Fatigue failure: de havelland comet

Ron Bishop was made Chief Designer to lead the project. The design changed a lot from its first phase, changing from a small aircraft with 6 seats to a long range jetliner with 36 seats. Eventually named the DH 106 Comet in December 1947, it was an all-metal, low-wing cantilever monoplane. Initial design was for it to be powered by four Rolls Royce Avon engines. However, de Havilland insisted on using the heavier De Havilland’s Mk 150 Ghosts which added a further challenge of trying to make the plane even lighter. The Comet’s thin metal skin was composed of advanced new alloys. Different panels were chemically bonded using an adhesive called Rivex, and were also riveted. This made the plane light and the Rivex added strength to the rivets which were prone to cracking from fatigue (Some experts claimed after 1954 crashes that the Rivex was the real cause of comets disintegrations). It also had a low noise pressurized cabin and quite large, square, windows. It was extremely luxurious by standards of the day. The plane was believed to cost GB£250,000. It was seen initially as a great success. The Comet’s first flight was at the Farnborough Air Show in July 1949. Unfortunately, during this exhibition one of its panels peeled off, showing problems with its thin skin. The first production aircraft flew in January 1951 and the Comet was recognized as airworthy in January 1952, resulting in the first ever jetliner flight with passengers to Johannesburg in May later that year. Around 30,000 passengers travelled in Comets in their first year of service, including notably Queen Elizabeth and many other members of the royal family.

The third incident was devastating. On the 2^nd of May 1953, a Comet Aircraft flying at 10,000ft encountered a tropical storm. It broke up in mid-air 50km from Calcutta killing all 43 people on board. Investigations were carried out into all of these incidents. The first two crashes were said to be down to the unfamiliarity of the pilots with the new aircraft design. While the third accident was blamed on the poor weather conditions, causing excessive stresses in the airframe.

Two further accidents then occurred within 3 months of each other which strongly suggested some serious issues with the Aircraft design. On 8^th January 1954, the Comet G-ALYP (“Yoke Peter”) exploded mid-flight and crashed into the Mediterranean Sea near Elba, Killing all 35 aboard. The fleet was grounded, voluntarily, while an investigation was carried out. However it began service again on the 23^rd of March after the investigation was inconclusive. A very similar accident then occurred a few weeks later. The Comet G-ALYY (“Yoke Yoke”) broke up just 30mins into its flight as it reached the top of its climb (10.6km). The wreckage of this aircraft was found in Stromboli Italy. The De Havilland Comet’s were once again grounded, this time by the Ministry of Transport and Civil Aviation.

Reason for failure: A full scale investigation was carried out by the RAE after this accident. Engineers subjected identical airframe models through repeated re-pressurization and over-pressurization. This test gave them their break through. It suggested that the aircraft may have failed due to metal fatigue failure at one of its escape hatches. Investigators began considering fatigue as the most likely cause of failure and initiated further research into measuring the strain on the aircraft skin. This theory was then confirmed as after thousands of experimental pressurized climbs and descents, the thin fuselage metal around the Comet's large, right-angled windows began to crack. The metal could not cope with the 315MPa stresses at the edge of the window and a 70Mpa stress at each bolt position. These windows were right angled and the stress concentration factor was very high at each of the corners. This small weakness would deteriorate rapidly under pressure, leading to the fuselage breaking up, causing an explosion.

Legacy of Comet 1 Disasters: Testing undertaken on the comet prior to launch was far in excess of what was legally required and also far in excess of situations, stresses and strains it was expected to endure in flight. The investigation caused a change in estimates of safe loading strength of cabin pressures. Comet 1 designers insisted on square windows to avoid resembling ships windows. The crash’s contributed to the absence of cornered windows from future jetliners.

With 60 alterations to design in response to the crashes, before the Cohen report and investigation was concluded, it is not possible to mention or list them all here. The primary ones were-

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  Full scale testing of future aircraft structures
  
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  Future Comets had improved range and performance due to more powerful Rolls Royce Avon engines being used.
  
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  Comets were built with heavier gauge skin and rounded openings.
  
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  The extensive testing led to a much better understanding of fatigue cracks in airlines.
  
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  The ‘one-bay’ crack tolerance in fuselage was formulated.
  

According to Cowell (1976) testing in 1953 revealed the potential for fatigue failure at window corners that could lead to catastrophic failure of the pressure cabin and thus unavoidable disintegration of plane at high altitudes. The, 1953, report indicated no further remedial action was required as trial pressure testing that led to this failure was far in excess of what would be encountered. The reason why these finding were accepted at face value was due to the national prestige invested in the De Havilland Comet. It is hard to imagine a developed country today accepting such. Indeed the Cohen inquiry in 1954 tried extremely hard not to apportion blame for the failures. The only area of De Havilland that was criticized was their stress calculations which were deemed to be inadequate. De Havilland was essentially exonerated with Cohen stating that what happened was not reasonably foreseeable, given scientific knowledge at the time.

The Comet disasters remind us that not all technological disasters have purely negative connotations.

Reference list

Cowell, J. G., (1976) DH Comet : the world's first jet airliner. Airline Publications and Sales Ltd, Hounslow, Uk.

Darling, K., (2005) De Havilland Comet. Crowood Press, UK.

http://www.oocities.org/capecanaveral/lab/8803/fcometcr.htm#local

http://www.superstock.co.uk/stock-photos-images/1895-38266

http://www.zoggavia.com/De_Havilland_Comet.html