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Aviation History
1978
1978 - 1136.PDF
manufacturer likes to see any of his aircraft, however old, involved in an accident. In any case', there is a very strong design incentive to monitor structural performance. We need in-service data for our new aircraft so that our design for inspectability is being constantly improved." Boeing admits that it is worried about proper inspection and maintenance information reaching all operators, par ticularly when aircraft have been resold several times. But Boeing emphasises that the aircraft it builds are designed for inspectability. Watson says: "We sell aircraft to cus tomers who know how to work out proper inspection pro grammes with the proper authority and with the right relationships. An important part of airworthiness approval is the inspection programme worked out with the airline, the manufacturer and the airworthiness authority. Air worthiness certification depends on the maintenance of an aircraft to an approved maintenance programme which is constantly updated and revised. Fail-safe philosophy de pends on inspection. There is no such thing as a fail-safe structure that needs no inspection." There will be a Supplementary Structural Inspection document for every Boeing model, each running to an average 100 pages. The first, covering the Boeing 707/720 series, is likely to appear in November, according to present Boeing internal planning. The second, on the 727 family, will be published about six months later, followed by the 737 and 747 material. These documents will be on the public record in the USA, like all safety publications and manufacturer's Service Bulletins (SBs). (In Britain the equivalent documents are protected by the CAA as "com mercial information.") Will the new documents be mandatory? This is up to the airworthiness authorities, in Boeing's view. "They are unlikely to be an airworthiness requirement and I don't think the FAA intends to make them such," says Yeager. He does not think that the authorities will issue publication deadlines or require approvals. "Instead Boeing is calling for some new inspections, re-emphasising some previous SBs, and organising it all into a single document." Boeing rejects the idea that British and American geriatric-airframe philosophies are in conflict. "We don't FLIGHT International, 8 Mr 1978 think it's the right question or the right approach," says Watson. Boeing feels that the British insistence on fatigue tests and the CAA's predilection for safe lives is as over- simple as the alleged American view that fatigue tests and calculated lives are unimportant. To the question "Should damage-tolerant structures have any finite limit?" Watson replies: "With proper inspections augmented by our airworthiness directive (AD) system, no." Boeing argues that aircraft structures must be not only damage-tolerant but inspectable. Why, asks Boeing, should there be a life at which a structure somehow suddenly becomes unsafe, especially when the fatigue scatter factor between one specimen and another off the same production line is five or even ten? "People who like applying factors can use a factor of two, but that's not our game." Why does Boeing limit its aircraft to a certain life war ranty, in other words a finite life, if at the same time it designs for damage-tolerance and inspectability—-which theoretically means a structure good for all time? Watson points out that stress fatigue is not the only source of damage. Loading accidents and scratches are another source. Another—perhaps the most important of all—is corrosion. "Corrosion is a much larger problem than mechanical fatigue," says Watson. Corrosion of the Boeing 707, in his opinion, is no worse and no better than that of 'People say to us you should build more durable aircraft like your competitors, and then they say the same thing to our competitors about our aeroplanes' ROBERT WATSON, BOEING other aircraft. "It depends on how the aircraft has been used and maintained. People say to us you should build more durable aircraft like your competitors, and then they say the same thing to our competitors about our aero planes. You can wet-seal, prime and paint all you like but feedback is that our structures compare favourably in total structural maintenance cost with competing aircraft. The 44,000hr sabotaged TWA 707 airframe which was salvaged by Boeing and subjected to a teardown showed virtually no ="«. •...••/>: 'j:;^.ir-.:--3iH a mm
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