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Aviation History
1989
1989 - 2331.PDF
Opposite At Hatfield on July 27,1949, the •world's first jet-propelled airliner is ready to fly. Above At Seattle 40 years later the best-selling jet airliner (about 2,000 ordered) takes off in its latest form are the gateways to every city. The pilots on that first flight, John Cunningham and John Wilson, agree with this recent comment of their flight-test observer, Tony Fairbrother: "I don't think it is too much to say that the world changed from the moment the Comet's wheels left the ground". Although the jet airliner was invented and developed in Europe, 7,000 of the 8,000 Western jet transports in service today were built in America. Few more than 100 Comets were built. That is another story. The jetliner production balance is chang ing. Forty years after the Comet l's first flight, Europe is getting 20 per cent of new jet- transport orders, thanks to co-operation called Airbus, and its share is rising. It could reach 30 per cent on the Comet l's 50th anniversary. The catastrophic fatigue failure of two Comet Is in 1954 diminished history's recog nition of the technical achievement. In the three years from general-arrangement draw-. ing to first flight, de Havilland doubled-the speed and altitude of air transport— designing, testing and producing the first practical passenger-rated jet engines, power- operated flying controls without manual reversion, structural bonding, integral wing fuel tanks, pressure refuelling, engine-bled air for pressurisation and deicing, and airbrakes. The Comets' designers achieved other innovations which today are taken for granted, such as swept-wing flexibility and upper-air gust analysis. The prototype, G-ALVG, flew 324 hours in its first 11 months with no major modifications—flight-test reliability unheard of in those days. Certifica tion was achieved in less than three years with two prototypes. The in-service accidents were caused by fatigue failure of the pressurised fuselage, in each case at about 1,000 flights. Fatigue cracks had probably started at the corner of a cutout, where normally higher stresses were aggravated by substandard riveting and were not contained by failsafe design, as we now call it. Fatigue was a known problem, and de Havilland had tested a Comet 1 fuselage section to 16,000 cycles in a water tank at Hatfield in the first-ever pressurisation fatigue test. In June 1953—six months before the first in-flight cabin failure—this test section failed in fatigue at the corner of a hatch after 16,000 applications of 8-25 lb/in2, the differ ential cabin working pressure (P). Why did two service fuselages fail at 1,000 flights? Why, after the accidents, did a test fuselage fail in the water tank at Farnborough long before 16,000 "flights", actually at 3,000? The Hatfield test section had been proof- tested before fatigue-cycling. In those days professional structures opinion was that, if you proved static strength, you proved fatigue strength. The bigger the margin the greater the fatigue strength. The actual airworthiness requirement was to demonstrate ultimate design strength at 2P, double the working pressure. Chief designer R. E. Bishop, tough as he was about weight-saving, actually increased this to 2-5P. Pumping up the test section to 2-5P had probably increased its fatigue life. The lessons, shared with the whole indus try, were profound. Boeing engineers visited Hatfield, and de Havilland engineers visited Seattle. The whole design culture changed. Structure life became as important as ultimate strength. The term failsafe, now part of the English vocabulary, was invented. A new dictionary of terms evolved: multipath load structure, inspectability, fracture toughness, damage tolerance, crack-stopper, tearstrap, inspectability. A whole new industry of non-destructive testing (NDT)—structural inspection with X-rays, dye penetrants, eddy currents, and ultrasonics—owes it origin to the Comet. The technical achievement was marred by the accidents, and the Comet and its designers paid the price which pioneering so often exacts, but the achievement deserves to stand proud. Creating the world's first jet- propelled airliner was a challenge as daunting in its day as the supersonic airliner and space shuttle were in theirs, and as Hotol is today. The Comet 4 redeemed the Comet 1, becoming in 1958 the first transatlantic jet transport, and giving a dozen airlines tech nically flawless service for 20 years. It flies on today as the RAF Nimrod, which may continue in service beyond the 60th anniver sary of the Comet 1. What will the daughters of the Comet be like in 40 years' time? That first flight 40 years' ago has led to the carriage of one billion jet passengers a year. The most successful jet transport in Halfway into the jet transport era the first super sonic airliner was born, and this year celebrated Its 20th birthday. FLIGHT INTERNATIONAL 29 July 1989 29
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