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The de Havilland DH.106 Comet was the world's first commercial jet airliner. Developed and manufactured by de Havilland in the 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.
Although sales never fully recovered, the improved Comet 2 and the prototype Comet 3 culminated in the redesigned Comet 4 series which debuted in 1958 and remained in commercial service until 1981. The Comet was also adapted for a variety of military roles such as VIP, medical and passenger transport, as well as surveillance; the last Comet 4, used as a research platform, made its final flight in 1997. The most extensive modification resulted in a specialised maritime patrol derivative, the Hawker Siddeley Nimrod, which remained in service with the Royal Air Force until 2011, over 60 years after the Comet's first flight.
The first prototype DH.106 Comet (carrying Class B markings G-5-1) was completed in 1949 and was initially used to conduct ground tests and brief early flights. The prototype's maiden flight, out of Hatfield Aerodrome, took place on 27 July 1949 and lasted 31 minutes. At the controls was de Havilland chief test pilot John "Cats Eyes" Cunningham, a famous night-fighter pilot of the Second World War, along with co-pilot Harold "Tubby" Waters, engineers John Wilson (electrics) and Frank Reynolds (hydraulics), and flight test observer Tony Fairbrother.
The prototype was registered G-ALVG just before it was publicly displayed at the 1949 Farnborough Airshow before the start of flight trials. A year later, the second prototype G-5-2 made its maiden flight. The second prototype was registered G-ALZK in July 1950 and it was used by the BOAC Comet Unit at Hurn from April 1951 to carry out 500 flying hours of crew training and route-proving. Australian airline Qantas also sent its own technical experts to observe the performance of the prototypes, seeking to quell internal uncertainty about its prospective Comet purchase. Both prototypes could be externally distinguished from later Comets by the large single-wheeled main landing gear, which was replaced on production models starting with G-ALYP by four-wheeled bogies.
The Comet's thin metal skin was composed of advanced new alloys[N 13] and was both riveted and chemically bonded, which saved weight and reduced the risk of fatigue cracks spreading from the rivets. The chemical bonding process was accomplished using a new adhesive, Redux, which was liberally used in the construction of the wings and the fuselage of the Comet; it also had the advantage of simplifying the manufacturing process.
When several of the fuselage alloys were discovered to be vulnerable to weakening via metal fatigue, a detailed routine inspection process was introduced. As well as thorough visual inspections of the outer skin, mandatory structural sampling was routinely conducted by both civil and military Comet operators. The need to inspect areas not easily viewable by the naked eye led to the introduction of widespread radiography examination in aviation; this also had the advantage of detecting cracks and flaws too small to be seen otherwise.
Media attention centred on potential sabotage; 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 flutter (which had led to the loss of DH 108 prototypes), structural failure due to high loads or 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.
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 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). 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. 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 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.  Once the crack initiated the skin failed from the point of the ADF cut out and propagated downwards and rearwards along a stringer resulting in an explosive decompression.
It was also found that 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.[N 20]
The issue of the lightness of Comet 1 construction (in order to not tax the relatively low thrust DeHavilland Ghost engines), had been noted by DeHavilland 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 DeHavilland Engine Company (that made the Comet 1's Ghost engines) 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."
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."
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. 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 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. Comet commercial flights would not resume until 1958.
According to de Havilland's chief test pilot John Cunningham, who had flown the prototype's first flight, representatives from American manufacturers such as Boeing and Douglas privately disclosed that if de Havilland had not experienced the Comet's pressurisation problems first, it would have happened to them. Cunningham likened the Comet to the later Concorde and added that he had assumed that the aircraft would change aviation, which it subsequently did. Aviation author Bill Withuhn concluded that the Comet had pushed "'the state-of-the-art' beyond its limits." 2b1af7f3a8