page_title: De Havilland Comet
section_title: Cohen Committee Court of Inquiry
hierarchical_section_title: De Havilland Comet / Operational history / Comet disasters of 1954 / Cohen Committee Court of Inquiry
caption_attribution_description: English: De Havilland Comet 1 G-ALYW of BOAC stored cocooned at London (Heathrow) Airport after the Comet crashes earlier that year. Français&#160;: Le Havilland Comet 1 G-ALYW de la BOAC, mis sous cocon et entreposé dans l'aire de maintenance de l'aéroport de Londres Heathrow, en septembre 1954, après les crashes survenus plus tôt cette année.
context_page_description: The de Havilland DH 106 Comet was the world's first commercial jet airliner. Developed and manufactured by de Havilland at its Hatfield Aerodrome in Hertfordshire, United Kingdom, the Comet 1 prototype first flew in 1949. It featured an aerodynamically clean design with four de Havilland Ghost turbojet engines buried in the wing roots, a pressurised cabin, and large square windows. For the era, it offered a relatively quiet, comfortable passenger cabin and was commercially promising at its debut in 1952.
  However, within a year of entering airline service, problems started to emerge, with three Comets lost within twelve months in highly publicised accidents, after suffering catastrophic in-flight break-ups. Two of these were found to be caused by structural failure resulting from metal fatigue in the airframe, a phenomenon not fully understood at the time; the other was due to overstressing of the airframe during flight through severe weather. The Comet was withdrawn from service and extensively tested. Design and construction flaws, including improper riveting and dangerous concentrations of stress around some of the square windows, were ultimately identified.
context_section_description: On 19 October 1954, the Cohen Committee was established to examine the causes of the Comet crashes. Chaired by Lord Cohen, the committee tasked an investigation team led by Sir Arnold Hall, Director of the RAE at Farnborough, to perform a more detailed investigation.  Hall's team began considering fatigue as the most likely cause of both accidents, and initiated further research into measurable strain on the aircraft's skin. With the recovery of large sections of G-ALYP from the Elba crash and BOAC's donation of an identical airframe, G-ALYU, for further examination, an extensive "water torture" test eventually provided conclusive results. This time, the entire fuselage was tested in a dedicated water tank that was built specifically at Farnborough to accommodate its full length. Stress around the window corners was found to be much higher than expected and stresses on the skin were generally more than previously expected or tested. The windows' square shape caused stress concentration by generating levels of stress two or three times greater than across the rest of the fuselage. In 2012 a finite element analysis was carried out to find the stress values in a digital model of the Comet's cabin window loaded to a pressure differential of 8.25 psi. In this model the maximum stress level at the margin of one of the outer row of rivet holes near the corner of the window was almost five times greater than in the areas of skin remote from the windows.
  In water tank testing, engineers subjected G-ALYU to repeated repressurisation and over-pressurisation, and on 24 June 1954, after 3,057 flight cycles (1,221 actual and 1,836 simulated), G-ALYU burst open. Hall, Geoffrey de Havilland and Bishop were immediately called to the scene, where the water tank was drained to reveal that the fuselage had ripped open at a bolt hole, forward of the forward left escape hatch cutout. The failure then occurred longitudinally along a fuselage stringer at the widest point of the fuselage (accident report Fig 7). The fuselage frames did not have sufficient strength to prevent the crack propagating. Although the fuselage failed after a number of cycles that represented three times the life of G-ALYP at the time of the accident, it was still much earlier than expected. A further test reproduced the same results. Based on these findings, Comet 1 structural failures could be expected at anywhere from 1,000 to 9,000 cycles. Before the Elba accident, G-ALYP had made 1,290 pressurised flights, while G-ALYY had made 900 pressurised flights before crashing. Dr P.B. Walker, Head of the Structures Department at the RAE, said he was not surprised by this, noting that the difference was about three to one, and previous experience with metal fatigue suggested a total range of nine to one between experiment and outcome in the field could result in failure.
  The RAE also reconstructed about two-thirds of G-ALYP at Farnborough and found fatigue crack growth from a rivet hole at the low-drag fibreglass forward aperture around the Automatic Direction Finder, which had caused a catastrophic break-up of the aircraft in high-altitude flight. The punch rivet construction technique employed in the Comet's design had exacerbated its structural fatigue problems; the aircraft's windows had been engineered to be glued and riveted, but had been punch riveted only. Unlike drill riveting, the imperfect nature of the hole created by punch riveting could cause fatigue cracks to start developing around the rivet. Principal investigator Hall accepted the RAE's conclusion of design and construction flaws as the likely explanation for G-ALYU's structural failure after 3,060 pressurisation cycles.
caption: BOAC Comet 1 cocooned and stored in the maintenance area at London Heathrow Airport in September 1954
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