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Aviation History
1978
1978 - 1138.PDF
90 RIGHT international, 8 July 1978 audit will be the withdrawal of arbitrary lives wherever, for lack of knowledge, the CAA has had to apply them. Every manufacturer who wants an indefinite life for his aircraft will have to produce an SIA. The SIA will not be mandatory. The CAA will not require it as part of an airworthiness certificate. But the authority will say that without an SIA, lives may have to be imposed. The auditing work will be done by the manu facturer, who will issue the operator with the appropriate Inspection Document. What aircraft will be affected? The CAA has already published structure lives for Boeing 707s, 720s and 727s and Douglas DC-8s. These are lives beyond which the CAA has said that it cannot renew airworthiness certificates. In the case of the Boeing 707 and 720, for example, the CAA's published lives fall between 50,000 and 60,000hr or 30,000 flight cycles. Again, these lives are not strictly speaking mandatory, but any operator flying beyond them would find himself without a certificate of airworthiness. Should he fly without that document, he would of course be in legal trouble. The aircraft not affected are those designed in the safe- life era and which have safe lives already enumerated and applied: the Britannia, Viscount, Comet and Herald, for example. Aircraft designed to fail-safe principles are the ones to which the structural integrity audit Will apply. They include, in addition to the Boeings and DC-8s on the UK register, VClOs, One-Elevens, HS.748s and Vanguards. Safe lives are in practice determined by the manufac turer, and are advised to the operator either by him or by the airworthiness authority. In practice, if a United King dom operator buys an ageing fail-safe aircraft, the CAA will now make sure the operator has the appropriate SIA and inspection document. The authority may otherwise put a finite life on his aircraft. Will there be much "junking" of probably quite sound aircraft as a result of the new policy? "No," says John Pardoe of the CAA. "In fact it will be the other Way round. Provided the makers produce the appropriate inspection documents, and provided the airlines comply with them properly, we will now have more confidence in older structures." What will be the effect of the new policy on the air line industry? Pardoe says that it will obviously mean more precise inspection. "We expect airlines for the first time to be given complete detailed information about the sort of damage to look for, how to look for it, and precisely where to find it." In the past, he says, inspection pro cedures have too often taken the form of "inspect the tail every 5,000hr and if you find a crack, repair it." Now, he says, the manufacturers' inspection briefings will have to be much more precise. What incentive is there for a manufacturer to carry out an obviously extensive new structural integrity audit on an aircraft type no longer in production? Clearly, the CAA does not expect that all manufacturers will do such audits, especially if they have got new aircraft to sell, or if noisy engines are going to make the old ones scrappable anyway. But manufacturers want to carry on selling spares and, as Pardoe says, "they will never sell another aeroplane if they let a customer down on their older ones." He feels sure that British Aerospace will put the One-Eleven through a SIA. Is Boeing producing its new Inspection Documents as a result of the CAA's and ABB's philosophy? The CAA be lieves that the stand which it has taken on cracks in fail safe structures has played a leading part in changing everyone's mind on the need for better inspection and more inspectable design. The authority strongly believes that the new approach to geriatric jet problems is typical of what can result from the existence of two strong, in dependent and intellectually competitive airworthiness systems. The CAA's position has never been the alleged one of "euthanasia": junk your jets after x thousand hours or cycles unless otherwise demonstrated by test or service experience. Its philosophy, now backed by the advice of the Airworthiness Requirements Board, is: "If you want an indefinite life, show us your structural integrity audit and related new inspection schedules." The popular conception of CAA and British structural philosophy is one of full-scale testing of airframes and the lifeing thereof—-one fifth a safe-life design, half a fail-safe design. In fact this is nowhere spelt out for fail-safe de signs in British Civil Airworthiness Requirements (BCARs). But, over the years, and especially since the Comet 1 fatigue disasters, testing has in practice been more a way of life in Britain and Europe than it has in the United States. The American manufacturers have never gone in for Whole-aircraft certification testing to the same extent, nor has testing been spelt out in Federal Airworthiness Be- quirements. But what the Americans now know is that while you can have multiple fail-safe load members, in actual service cracks often occur at the same time in more than one, and possibly even in all, the members. A certain airliner's fin is held on by four fittings; these have fatigued simultaneously so that one day a gust could have taken the fin off. Inspection for this sort of multiple damage is clearly a priority of the new philosophy. The CAA thinks that both the British and the Americans will go for more testing, though not of whole-aircraft structures. Pardoe believes in the testing of representative sections to very high lives. In this way, testing is easier, quicker and cheaper. Given the right loads and avoiding the sort of miscalculations which brought early cracks to the Trident wings, the testing of smaller representative sections is expected to produce a much more realistic data base. Supplementing test data with service experience, the manufacturer will be able to advise the operator much more precisely about the sort of damage to look for and where to find it. The Trident wing-fatigue problem was, in the CAA's opinion, the result of small errors in the calculation of wing load-distribution, and much has been learnt as a result about the need for flight-measured loadings. Clearly too, in the CAA view, an American manufacturer got his tail fatigue loadings fatally wrong, failing to allow for the heavy loadings of spoiler buffet after touchdown. While the CAA finds it incredible that an Argentine operator could have missed 30in cracks in its 748s, the existence of the cracks remains, says Pardoe, "a disappoint ing fact." The publication of more detailed inspection advice is, he believes, one of the main contributions of the ARB technical committee and the CAA to the help now being provided for the aged airliner. Q
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