Editing De Havilland Comet (section)

==Operational history==

===Introduction===
The earliest production aircraft, registered G-ALYP ("Yoke Peter"), first flew on 9 January 1951 and was subsequently lent to BOAC for development flying by its Comet Unit.<ref name=DaviesandBirtles31>Davies and Birtles 1999, p. 31.</ref> On 22 January 1952, the fifth production aircraft, registered G-ALYS, received the first Certificate of Airworthiness awarded to a Comet, six months ahead of schedule.<ref name=DaviesandBirtles34>Davies and Birtles 1999, p. 34.</ref> On 2 May 1952, as part of BOAC's route-proving trials, G-ALYP took off on the world's first jetliner{{refn|The [[Avro Canada C102 Jetliner]], for which it was [[neologism|coined]], first used the term; "jetliner" later became a generic term for all jet airliners.<ref>Floyd 1986, p. 88.</ref>|group=N}} flight with fare-paying passengers and inaugurated scheduled service from [[London]] to [[Johannesburg]].<ref name=McNeil2002,39>McNeil 2002, p. 39.</ref><ref name=bbc>[http://news.bbc.co.uk/onthisday/hi/dates/stories/may/2/newsid_2480000/2480339.stm "On This Day: Comet inaugurates the jet age."] ''BBC News,'' 2 May 1952. Retrieved 26 April 2012.</ref><ref name=cookman195207>Cookman, Aubrey O. Jr. [https://books.google.com/books?id=WNwDAAAAMBAJ&pg=PA90 "I Rode The First Jet Airliner."] ''Popular Mechanics'', July 1952, pp. 90–94. Retrieved 26 April 2012.</ref> The final Comet from BOAC's initial order, registered G-ALYZ, began flying in September 1952 and carried cargo along South American routes while simulating passenger schedules.<ref>Jackson 1988, pp. 173–174.</ref>

[[File:BOAC Comet 1952 Entebbe.jpg|thumb|left|[[British Overseas Airways Corporation|BOAC]] Comet 1 at [[Entebbe International Airport|Entebbe Airport]], Uganda in 1952]]

Prince Philip returned from the Helsinki Olympic Games with G-ALYS on 4 August 1952. [[Elizabeth Bowes-Lyon|Queen Elizabeth, the Queen Mother]] and [[Princess Margaret, Countess of Snowdon|Princess Margaret]] were guests on a special flight of the Comet on 30 June 1953 hosted by Sir Geoffrey and Lady de Havilland.<ref>Lane 1979, p. 205.</ref> Flights on the Comet were about twice as fast as advanced piston-engined aircraft such as the [[Douglas DC-6]] ({{cvt|490|mph}}
vs {{cvt|315|mph}}, respectively), and a faster rate of climb further cut flight times. In August 1953 BOAC scheduled the nine-stop London to [[Tokyo]] flights by Comet for 36 hours, compared to 86 hours and 35 minutes on its [[Canadair North Star|Argonaut]] (a DC-4 variant) piston airliner. ([[Pan American World Airways|Pan Am]]'s DC-6B was scheduled for 46 hours 45 minutes.) The five-stop flight from London to Johannesburg was scheduled for 21 hr 20 min.<ref>{{cite magazine |magazine=Flight |title=Jet Air-Routes |url=http://www.flightglobal.com/pdfarchive/view/1953/1953%20-%200551.html |archive-url=https://web.archive.org/web/20160305070326/http://www.flightglobal.com/pdfarchive/view/1953/1953%20-%200551.html |archive-date=5 March 2016 |date=1 May 1953 |page=547}}</ref>

In their first year, Comets carried 30,000 passengers. As the aircraft could be profitable with a load factor as low as 43 per cent, commercial success was expected.<ref name=Walker25>Walker 2000, p. 25.</ref> The Ghost engines allowed the Comet to fly above weather that competitors had to fly through. They ran smoothly and were less noisy than piston engines, had low maintenance costs <!--lower for parts, lower for labour, & also faster turnaround times--> and were fuel-efficient above {{cvt|30000|ft}}.{{refn|Depending on weight and temperature, cruise fuel consumption was {{cvt|6|to|10|kg}} per nautical mile (1.2 miles; 1.9 km), the higher figure being at the lower altitude needed at high weight.{{citation needed|date=April 2021}}|group=N}} In summer 1953, eight BOAC Comets left London each week: three to Johannesburg, two to Tokyo, two to [[Singapore]] and one to [[Colombo]].<ref>Davies and Birtles 1999, p. 22 (Route map illustration).</ref>

In 1953, the Comet appeared to have achieved success for de Havilland.<ref>Schnaars 2002, p. 71.</ref> ''[[Popular Mechanics]]'' wrote that Britain had a lead of three to five years on the rest of the world in jetliners.<ref name=cookman195207/> As well as the sales to BOAC, two French airlines, [[Union Aéromaritime de Transport]] and Air France, each acquired three Comet 1As, an upgraded variant with greater fuel capacity, for flights to West Africa and the Middle East.<ref name=1A>Schnaars 2002, p. 70.</ref><ref>{{cite magazine |url=http://www.flightglobal.com/pdfarchive/view/1953/1953%20-%201135.html |title=Preludes and Overtures: de Havilland Comet 1 |magazine=Flight |date=4 September 1953 |access-date=30 May 2012 |url-status=dead |archive-date=14 January 2015 |archive-url=https://web.archive.org/web/20150114231808/http://www.flightglobal.com/pdfarchive/view/1953/1953%20-%201135.html}}</ref>{{page needed|date=April 2023}} A slightly longer version of the Comet 1 with more powerful engines, the Comet 2, was being developed,<ref>Darling 2001, p. 20.</ref> and orders were placed by [[Air India]],<ref>Cacutt 1989, p. 146.</ref> [[British Commonwealth Pacific Airlines]],<ref name=darling119/> [[Japan Air Lines]],<ref name=JAPL/> [[Linea Aeropostal Venezolana]],<ref name=JAPL/> and [[Panair do Brasil]].<ref name=JAPL/> American carriers [[Capital Airlines (United States)|Capital Airlines]], [[National Airlines (NA)|National Airlines]] and Pan Am placed orders for the planned Comet 3, an even-larger, longer-range version for transatlantic operations.<ref name=darling128>Darling 2005, p. 128.</ref><ref>Proctor et al. 2010, p. 23.</ref> Qantas was interested in the Comet 1 but concluded that a version with more range and better takeoff performance was needed for the London to Canberra route.<ref>Gunn 1987, pp. 268–270.</ref>

===Early hull losses===

On 26 October 1952, the Comet suffered its first hull loss when a BOAC flight departing Rome's [[Rome Ciampino Airport|Ciampino airport]] failed to become airborne and ran into rough ground at the end of the runway. Two passengers sustained minor injuries, but the aircraft, G-ALYZ, was a write-off. On 3 March 1953, a new [[Canadian Pacific Airlines]] Comet 1A, registered CF-CUN and named ''Empress of Hawaii,'' failed to become airborne while attempting a night takeoff from [[Karachi, Pakistan]], on a delivery flight to [[Australia]]. The aircraft plunged into a dry drainage canal and collided with an embankment, killing all five crew and six passengers on board.<ref>{{Cite web |title=Comet Accident Record |url=https://asn.flightsafety.org/asndb/335403 |access-date=22 September 2010 |website=Aviation Safety Network}}</ref><ref>{{Cite web |title=CF-CUN |url=http://www.edcoatescollection.com/ac4/CF-CUN.html |access-date=18 February 2011 |website=Ed Coates' Civil Aircraft Photograph Collection}}</ref> The accident was the first fatal jetliner crash.<ref name=JAPL/> In response, Canadian Pacific cancelled its remaining order for a second Comet 1A and never operated the type in commercial service.<ref name=JAPL/>

[[File:De Havilland Comet 1 BOAC Heathrow G-ALYX 1953.jpg|thumb|BOAC Comet 1 ''G-ALYX'' (Yoke X-Ray) at [[London Heathrow Airport]] in 1953 prior to a scheduled flight]]

Both early accidents were originally attributed to pilot error, as [[rotation (aeronautics)|overrotation]] had led to a loss of lift from the [[leading edge]] of the aircraft's wings. It was later determined that the Comet's wing profile experienced a loss of lift at a high [[angle of attack]], and its engine inlets also suffered a lack of pressure recovery in the same conditions. As a result, de Havilland re-profiled the wings' leading edge with a pronounced "droop",<ref name=Withuhn85>Withuhn 1976, p. 85.</ref> and [[wing fence]]s were added to control spanwise flow.<ref name=Birtles127>Birtles 1970, p. 127.</ref> A fictionalised investigation into the Comet's takeoff accidents was the subject of the novel ''Cone of Silence'' (1959) by [[Arthur David Beaty]], a former BOAC captain. ''Cone of Silence'' was made into a [[Cone of Silence (film)|film]] in 1960, and Beaty also recounted the story of the Comet's takeoff accidents in a chapter of his non-fiction work, ''Strange Encounters: Mysteries of the Air'' (1984).<ref>Beaty 1984, pp. 113–114.</ref>

The Comet's second fatal accident occurred on 2 May 1953, when [[BOAC Flight 783]], a Comet 1, registered G-ALYV, crashed in a severe [[squall|thundersquall]] six minutes after taking off from Calcutta-Dum Dum (now [[Netaji Subhash Chandra Bose International Airport]]), India,<ref>Darling 2005, p. 36.</ref> killing all 43 on board. Witnesses observed the wingless Comet on fire plunging into the village of Jagalgori,<ref name=lokurNS>{{cite web |last1=Lokur |first1=N. S. |title=Report of the court investigation on the accident to COMET G-ALYV |url=http://lessonslearned.faa.gov/Comet1/G-ALYV_Report.pdf |website=Lessons Learned |publisher=Federal Aviation Administration |access-date=23 February 2015 |url-status=dead |archive-url=https://web.archive.org/web/20150415121817/http://lessonslearned.faa.gov/Comet1/G-ALYV_Report.pdf |archive-date=15 April 2015}}</ref> leading investigators to suspect structural failure.<ref name=Walker37/>

====India Court of Inquiry====
After the loss of G-ALYV, the [[Government of India]] convened a court of inquiry<ref name=lokurNS/> to examine the cause of the accident.{{refn|The court acted under the provisions of Rule 75 of the Indian Aircraft Rules 1937.<ref name=Walker37>Walker 2000, p. 37.</ref>|group=N}} Professor [[Natesan Srinivasan]] joined the inquiry as the main technical expert. A large portion of the aircraft was recovered and reassembled at Farnborough,<ref name=Walker37/> during which the break-up was found to have begun with a left elevator spar failure in the [[tailplane|horizontal stabilizer]]. The inquiry concluded that the aircraft had encountered extreme negative [[g-force]]s during takeoff; severe turbulence generated by adverse weather was determined to have induced down-loading, leading to the loss of the wings. Examination of the cockpit controls suggested that the pilot may have inadvertently over-stressed the aircraft when pulling out of a steep dive by over-manipulation of the fully powered flight controls. Investigators did not consider metal fatigue as a contributory cause.<ref name=LoBao7>Lo Bao 1996, p. 7.</ref>

The inquiry's recommendations revolved around the enforcement of stricter speed limits during turbulence, and two significant design changes also resulted: all Comets were equipped with [[weather radar]] and the "Q feel" system was introduced, which ensured that control column forces (invariably called stick forces) would be proportional to control loads. This [[aircraft flight control system#Artificial feel devices|artificial feel]] was the first of its kind to be introduced in any aircraft.<ref name=Walker37/> The Comet 1 and 1A had been criticised for a lack of "[[feedback|feel]]" in their controls,<ref>Job 1996, p. 14.</ref> and investigators suggested that this might have contributed to the pilot's alleged over-stressing of the aircraft;<ref>Darling 2001, p. 26.</ref> Comet chief test pilot John Cunningham contended that the jetliner flew smoothly and was highly responsive in a manner consistent with other de Havilland aircraft.<ref name=Faith>Faith 1996, pp. 63–64.</ref>{{refn|Cunningham: "[the Comet] flew extremely smoothly and responded to the controls in the best way de Havilland aircraft usually did."<ref name=Faith/>|group=N}}

===Comet disasters of 1954===
{{Main|BOAC Flight 781|South African Airways Flight 201}}
Just over a year later, Rome's Ciampino airport, the site of the first Comet hull loss, was the origin of a more-disastrous Comet flight. On 10 January 1954, 20&nbsp;minutes after taking off from Ciampino, the first production Comet, G-ALYP, broke up in mid-air while operating [[BOAC Flight 781]] and crashed into the Mediterranean off the Italian island of [[Elba]] with the loss of all 35 on board.<ref name=witheyfatigue>{{Cite journal |title=Fatigue Failure of the de Havilland Comet I |journal=Engineering Failure Analysis |volume=4 |issue=2 |page=147 |year=1997 |last1=Withey |first1=P.A. |doi=10.1016/S1350-6307(97)00005-8}}</ref><ref>[http://www.flightglobal.com/pdfarchive/view/1954/1954%20-%200130.html "B.O.A.C. Comet Lost: Services Suspended."] ''Flight,'' January 1954, p. 58. Retrieved 26 April 2012.</ref> With no witnesses to the disaster and only partial radio transmissions as incomplete evidence, no obvious reason for the crash could be deduced. Engineers at de Havilland immediately recommended 60 modifications aimed at any possible design flaw, while the Abell Committee met to determine potential causes of the crash.<ref>Faith 1996, p. 66.</ref>{{refn|The Abell Committee, named after chairman C. Abell, Deputy Operations Director (Engineering) of BOAC, consisted of representatives of the Allegation Review Board (A.R.B.), BOAC, and de Havilland.<ref>Keith 1997, p. 288.</ref>|group=N}} BOAC also voluntarily grounded its Comet fleet pending investigation into the causes of the accident.<ref name=d29>Darling 2001, pp. 28–30.</ref>

====Abell Committee Court of Inquiry====
Media attention centred on potential [[sabotage]];<ref name=Withuhn85/> other speculation ranged from [[clear-air turbulence]] to an explosion of vapour in an empty fuel tank. The Abell Committee focused on six potential aerodynamic and mechanical causes: control [[wing flutter|flutter]] (which had led to the loss of DH 108 prototypes), structural failure due to high loads or [[fatigue (material)|metal fatigue]] of the wing structure, failure of the powered flight controls, failure of the window panels leading to explosive decompression, or fire and other engine problems. The committee concluded that fire was the most likely cause of the problem, and changes were made to the aircraft to protect the engines and wings from damage that might lead to another fire.<ref>[https://web.archive.org/web/20080703165044/http://www.geocities.com/CapeCanaveral/Lab/8803/comgalyp.htm "Report of the Public Inquiry into the causes and circumstances of the accident which occurred on the 10 January 1954, to the Comet aircraft G-ALYP, Part IX (d)."] ''geocities.com.'' Retrieved: 3 September 2010.</ref>

{{Quote box|align=left|width=20%|quote=The cost of solving the Comet mystery must be reckoned neither in money nor in manpower.|source=<small>[[Prime Minister]] [[Winston Churchill]], 1954.<ref>Job 1996, p. 11.</ref></small>}}

During the investigation, the [[Royal Navy]] conducted recovery operations.<ref>[http://www.flightglobal.com/pdfarchive/view/1954/1954%20-%200224.html "Elba Accident Developments."] ''Flight,'' January 1954, p. 108. Retrieved 26 April 2012.</ref> The first pieces of wreckage were discovered on 12 February 1954<ref>[https://web.archive.org/web/20080703165044/http://www.geocities.com/CapeCanaveral/Lab/8803/comgalyp.htm "Report of the Public Inquiry into the causes and circumstances of the accident which occurred on the 10 January 1954, to the Comet aircraft G-ALYP, Part IX (c): Action taken after the accident and prior to the accident to Comet G-ALYY: Naval search for wreckage."] ''geocities.com.'' Retrieved: 3 September 2010.</ref> and the search continued until September 1954, by which time 70 per cent by weight of the main structure, 80 per cent of the power section, and 50 per cent of the aircraft's systems and equipment had been recovered.<ref>[http://www.rafmuseum.org.uk/online-exhibitions/comet/comet5.cfm "Comet Failure."] {{webarchive|url=https://web.archive.org/web/20090923173849/http://www.rafmuseum.org.uk/online-exhibitions/comet/comet5.cfm |date=23 September 2009}} ''Royal Air Force Museum Cosford''. Retrieved 1 November 2010.</ref><ref name=flight54p652>''Flight'' 29 October 1954, p. 652.</ref> The forensic reconstruction effort had just begun when the Abell Committee reported its findings. No apparent fault in the aircraft was found,{{refn|On 4 April, [[John Moore-Brabazon, 1st Baron Brabazon of Tara|Lord Brabazon]] wrote to the Minister of Transport, "Although no definite reason for the accident has been established, modifications are being embodied to cover every possibility that imagination has suggested as a likely cause of the disaster. When these modifications are completed and have been satisfactorily flight-tested, the Board sees no reason why passenger services should not be resumed."<ref name=d29/>|group=N}} and the British government decided against opening a further public inquiry into the accident.<ref name=d29/> The prestigious nature of the Comet project, particularly for the British aerospace industry, and the financial impact of the aircraft's grounding on BOAC's operations both served to pressure the inquiry to end without further investigation.<ref name=d29/> Comet flights resumed on 23 March 1954.<ref name=Birtlespp.128–129>Birtles 1970, pp. 128–129.</ref>

On 8 April 1954, Comet G-ALYY ("Yoke Yoke"), on charter to [[South African Airways]], was on a leg from Rome to [[Cairo]] (of a longer route, [[South African Airways Flight 201|SA Flight 201]] from London to Johannesburg), when it crashed in the Mediterranean near [[Naples]] with the loss of all 21 passengers and crew on board.<ref name=witheyfatigue/> The Comet fleet was immediately grounded once again and a large investigation board was formed under the direction of the [[Royal Aircraft Establishment]] (RAE).<ref name=witheyfatigue/> Prime Minister Winston Churchill tasked the Royal Navy with helping to locate and retrieve the wreckage so that the cause of the accident could be determined.<ref name=DaviesandBirtles30–31/> The Comet's Certificate of Airworthiness was revoked, and Comet 1 line production was suspended at the Hatfield factory while the BOAC fleet was permanently grounded, [[cocooning (aircraft)|cocooned]] and stored.<ref name=Withuhn85/>

====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}}

<gallery widths="200px" heights="170px">
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>

====Earlier structural indications====
The issue of the lightness of Comet 1 construction (in order to not tax the relatively low thrust de Havilland Ghost engines), had been noted by de Havilland test pilot John Wilson, while flying the prototype during a Farnborough flypast in 1949. On the flight, he was accompanied by Chris Beaumont, Chief Test Pilot of the de Havilland Engine Company who stood in the entrance to the cockpit behind the Flight Engineer. He stated "Every time we pulled 2 1/2-3G to go around the corner, Chris found that the floor on which he was standing, bulging up and there was a loud bang at that speed from the nose of the aircraft where the skin 'panted' (flexed), so when we heard this bang we knew without checking the airspeed indicator, that we were doing 340 knots. In later years we realised that these were the indications of how flimsy the structure really was."<ref>{{cite book |first=James |last=Hamilton-Paterson |date=2010 |title=Empire of the Clouds |publisher=Faber and Faber |pages=39–40}}</ref>

====Square window myths====
[[File:Comet 1 windows.png|thumb|Surviving DeHavilland Comet 1 showing rectangular windows with rounded corners not 'square' as commonly described.]]

Despite findings of the Cohen Inquiry, a number of myths have evolved around the cause of the Comet 1's accidents. Most commonly quoted are the 'square' passenger windows. While the report noted that stress around fuselage cut-outs, emergency exits and windows was found to be much higher than expected due to DeHavilland's assumptions and testing methods<ref>Cohen Report P 26 – para 118-123</ref> the passenger windows shape has been commonly misunderstood and cited as a cause of the fuselage failure. In fact the mention of 'windows' in the Cohen report's conclusion, refers specifically to the origin point of failure in the ADF Antenna cut-out 'windows', located above the cockpit, not passenger windows.<ref>Report of the Court of Inquiry into the Accidents to Comet G-ALYP on 10th January 1954 and Comet G-ALYY on 8th April 1954-HM Stationery Office 1955-p 20 – para 78-79</ref> The shape of the passenger windows were not indicated in any failure mode detailed in the accident report and were not viewed as a contributing factor. A number of other pressurised airliners of the period including the Boeing 377 Stratocruiser, Douglas DC-7, and DC-8 had larger and more 'square' windows than the Comet 1, and experienced no such failures.<ref name=autogenerated1>The DeHavilland Comet Disaster – Aerospace Engineering – Paul Withey Professor of Casting at the University of Birmingham School of Metallurgy – Video presentation retrieved 30NOV22</ref> In fact, the Comet 1's window general shape resembles a slightly larger Boeing 737 window mounted horizontally. They are rectangular ''not square'', have rounded corners and are within 5% of the radius of the Boeing 737 windows and virtually identical to modern airliners.<ref name=autogenerated1/> Paul Withey, Professor of Casting at the University of Birmingham School of Metallurgy states in a video presentation delivered in 2019, analysing all available data that: "The fact that DeHavilland put oval windows into later marks, is not because of any 'squareness' of the windows that caused failure."<ref>The deHavilland Comet Disaster – Aerospace Engineering - Paul Withey Professor of Casting at the University of Birmingham School of Metallurgy – Video presentation retrieved 30NOV22 Time stamp 42:07</ref> "DeHavilland went to oval windows on the subsequent Marks because it was easier to Redux them in (use adhesive) – nothing to do with the [[stress concentration]] and it's purely to remove rivets." (from the structure)<ref>The DeHavilland Comet Disaster – Aerospace Engineering – Paul Withey, Professor of Casting at the University of Birmingham School of Metallurgy – Video presentation retrieved 30NOV22 Time stamp 58:27</ref>

Surviving Comet 1s can be seen on view at the Royal Air Force Museum Midlands and the De Havilland Museum at Salisbury Hall, London Colney.

====Response====
In responding to the report de Havilland stated: "Now that the danger of high level fatigue in pressure cabins has been generally appreciated, de Havillands will take adequate measures to deal with this problem. To this end we propose to use thicker gauge materials in the pressure cabin area and to strengthen and redesign windows and cut outs and so lower the general stress to a level at which local stress concentrations either at rivets and bolt holes or as such may occur by reason of cracks caused accidentally during manufacture or subsequently, will not constitute a danger."<ref>Cohen Inquiry Report p 42</ref>

The Cohen inquiry closed on 24 November 1954, having "found that the basic design of the Comet was sound",<ref name=Jones68/> and made no observations or recommendations regarding the shape of the windows. De Havilland nonetheless began a refit programme to strengthen the fuselage and wing structure, employing thicker-gauge skin and replacing the rectangular windows and panels with rounded versions, although this was not related to the erroneous 'square' window claim, as can be seen by the fact that the fuselage escape hatch cut-outs (the source of the failure in test aircraft G-ALYU) retained their rectangular shape.<ref name=DaviesandBirtles30–31>Davies and Birtles 1999, pp. 30–31.</ref><ref>{{cite web |title=XS235 – De Havilland DH-106 Comet 4C – United Kingdom – Royal Air Force (RAF) – David Oates |website=JetPhotos |url=https://www.jetphotos.com/photo/7193453 |access-date=22 March 2019}}</ref>

Following the Comet enquiry, aircraft were designed to "[[fail-safe]]" or [[safe-life design|safe-life]] standards,<ref>{{cite web |url=https://www.researchgate.net/publication/287199920 |title=Milestones in Aircraft Structural Integrity |website=[[ResearchGate]] |access-date=22 March 2019}}</ref> though several subsequent catastrophic fatigue failures, such as [[Aloha Airlines Flight 243]] of April 28, 1988 have occurred.<ref>{{cite web |url=https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR8903.pdf |title=Aircraft Accident Report AAR8903: Aloha Airlines, Flight 243, Boeing 737-200, N73711 |publisher=[[NTSB]] |date=14 June 1989}}</ref>

===Resumption of service===
With the discovery of the structural problems of the early series, all remaining Comets were withdrawn from service, while de Havilland launched a major effort to build a new version that would be both larger and stronger. All outstanding orders for the Comet 2 were cancelled by airline customers.<ref name=d33>Darling 2001, p. 33.</ref> All production Comet 2s were also modified with thicker gauge skin to better distribute loads and alleviate the fatigue problems (most of these served with the [[Royal Air Force|RAF]] as the Comet C2); a programme to produce a Comet 2 with more powerful Avons was delayed. The prototype Comet 3 first flew in July 1954 and was tested in an unpressurised state pending completion of the Cohen inquiry.<ref name=d33/> Comet commercial flights would not resume until 1958.<ref>Swanborough 1962, pp. 47–48.</ref>

Development flying and route proving with the Comet 3 allowed accelerated certification of what was destined to be the most successful variant of the type, the Comet 4. All airline customers for the Comet 3 subsequently cancelled their orders and switched to the Comet 4,<ref name=d33/> which was based on the Comet 3 but with improved fuel capacity. BOAC ordered 19 Comet 4s in March 1955, and American operator Capital Airlines ordered 14 Comets in July 1956.<ref>"Capital Comet." ''Canadian Aviation'', 29(9–12), 1956, p. 51.</ref> Capital's order included 10 Comet 4As, a variant modified for short-range operations with a stretched fuselage and short wings, lacking the pinion (outboard wing) fuel tanks of the Comet 4.<ref name=darling128/> Financial problems and a takeover by [[United Airlines]] meant that Capital would never operate the Comet.{{citation needed|date=October 2021}}

The Comet 4 first flew on 27 April 1958 and received its Certificate of Airworthiness on 24 September 1958; the first was delivered to BOAC the next day.<ref name=bao11>Lo Bao 1996, p. 11.</ref><ref>Walker 2000, pp. 187–188.</ref> The base price of a new Comet 4 was roughly £1.14 million (£{{Inflation|UK-CAP|1.14|1958|r=2|fmt=c}} million in {{Inflation/year|UK-CAP}}).<ref>[http://www.flightglobal.com/pdfarchive/view/1960/1960%20-%202684.html "De Havilland."] ''Flightglobal.com,'' 18 November 1960. Retrieved 13 August 2012.</ref> The Comet 4 enabled BOAC to inaugurate the first regular jet-powered transatlantic services on 4 October 1958 between London and New York (albeit still requiring a fuel stop at [[Gander International Airport]], Newfoundland, on westward North Atlantic crossings).<ref name=McNeil2002,39/> While BOAC gained publicity as the first to provide transatlantic jet service, by the end of the month rival Pan American World Airways was flying the [[Boeing 707]] on the New York-Paris route, with a fuel stop at Gander in both directions,<ref>[https://books.google.com/books?id=BZdQhOW-ys0C&pg=PA940 "The Comet's chance to shine."]{{Dead link|date=January 2024 |bot=InternetArchiveBot |fix-attempted=yes }} ''[[The New Scientist]]'', 4(98), 2 October 1958, p. 940. Retrieved 26 April 2012.</ref> and in 1960 began flying [[Douglas DC-8|Douglas DC-8's]] on its transatlantic routes as well. The American jets were larger, faster, longer-ranged and more cost-effective than the Comet.<ref>Haddon-Cave 2009, p. 16.</ref> After analysing route structures for the Comet, BOAC reluctantly cast-about for a successor, and in 1956 entered into an agreement with Boeing to purchase the 707.<ref>Lo Bao 1996, p. 12.</ref>

The Comet 4 was ordered by two other airlines: [[Aerolíneas Argentinas]] took delivery of six Comet 4s from 1959 to 1960, using them between Buenos Aires and Santiago, New York and Europe, and [[East African Airways]] received three new Comet 4s from 1960 to 1962 and operated them to the United Kingdom and to Kenya, Tanzania, and Uganda.<ref name=darling114>Darling 2005, p. 114.</ref> The Comet 4A ordered by Capital Airlines was instead built for BEA as the Comet 4B, with a further fuselage stretch of {{cvt|38|in}} and seating for 99 passengers. The first Comet 4B flew on 27 June 1959 and BEA began Tel Aviv to London-Heathrow services on 1 April 1960.<ref name=DaviesandBirtles62/> [[Olympic Airways]] was the only other customer to order the type.<ref name=Jacksondhp459>Jackson 1987, p. 459.</ref> The last Comet 4 variant, the Comet 4C, first flew on 31 October 1959 and entered service with Mexicana in 1960.<ref>[http://www.flightglobal.com/pdfarchive/view/1962/1962%20-%200749.html "Comet 4Cs for Mexicana."] ''Flight International'', 76, 6 November 1959, p. 491. Retrieved 26 April 2012.</ref> The Comet 4C had the Comet 4B's longer fuselage and the longer wings and extra fuel tanks of the original Comet 4, which gave it a longer range than the 4B. Ordered by [[Kuwait Airways]], [[Middle East Airlines]], [[EgyptAir|Misrair]] (later Egyptair), and [[Sudan Airways]], it was the most popular Comet variant.<ref name=JAPL/><ref>Howard, Paul. [https://abpic.co.uk/pictures/view/1101609/ "De Havilland DH.106 Comet 4C, OD-ADT, MEA – Middle East Airlines."] ''Air-Britain Photographic Images Collection''. Retrieved 19 November 2010.</ref>

<gallery widths="200px" heights="145px">
File:MSA Comet Groves.jpg|[[Malaysia-Singapore Airlines]] Comet 4 at [[Kai Tak Airport]] in 1966
File:United Arab Airlines Comet Soderstrom.jpg|[[EgyptAir|United Arab Airlines]] Comet 4C at [[Geneva International Airport|Geneva Airport]] in 1968
File:DH.106 Comet 5H-AAF EAA LHR 10.05.64 edited-3.jpg|Comet 4 of East African Airways at London Heathrow in 1964
</gallery>

===Later service===
[[File:Comet Canopus.jpg|thumb|Comet 4C ''Canopus'' on display at the [[Bruntingthorpe Aerodrome]] in [[Leicestershire]], England]]

In 1959, BOAC began shifting its Comets from transatlantic routes{{refn|The Feb 1959 OAG shows eight transatlantic Comets a week out of London, plus 10 BOAC Britannias and 11 DC-7Cs. In April 1960, 13 Comets, 19 Britannias and 6 DC-7Cs. Comets quit flying the North Atlantic in October 1960 (but reportedly made a few flights in summer 1964).{{citation needed|date=April 2021}} |group=N}} and released the Comet to associate companies, making the Comet 4's ascendancy as a premier airliner brief. Besides the 707 and DC-8, the introduction of the [[Vickers VC10]] allowed competing aircraft to assume the high-speed, long-range passenger service role pioneered by the Comet.<ref>Lo Bao 1996, p. 13.</ref> In 1960, as part of a government-backed consolidation of the British aerospace industry, de Havilland itself was acquired by Hawker Siddeley, within which it became a wholly owned division.<ref name=post>[http://www.rafmuseum.org.uk/online-exhibitions/dehavilland/post_war.cfm "De Havilland – Post War"] {{webarchive|url=https://web.archive.org/web/20110625135746/http://www.rafmuseum.org.uk/online-exhibitions/dehavilland/post_war.cfm |date=25 June 2011}}, ''rafmuseum.org.uk''. Retrieved 30 May 2012</ref>

In the 1960s, orders declined, a total of 76 Comet 4s being delivered from 1958 to 1964. In November 1965, BOAC retired its Comet 4s from revenue service; other operators continued commercial passenger flights with the Comet until 1981. [[Dan-Air]] played a significant role in the fleet's later history and, at one time, owned all 49 remaining airworthy civil Comets.<ref>Swanborough 1980, p. 35.</ref> On 14 March 1997 a Comet 4C [[United Kingdom military aircraft serials|serial]] ''XS235'' and named ''Canopus'',<ref name=d5/> which had been acquired by the British [[Minister of Technology|Ministry of Technology]] and used for radio, radar and avionics trials, made the last documented production Comet flight.<ref name=walker169>Walker 2000, p. 169.</ref>