review of progress in one of the most significant and difficult projects ever undertaken jointly by manufacturers and operators—that of the achievement in regular commercial service of automatic landing, in what has come to be known as Category 3B (i.e., nearly blind) conditions? The project has from the outset needed the closest possible partnership between manufacturers of aircraft and systems, operators of aircraft and ground services and the research establishments. It has established a world lead for the partnership in a development which shows every promise of being the final answer to a fundamental problem of air transport which has beaten all-comers until today. Sir Geoffrey himself described a flight that he made in fog as "the most unpleasant experience in 50 years' flying." I am sure he would have approved of a lecture dealing with an attack on this problem which has already achieved major success and is pretty well on target for full "achievement in commercial service in two to three years' time—particularly so as his firm has played such an outstanding part in the work which promises to eliminate this kind of unpleasantness. When BEA ordered the Trident the specification included Autoflare (i.e., automatic control in pitch only at touchdown) but not fully automatic landing, blind or otherwise. Radio guidance systems and the general state of the art were not sufficiently advanced at the time for us to place an order for automatic landing or for de Havilland to accept such an order. So while we had blind landing very much in mind as the ulti mate objective, we ordered the Trident as an aircraft initially capable only of flaring automatically, with pilot controlling it in roll and azimuth. Provision for this so-called "Autoflare" was included in the original price of the aircraft. Why Autoflare? Without a guarantee of final blind landing it may appear pointless. To understand this we must go back about ten years. In 1956-57 BEA had a first-line fleet of propeller-turbine aircraft, but we were still operating some piston-engined propeller aircraft with approach speeds of less than 80kt. It was in this era that we were preparing our specification for the short-haul jet aircraft that became the Trident. One of our big worries at the time was that not only would it be difficult to improve on our existing weather minima but. with the higher approach speed, tower manoeuvrability, slower engine response and loss of "instant lift" provided by the propeller slipstream over the wing, it might not be possible to maintain our existing weather minima,with safety. In the transition from piston-engined aircraft to propeller- turbine aircraft we managed to maintain or improve our weather minima and safety by the introduction of "flight directors," which provided the pilot with control demand signals computed from basic flight parameters, and with auto pilots that could be coupled to VOR and ILS. This latter feature did not always work satisfactorily, but did relieve the pilots of some workload. The disturbing fact remained that within ten years we would be introducing aircraft having approximately double the approach and landing speed of some of our existing aircraft, which had weather minima which we needed to improve. There was nothing to give us confidence that we should be able to improve or even maintain our existing weather minima and standards of safety without a breakthrough in approach and landing techniques. Information about the ability of a human pilot to control an aircraft of the kind we had in mind in conditions of low visibility, or about pilot workload involved, was scanty. From our own and other people's experience with existing aircraft and from the excellent work emerging in a steady stream from E. S. Calvert at the RAE and from the Blind Landing Experimental Unit, we came to the conclusion that if an aircraft of the Trident performance were accurately controlled, both in speed and relative to the ILS localiser down to 200ft, the pilot had adequate information to carry on the approach in roll and azimuth, provided at least one crossbar of the Calvert lighting system was visible. At this point in an approach, a pilot would not, however, have adequate informa tion to control an aircraft in pitch unless the aiming point was visible. It therefore appeared that unless the Trident was to be operated on the basis that the pilot must have the aiming point in view by 200ft, or overshoot, we would have to provide him with some extra aid in pitch. The use of various types of head-up flight directors was considered—and a provision was, in fact, finally made for the para-visual director (PVD)—but again very little information on pilots' ability to use flight directors in this phase of flight was available—certainly not enough on which to base the future operations of the Trident. BLEU had, however, amply demonstrated that, without the use of any new external aids, an aircraft could be made to flare and touch down automatically with acceptable accuracy, making use of radio altimeters to measure height above runway. This had been accomplished with a "simplex" system, but the performance standard had been demonstrated. This meant that in spite of the fact that the ILS of that time was not satisfactory below 200ft it should be possible, with Fig I Relationship between estimated manual landing risk ("chances against") and runway visual range
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