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The Real Story of the Comet Disaster (De Havilland Comet - Structural Fatigue)

Withey, Paul

Editor(s)
Scholz, Dieter

Design work began on the De Havilland Comet in September 1946. Key features: 36 passengers, range 2800 km, cruising speed 780 km/h, cruising altitude 35000 ft,  aluminium construction,  four de Havilland Ghost Jet engines. The pressurised cabin was designed for a cabin pressure equivalent to an altitude of 8000 ft. De Havilland were aware of the likelihood of fatigue and had installed several safety measures and tests in line with certification requirements. First flight was on 27th July 1949. The aircraft entered service on 2nd May 1952 (G-ALYP) and put the UK aircraft industry at the forefront of technology. First accident of a De Havilland Comet was on 10th January 1954. G-ALYP crashed into the sea leaving Rome. Flight services was resumed on 23rd March 1954. The second accident was on 8th April 1954. G-ALYY crashed also into the sea leaving Rome. The Certificate of Airworthiness was withdrawn 12th April 1954. Intensive research followed, concentrating on the understanding of structural fatigue. A Comet fuselage was pressurized in a water tank. The recovered wreckage of the "YP" was assembled on frames by the RAE. It was found that the aircraft disassembled in the air. The accident was caused by structural failure of the pressure cabin, brought about by fatigue. The square windows were the cause of high stresses. The bolt hole which failed on "YP" had a defect in the chamfer which indicated the potential for manufacturing defects on all skin holes. The interaction of the skin stresses and the manufacturing defects was beyond the scientific knowledge base of the early 1950s. The Comet flew again as the Comet IV with different window design. The Comet was the first airliner to fly a scheduled service across the Atlantic on 4th October 1958. It remained in service as the Nimrod until 60 years after first Comet flight. The presentation revisits the Comet case and shows a modern investigation based on the research done in the 1950th and the "Paris Law" from 1963 which allows the calculation of crack growth. Using the data from the 1950’s, it was calculated: parameter A = 49.5 MPa m^(1/2) and exponent m = 5.  As such, the material behaved slightly worse than current similar alloys. Crack growth analysis calculated the life of "YP" as 1272 cycles. The actual number of pressurised flights was 1290.

http://hamburg.dglr.de --- http://www.raes-hamburg.de --- http://www.hav-connect.aero/Group/Lectures
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