Editing De Havilland Comet (section)

====Cohen Committee Court of Inquiry====
On 19 October 1954, the Cohen Committee was established to examine the causes of the Comet crashes.<ref name=Jones68/> Chaired by [[Lionel Cohen, Baron Cohen|Lord Cohen]], the committee tasked an investigation team led by [[Arnold Alexander Hall|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.<ref name=witheyfatigue/> 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.<ref name=Groh-comet-crash>{{cite web |last1=Groh |first1=Rainer |title=The DeHavilland Comet Crash |url=https://aerospaceengineeringblog.com/dehavilland-comet-crash/ |website=Aerospace Engineering Blog |access-date=31 July 2022 |date=9 June 2012 |archive-date=10 September 2022 |archive-url=https://web.archive.org/web/20220910013234/http://aerospaceengineeringblog.com/dehavilland-comet-crash/ |url-status=dead }}</ref> This time, the entire fuselage was tested in a dedicated water tank that was built specifically at Farnborough to accommodate its full length.<ref name=d29/>

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),<ref name=rafwebarchive>[https://web.archive.org/web/20061002072008/http://www.rafmuseum.org.uk/london/exhibitions/comet/comet5.cfm "Comet."] ''RAF Museum''. Retrieved 3 September 2010.</ref> 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 cut out. The failure then occurred longitudinally along a fuselage stringer at the widest point of the fuselage and through a cut out for an escape hatch. The skin thickness was discovered to be insufficient to distribute the load across the structure, leading to overloading of fuselage frames adjacent to fuselage cut outs. (Cohen Inquiry accident report Fig 7).<ref>Cohen Inquiry Report P 31</ref> The fuselage frames did not have sufficient strength to prevent the crack from 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.<ref>Cohen Inquiry Report p 27</ref> A further test reproduced the same results.<ref name=Withuhn87>Withuhn 1976, p. 87.</ref> 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.<ref name=rafwebarchive/>

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.<ref>"Summary: Aircraft Investigation". ''Aircraft Engineering'', 37, 1965, p. 38.</ref> The exact origin of the fatigue failure could not be identified but was localised to the ADF antenna cut out. A countersunk bolt hole and manufacturing damage that had been repaired at the time of construction using methods that were common, but were likely insufficient allowing for the stresses involved, were both located along the failure crack.<ref>Cohen Inquiry Report P 28 Para 131-136</ref> Once the crack initiated the skin failed from the point of the ADF cut out and propagated downward and rearward along a stringer resulting in an explosive decompression.<ref>Cohen report P 20 para 77-79</ref>

It was also found that the punch-rivet construction technique employed in the Comet's design had exacerbated its structural fatigue problems;<ref name=witheyfatigue/> the aircraft's ADF antenna 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.{{refn|Hall: "In the light of known properties of the aluminium alloy D.T.D. 546 or 746 of which the skin was made and in accordance with the advice I received from my Assessors, I accept the conclusion of RAE that this is a sufficient explanation of the failure of the cabin skin of Yoke Uncle by fatigue after a small number, namely, 3,060 cycles of pressurisation."<ref>Cohen Inquiry Report, Part XI (a. 69)</ref>|group=N}}

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File:DH.106 Comet 1 G-ALYW BOAC LAP (2) 12.09.54 edited-3.jpg|BOAC Comet 1 cocooned and stored in the maintenance area at London Heathrow Airport in September 1954
File:Comet G-ALYU escape hatch failure.png|Image from the Cohen Inquiry Report showing fuselage failure under water pressure test of Comet 1 G-ALYU. Note intact escape hatch window frame
File:Fuselage of de Havilland Comet Airliner G-ALYP.JPG|The [[automatic direction finder|ADF]] antenna penetration that failed on G-ALYP. On display at the [[Science Museum (London)|Science Museum]] in London
File:Comet G-ALYP ADF windows.png|Image (Fig 12) from the Cohen Inquiry showing the location of the ADF antenna cut out 'windows' in the roof above the cockpit of Comet 1 G-ALYP
</gallery>