De Havilland Comet

Origins On 11 March 1943, the Cabinet of the United Kingdom formed the Brabazon Committee, which was tasked with determining the UK's airliner needs after the conclusion of the Second World War. One of its recommendations was for the development and production of a pressurised, transatlantic mailplane that could carry of payload at a cruising speed of non-stop. Aviation company de Havilland was interested in this requirement, but chose to challenge the then widely held view that jet engines were too fuel-hungry and unreliable for such a role. As a result, committee member Sir Geoffrey de Havilland, head of the de Havilland company, used his personal influence and his company's expertise to champion the development of a jet-propelled aircraft, proposing a specification for a pure turbojet-powered design. First-phase development of the DH.106 focused on short- and intermediate-range mailplanes with small passenger compartments and as few as six seats, before being redefined as a long-range airliner with a capacity of 24 seats. A design team was formed in 1946 under the leadership of chief designer Ronald Bishop, who had been responsible for the Mosquito fighter-bomber. During flight tests, the DH 108 gained a reputation for being accident-prone and unstable, leading de Havilland and BOAC to gravitate to conventional configurations and, necessarily, designs with less technical risk. The DH 108s were later modified to test the DH.106's power controls. Testing and prototypes in October 1949 As the Comet represented a new category of passenger aircraft, more rigorous testing was a development priority. The entire forward fuselage section was tested for metal fatigue by repeatedly pressurising to overpressure and depressurising through more than 16,000 cycles, equivalent to about 40,000 hours of airline service. The windows were also tested under a pressure of , above expected pressures at the normal service ceiling of . about 1,250 per cent over the maximum pressure it was expected to encounter in service. 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. ==Design==

Overview Comet 4C cabin at the National Museum of Flight The Comet was an all-metal low-wing cantilever monoplane powered by four jet engines; it had a four-place cockpit occupied by two pilots, a flight engineer, and a navigator. Two pairs of turbojet engines (on the Comet 1s, Halford H.2 Ghosts, subsequently known as de Havilland Ghost 50 Mk1s) were buried in the wings. The original Comet was the approximate length of, but not as wide as, the later Boeing 737-100, and carried fewer people in a significantly more-spacious environment. BOAC installed 36 reclining "slumberseats" with centres on its first Comets, allowing for greater leg room in front and behind; Air France had 11 rows of seats with four seats to a row installed on its Comets. Large picture window views and table seating accommodations for a row of passengers afforded a feeling of comfort and luxury unusual for transportation of the period. Amenities included a galley that could serve hot and cold food and drinks, a bar, and separate men's and women's toilets. Provisions for emergency situations included several life rafts stored in the wings near the engines, and individual life vests were stowed under each seat. For passengers used to piston-engined propeller-driven airliners, smooth and quiet jet flight was a novel experience. Avionics and systems For ease of training and fleet conversion, de Havilland designed the Comet's flight deck layout with a degree of similarity to the Lockheed Constellation, an aircraft that was popular at the time with key customers such as BOAC. The navigator occupied a dedicated station, with a table across from the flight engineer. Several of the Comet's avionics systems were new to civil aviation. One such feature was irreversible, powered flight controls, which increased the pilot's ease of control and the safety of the aircraft by preventing aerodynamic forces from changing the directed positions and placement of the aircraft's control surfaces. Many of the control surfaces, such as the elevators, were equipped with a complex gearing system as a safeguard against accidentally over-stressing the surfaces or airframe at higher speed ranges. The Comet had a total of four hydraulic systems: two primaries, one secondary, and a final emergency system for basic functions such as lowering the undercarriage. The undercarriage could also be lowered by a combination of gravity and a hand-pump. Power was syphoned from all four engines for the hydraulics, cabin air conditioning, and the de-icing system; these systems had operational redundancy in that they could keep working even if only a single engine was active. The majority of hydraulic components were centred in a single avionics bay. A pressurised refuelling system, developed by Flight Refuelling Ltd, allowed the Comet's fuel tanks to be refuelled at a far greater rate than by other methods. The cockpit was significantly altered for the Comet 4's introduction, on which an improved layout focusing on the onboard navigational suite was introduced. An EKCO E160 radar unit was installed in the Comet 4's nose cone, providing search functions as well as ground and cloud-mapping capabilities, In 1969, when the Comet 4's design was modified by Hawker Siddeley to become the basis for the Nimrod, the cockpit layout was completely redesigned and bore little resemblance to its predecessors except for the control yoke. Fuselage Diverse geographic destinations and cabin pressurisation alike on the Comet demanded the use of a high proportion of alloys, plastics, and other materials new to civil aviation across the aircraft to meet certification requirements. The Comet's high cabin pressure and high operating speeds were unprecedented in commercial aviation, making its fuselage design an experimental process. 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. Operationally, the design of the cargo holds led to considerable difficulty for the ground crew, especially baggage handlers at the airports. The cargo hold had its doors located directly underneath the aircraft, so each item of baggage or cargo had to be loaded vertically upward from the top of the baggage truck, then slid along the hold floor to be stacked inside. The individual pieces of luggage and cargo also had to be retrieved in a similarly slow manner at the arriving airport. Propulsion The Comet was powered by two pairs of turbojet engines buried in the wings close to the fuselage. Chief designer Bishop chose the Comet's embedded-engine configuration because it avoided the drag of podded engines and allowed for a smaller fin and rudder since the hazards of asymmetric thrust were reduced. The engines were outfitted with baffles to reduce noise emissions, and extensive soundproofing was also implemented to improve passenger conditions. engine intakes Placing the engines within the wings had the advantage of a reduction in the risk of foreign object damage, which could seriously damage jet engines. The low-mounted engines and good placement of service panels also made aircraft maintenance easier to perform. The Comet 1 featured de Havilland Ghost 50 Mk1 turbojet engines. Two hydrogen peroxide-powered de Havilland Sprite booster rockets were originally intended to be installed to boost takeoff under hot and high altitude conditions from airports such as Khartoum and Nairobi. These were tested on 30 flights, but the Ghosts alone were considered powerful enough and some airlines concluded that rocket motors were impractical. Comet 1s subsequently received more powerful Ghost DGT3 series engines. ==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. 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. On 2 May 1952, as part of BOAC's route-proving trials, G-ALYP took off on the world's first jetliner flight with fare-paying passengers and inaugurated scheduled service from London to Johannesburg. 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. Comet 1 at Entebbe Airport, Uganda in 1952, this aircraft later crashed as BOAC Flight 115 in the same year. Prince Philip returned from the Helsinki Olympic Games with G-ALYS on 4 August 1952. Queen Elizabeth, the Queen Mother, Elizabeth II and Princess Margaret were guests on a special flight of the Comet on 30 June 1953 hosted by Sir Geoffrey and Lady de Havilland. Flights on the Comet were about twice as fast as advanced piston-engined aircraft such as the Douglas DC-6 ( vs , 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 Argonaut (a DC-4 variant) piston airliner. (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. 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. 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 and were fuel-efficient above . In summer 1953, eight BOAC Comets left London each week: three to Johannesburg, two to Tokyo, two to Singapore and one to Colombo. In 1953, the Comet appeared to have achieved success for de Havilland. Popular Mechanics wrote that Britain had a lead of three to five years on the rest of the world in jetliners. A slightly longer version of the Comet 1 with more powerful engines, the Comet 2, was being developed, and orders were placed by Air India, British Commonwealth Pacific Airlines, 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. Early hull losses On 26 October 1952, the Comet suffered its first hull loss when BOAC Flight 115 departing Rome's 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. The accident was the first fatal jetliner crash. and wing fences were added to control spanwise flow. 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 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). 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 thundersquall six minutes after taking off from Calcutta-Dum Dum airport, India, killing all 43 on board. Witnesses observed the wingless Comet on fire plunging into the village of Jagalgori, leading investigators to suspect structural failure. 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 artificial feel was the first of its kind to be introduced in any aircraft. and investigators suggested that this might have contributed to the pilot's alleged over-stressing of the aircraft; 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. Comet disasters of 1954 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 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. 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. BOAC also voluntarily grounded its Comet fleet pending investigation into the causes of the accident. Abell Committee Court of Inquiry Media attention centred on potential sabotage; During the investigation, the Royal Navy conducted recovery operations. The first pieces of wreckage were discovered on 12 February 1954 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. The forensic reconstruction effort had just begun when the Abell Committee reported its findings. No apparent fault in the aircraft was found, and the British government decided against opening a further public inquiry into the accident. 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, 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. This time, the entire fuselage was tested in a dedicated water tank that was built specifically at Farnborough to accommodate its full length. 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. At the opposite port forward corner of the same window, a small crack resulting from manufacturing damage - that had been repaired at the time of construction - was also located along the failure, but the investigation found no evidence of metal fatigue at this point. Once the crack initiated the skin failed from the point of the ADF cut out and propagated downward and rearward, resulting in an explosive decompression. 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. 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 ADF antenna penetration that failed on G-ALYP. On display at the 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 Earlier structural indications The issue of the lightness of Comet 1 construction (in order not to 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." Square window myths 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 De Havilland's assumptions and testing methods 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. The shape of the passenger windows was not indicated in any failure mode detailed in the accident report and was 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. 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. "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) 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." The Cohen inquiry closed on 24 November 1954, having "found that the basic design of the Comet was sound", Following the Comet enquiry, aircraft were designed to "fail-safe" or safe-life standards, though several subsequent catastrophic fatigue failures, such as Aloha Airlines Flight 243 of April 28, 1988 have occurred. 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. 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. 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, 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. The base price of a new Comet 4 was roughly £1.14 million (£ million in ). 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). and in 1960 began flying 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. 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. 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. The Comet 4A ordered by Capital Airlines was instead built for BEA as the Comet 4B, with a further fuselage stretch of 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. The last Comet 4 variant, the Comet 4C, first flew on 31 October 1959 and entered service with Mexicana in 1960. 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, Misrair (later Egyptair), and Sudan Airways, it was the most popular Comet variant. File:MSA Comet Groves.jpg|Malaysia-Singapore Airlines Comet 4 at Kai Tak Airport in 1966 File:United Arab Airlines Comet Soderstrom.jpg|United Arab Airlines Comet 4C at 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 Later service in Leicestershire, England In 1959, BOAC began shifting its Comets from transatlantic routes 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. 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. 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. On 14 March 1997 a Comet 4C serial XS235 and named Canopus, ==Legacy==

The Comet is widely regarded as both an adventurous step forward and a supreme tragedy; the aircraft's legacy includes advances in aircraft design and in accident investigations. The inquiries into the accidents that plagued the Comet 1 were perhaps some of the most extensive and revolutionary that have ever taken place, establishing precedents in accident investigation; many of the deep-sea salvage and aircraft reconstruction techniques employed have remained in use within the aviation industry. In spite of the Comet being subjected to what was then the most rigorous testing of any contemporary airliner, pressurisation and the dynamic stresses involved were not thoroughly understood at the time of the aircraft's development, nor was the concept of metal fatigue. Though these lessons could be implemented on the drawing board for future aircraft, corrections could only be retroactively applied to the Comet. 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. Aeronautical-engineering firms were quick to respond to the Comet's commercial advantages and technical flaws alike; other aircraft manufacturers learned from, and profited by, the hard-earned lessons embodied by de Havilland's Comet. The Comet's buried engines were used on some other early jet airliners, such as the Tupolev Tu-104, but later aircraft, such as the Boeing 707 and Douglas DC-8, differed by employing podded engines held on pylons beneath the wings. Boeing stated that podded engines were selected for their passenger airliners because buried engines carried a higher risk of catastrophic wing failure in the event of engine fire. In response to the Comet tragedies, manufacturers also developed ways of pressurisation testing, often going so far as to explore rapid depressurisation; subsequent fuselage skins were of a greater thickness than the skin of the Comet. ==Variants==

Comet 1 Comet 1A at Le Bourget Airport in 1952 The Comet 1 was the first model produced, a total of 12 aircraft in service and test. Following closely the design features of the two prototypes, the only noticeable change was the adoption of four-wheel bogie main undercarriage units, replacing the single main wheels. Four Ghost 50 Mk 1 engines were fitted (later replaced by more powerful Ghost DGT3 series engines). The span was , and overall length ; the maximum takeoff weight was over and over 40 passengers could be carried. • An updated Comet 1A was offered with higher-allowed weight, greater fuel capacity, All were substantially damaged in stress testing or were scrapped entirely. • Comet 1XB: Four Comet 1As were upgraded to a 1XB standard with a reinforced fuselage structure and oval windows. Both 1X series were limited in number of pressurisation cycles. • The DH 111 Comet Bomber, a nuclear bomb-carrying variant developed to Air Ministry specification B35/46, was submitted to the Air Ministry on 27 May 1948. It had been originally proposed in 1948 as the "PR Comet", a high-altitude photo reconnaissance adaptation of the Comet 1. The Ghost DGT3-powered airframe featured a narrowed fuselage, a bulbous nose with H2S Mk IX radar, and a four-crewmember pressurised cockpit under a large bubble canopy. Fuel tanks carrying were added to attain a range of . The proposed DH 111 received a negative evaluation from the Royal Aircraft Establishment over serious concerns regarding weapons storage; this, along with the redundant capability offered by the RAF's proposed V bomber trio, led de Havilland to abandon the project on 22 October 1948. Comet 2 , fitted with revised round windows The Comet 2 had a slightly larger wing, higher fuel capacity and more-powerful Rolls-Royce Avon engines, which all improved the aircraft's range and performance; Following the Comet 1 disasters, these models were rebuilt with heavier-gauge skin and rounded windows, and the Avon engines featuring larger air intakes and outward-curving jet tailpipes. A total of 12 of the 44-seat Comet 2s were ordered by BOAC for the South Atlantic route. The first production aircraft (G-AMXA) flew on 27 August 1953. Although these aircraft performed well on test flights on the South Atlantic, their range was still not suitable for the North Atlantic. All but four Comet 2s were allocated to the RAF, deliveries beginning in 1955. Modifications to the interiors allowed the Comet 2s to be used in several roles. For VIP transport, the seating and accommodations were altered and provisions for carrying medical equipment including iron lungs were incorporated. Specialised signals intelligence and electronic surveillance capability was later added to some airframes. • Comet C2: Eight Comet 2s originally destined for the civil market were completed for the RAF and assigned to No. 216 Squadron. The Comet 3 was destined to remain a development series since it did not incorporate the fuselage-strengthening modifications of the later series aircraft, and was not able to be fully pressurised. Only two Comet 3s began construction; G-ANLO, the only airworthy Comet 3, was demonstrated at the Farnborough SBAC Show in September 1954. The other Comet 3 airframe was not completed to production standard and was used primarily for ground-based structural and technology testing during development of the similarly sized Comet 4. Another nine Comet 3 airframes were not completed and their construction was abandoned at Hatfield. In BOAC colours, G-ANLO was flown by John Cunningham in a marathon round-the-world promotional tour in December 1955. Comet 4 Comet 4B at Berlin Tempelhof Airport in October 1968 The Comet 4 was a further improvement on the stretched Comet 3 with even greater fuel capacity, The fuselage skin has been thickened considerably. The design had progressed significantly from the original Comet 1, growing by and typically seating 74 to 81 passengers compared to the Comet 1's 36 to 44 (119 passengers could be accommodated in a special charter seating package in the later 4C series). The Comet 4 was considered the definitive series, having a longer range, higher cruising speed and higher maximum takeoff weight. These improvements were possible largely because of Avon engines, with twice the thrust of the Comet 1's Ghosts. Deliveries to BOAC began on 30 September 1958 with two 48-seat aircraft, which were used to initiate the first scheduled transatlantic services. • Comet 4B: Originally developed for Capital Airlines as the 4A, the 4B featured greater capacity through a 2m longer fuselage, and a shorter wingspan; 18 were produced. • Comet 4C: This variant featured the Comet 4's wings and the 4B's longer fuselage; 28 were produced. The last two Comet 4C fuselages were used to build prototypes of the Hawker Siddeley Nimrod maritime patrol aircraft. Comet 5 proposal The Comet 5 was proposed as an improvement over previous models, including a Wider and longer fuselage with five-abreast seating, a wing with greater sweep and Still buried in the wing root Rolls-Royce Conway engines (This is actually very similar to the Vickers V-1000). Without support from the Ministry of Transport, the proposal languished as a hypothetical aircraft and was never realised. Hawker Siddeley Nimrod The last two Comet 4C aircraft produced were modified as prototypes (XV148 & XV147) to meet a British requirement for a maritime patrol aircraft for the Royal Air Force; initially named "Maritime Comet", the design was designated Type HS 801. This variant became the Hawker Siddeley Nimrod and production aircraft were built at the Hawker Siddeley factory at Woodford Aerodrome. Entering service in 1969, five Nimrod variants were produced. The final Nimrod aircraft were retired in June 2011. ==Operators==

s at London Gatwick Airport in 1976 The original operators of the early Comet 1 and the Comet 1A were BOAC, Union Aéromaritime de Transport and Air France. All early Comets were withdrawn from service for accident inquiries, during which orders from British Commonwealth Pacific Airlines, Japan Air Lines, Linea Aeropostal Venezolana, National Airlines, Pan American World Airways and Panair do Brasil were cancelled. while the Comet 4B variant was operated by customers BEA and Olympic Airways and Qantas; after 1965 they were sold to AREA Ecuador, Dan-Air, Mexicana, Malaysian Airways, and the Ministry of Defence. Channel Airways obtained five Comet 4Bs from BEA in 1970 for inclusive tour charters. Dan-Air bought all of the surviving flyable Comet 4s from the late 1960s into the 1970s; some were for spares reclamation, but most were operated on the carrier's inclusive-tour charters; a total of 48 Comets of all marks were acquired by the airline. In military service, the United Kingdom's Royal Air Force was the largest operator, with 51 Squadron (1958–1975; Comet C2, 2R), 192 Squadron (1957–1958; Comet C2, 2R), 216 Squadron (1956–1975; Comet C2 and C4), and the Royal Aircraft Establishment using the aircraft. The Royal Canadian Air Force also operated Comet 1As (later retrofitted to 1XB) through its 412 Squadron from 1953 to 1963. ==Accidents and incidents==

The Comet was involved in 25 hull-loss accidents, including 13 fatal crashes which resulted in 492 fatalities. Pilot error was blamed for the type's first fatal accident, which occurred during takeoff at Karachi, Pakistan, on 3 March 1953 and involved a Canadian Pacific Airlines Comet 1A. Three fatal Comet 1 crashes were due to structural problems, specifically British Overseas Airways Corporation flight 783 on 2 May 1953, British Overseas Airways Corporation flight 781 on 10 January 1954, and South African Airways flight 201 on 8 April 1954. These accidents led to the grounding of the entire Comet fleet. After design modifications were implemented, Comet services resumed on October 4, 1958, with Comet 4s. in the Spanish Montseny range in July 1970 during a Dan-Air flight. Other fatal Comet 4 accidents included a British European Airways crash in Ankara, Turkey, following instrument failure on 21 December 1961, a United Arab Airlines Flight 869 crash during inclement weather near Bombay, India, on 28 July 1963, and the terrorist bombing of Cyprus Airways Flight 284 off the Turkish coast on 12 October 1967. Nine Comets, including Comet 1s operated by BOAC and Union Aeromaritime de Transport and Comet 4s flown by Aerolíneas Argentinas, Dan-Air, Malaysian Airlines and United Arab Airlines, were irreparably damaged during takeoff or landing accidents that were survived by all on board. A hangar fire damaged a No. 192 Squadron RAF Comet 2R beyond repair on 13 September 1957, and three Middle East Airlines Comet 4Cs were destroyed by Israeli troops at Beirut, Lebanon, on 28 December 1968. ==Aircraft on display==

in Shropshire in Cambridgeshire; this aircraft was later painted in BOAC's livery and placed inside the museum's AirSpace hall. Since retirement, three early-generation Comet airframes have survived in museum collections. The only complete remaining Comet 1, a Comet 1XB with the registration G-APAS, the last Comet 1 built, is displayed at the Royal Air Force Museum Midlands. this aircraft also served with the RAF as XM823. The sole surviving Comet fuselage with the original square-shaped windows, part of a Comet 1A registered F-BGNX, has undergone restoration and is on display at the de Havilland Aircraft Museum near St Albans in Hertfordshire, England. A Comet C2 Sagittarius with serial XK699, later maintenance serial 7971M, was on display at the gate of RAF Lyneham in Wiltshire, England from 1987. In 2012, with the planned closure of RAF Lyneham, the aircraft was slated to be dismantled and shipped to the RAF Museum Cosford where it was to be re-assembled for display. The move was cancelled due to the level of corrosion and the majority of the airframe was scrapped in 2013, the cockpit section going to the Boscombe Down Aviation Collection at Old Sarum Airfield. Six complete Comet 4s are housed in museum collections. The Imperial War Museum Duxford has a Comet 4 (G-APDB), originally in Dan-Air colours as part of its Flight Line Display, and later in BOAC livery at its AirSpace building. A Comet 4B (G-APYD) is stored in a facility at the Science Museum at Wroughton in Wiltshire, England. Comet 4Cs are exhibited at the Flugausstellung Peter Junior at Hermeskeil, Germany (G-BDIW), the Museum of Flight Restoration Center near Everett, Washington (N888WA), and the National Museum of Flight near Edinburgh, Scotland (G-BDIX). The last Comet to fly, Comet 4C Canopus (XS235), Since the 2000s, several parties have proposed restoring Canopus, which is maintained by a staff of volunteers, to airworthy, fully flight-capable condition. The Bruntingthorpe Aerodrome also displays a related Hawker Siddeley Nimrod MR2 aircraft. ==Specifications==