Avro wrote:Don't forget to mention that the Comet had a very bad crash record. This had mainly to do with the engines which were fixed in the wing.
I agree with Comet on the Comets: the major (if not the only) cause of the crashes was metal fatigue. A few years later I was studying materials at the University and the professor took the example of the Comet to explain what metal fatigue was. Nothing to do with the position of the engines.
Metal fatigue is the weakening of a metal part due to repeated cyclical movement such as bending or torsion (twisting). This usually happens as a result of the movement of a component in the course of its operation, such as the slight flapping of an airplane's wings when the plane is airborne, but it can also be pressurisation and depressurisation.
The dangers of metal fatigue were dramatically highlighted in the early 1950s, when the first jet airliner, the British de Havilland Comet, went into service. In early 1954, less than 2 years after it had first begun to carry passengers, a Comet broke up in midair over the Mediterranean Sea. A short time later, a second Comet went down over the coast of Italy. Prior to these crashes, two other accidents with fatalities had occurred, but these had been attributed to structural failure brought on by severe weather conditions. In fact, the structural failures were the result of metal fatigue. This was demonstrated when the fuselage of a Comet was immersed in a large tank. The plane's cabin was filled with water and then emptied to simulate the cycle of pressurisation and depressurisation that it would experience in the course of actual operation. After 3,000 pressurisation and depressurisation cycles, the investigators found severe metal fatigue in the form of a 2.4-m crack that extended from an escape hatch window through the frame of another window. It was then surmised that similar cracks in the ill-fated Comets led to rapid structural failures and tragic loss of life.
In response to the hazards of metal fatigue, the aircraft industry substantially modified some of its design and construction practices, among them the use of thicker aluminum skins that were provided with crack stoppers welded in the fuselage interior. Static testing procedures were stiffened, including subjecting the airframe to 50,000 pressurisation cycles in order to simulate the number of takeoffs and landings that might be expected in the course of an airplane's operational life.
Starting new technologies (the jet age for aeroplanes) always involves a learning curve that sometimes ends dramatically. Neverthelesss, everyone recognises that De Havilland (not Boeing) were the pioneers of commercial jets with their Comet.
