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Capital Airlines de Havilland DH 4A Comet

Image by james_gordon_losangeles
CAPITAL AIRLINES PURCHASE COMETS’ was the headline in the Enterprise magazine – the internal magazine of the de Havilland Company. It referred to a (then) recent joint announcement by Capital and de Havilland which disclosed an order for 14 Comet aircraft.
Thus it appeared that de Havilland had done what every other non-American manufacturer needed to do, broken into the United States airliner market in the face of home competition. The argument went: with a foot hold in the U.S. market many more ‘knock on’ sales could be hoped for. So the announcement was of very great significance.
The agreement specified that the Comets would be powered by Rolls-Royce engines, and including spares, the cost was put at some £19 million/USD 7 million (year 2000 = £263/43). Deliveries were to commence in late 1958 with four Comet Mk.4s and late in 1959 with ten of the special variant the Mk.4A.
J.H. Slim Carmichael, who was President of Capital Airlines, said of the deal, The decision to purchase the Comet has been made after a most comprehensive and detailed study of all flight equipment either in production or projected, both in the United States and England. The economical and operating characteristics of the Comet 4A are ideally suited to the Capital system. The Comets will go into service on our major and most competitive routes.
Apparently the same basis for determining economic criteria were used when Capital purchased Viscounts. (ed note – The only financing Capital had available was through the Bank Of England and only for British built aircraft). Projections made before the Viscount purchase had proved accurate when it was introduced on Capital routes in 1955. The Comet order was placed because Capital now wanted a range of pure-jets to operate some 200 mph faster than anything else they then had in use. Capital was one of the biggest domestic carriers in the USA as was illustrated by figures for 1955 which showed that Capital carried 2½ million passengers over some 31 million miles!
Capital’s Mk.4As were to be furnished to accommodate 74 passengers in the utmost luxury by having 68 persons seated four abreast in two large cabins and six in a forward lounge. The expectation was that passengers would be carried in unprecedented smoothness and quietude, even surpassing the qualities of the earlier Comet models while the speed and economy also show a marked advance. The 4A was to be assembled at Chester as well as Hatfield, England.
The Mk.4A was launched in June 1956 as a short range version of the Comet. The fuselage was stretched and the wing span was reduced. Maximum takeoff weight was reduced to 152.5Klb. The Mk.4A died, when the launch customer Capital Airlines cancelled the order. As a result no Mk 4A was ever built.
Unfortunately Capital suffered sudden financial difficulties, a period of uncertainty and numerous fatal incidents, it was forced to give up some of its routes to rival carriers and was absorbed into United Airlines. The foothold into the US market was lost and the Mk.4A was never produced.
The de Havilland DH 106 Comet was the world’s first commercial jet airliner to reach production. Developed and manufactured by de Havilland at the Hatfield, Hertfordshire, United Kingdom headquarters, it first flew in 1949 and was a landmark in aeronautical design. It featured an extremely aerodynamically clean design with its four de Havilland Ghost turbojet engines buried into the wings, a low-noise pressurised cabin, and large windows; for the era, it was an exceptionally comfortable design for passengers and showed signs of being a major success in the first year upon launching.
However, a few years after introduction into commercial service, Comet airframes began suffering from catastrophic metal fatigue, which in combination with cabin pressurisation cycles, caused two well-publicised accidents where the aircraft tore apart in mid-flight. The Comet had to be withdrawn and extensively tested to discover the cause; the first incident had been incorrectly identified as having been caused by an onboard fire. Several contributory factors, such as window installation methodology, were also identified as exacerbating the problem. The Comet was extensively redesigned to eliminate this design flaw. Rival manufacturers meanwhile developed their own aircraft and heeded the lessons learned from the Comet.
Although sales never fully recovered, the redesigned Comet 4 series subsequently enjoyed a long and productive career of over 30 years. The Comet was adapted for a variety of military roles, such as surveillance, VIP, medical and passenger transport; the most extensive modification resulted in a specialised maritime patrol aircraft variant, the Hawker Siddeley Nimrod. Nimrods remained in service with the Royal Air Force (RAF) until they were retired in June 2011, over 60 years after the Comet’s first flight.
Development
Design studies for the DH 106 Comet 1944–1947
During the Second World War, the Brabazon Committee was formed on 11 March 1943 to determine Britain’s postwar airliner needs. One of the recommendations set a design target of a pressurised, transatlantic mailplane that could carry a ton of payload at a cruising speed of 400 mph (640 km/h).[8] Challenging the widely held scepticism of jet engines as too fuel-hungry and unreliable, committee member Sir Geoffrey de Havilland, head of the de Havilland company, used his influence and the company’s expertise with jets to specify a turbojet-powered design.[7] The committee accepted the proposal, calling it the "Type IV" (of five designs), and awarded the production contract to de Havilland’s Type 106. The first-phase designs focused on short and intermediate range mailplanes with a small passenger compartment and as few as six seats, later redefined as a long-range airliner with 24-seat capacity. Out of all the Brabazon designs, the DH 106 was seen as the riskiest in terms of both introducing untried design elements and for the financial commitment involved.[7] Nevertheless, British Overseas Airways Corporation (BOAC) found the Type IV’s specifications attractive, initially proposing a purchase of 25 aircraft and, in December 1945, when a "firm contract" was laid out, revising the number to 10.
A design team was formed in 1946 under the leadership of Chief Designer Ronald Bishop, who had been responsible for the Mosquito fighter-bomber. A number of unorthodox configurations were considered, all of which were subsequently rejected. The Ministry of Supply was interested in the most radical of the proposed designs and issued Operational Requirement OR207 to Specification E.18/45 for two experimental DH 108s ordered as proof-of-concept aircraft to test swept-wing configurations in both low-speed and high-speed flight.
Even before the DH 108s were completed, further requests from BOAC necessitated a redesign of the DH 106 from the original four-engined (Halford H.1 Goblin-powered) 24-seat airliner to a larger 36-seat version to specification 22/46 in September 1946. With no time to develop the technology required for the tailless configuration, Bishop opted for a more conventional 20˚ swept-wing design with unswept tail surfaces, married to an enlarged fuselage accommodating 36 passengers, arranged four abreast with a central aisle. Four new, more powerful Rolls-Royce Avons were to be incorporated in pairs buried in the wing roots, but Halford H.2 Ghost engines were eventually specified as an interim solution while the Avons cleared certification. The redesigned DH 106 was named the DH 106 Comet in December 1947. First revised orders for both BOAC and British South American Airways for a combined total of 14 aircraft had a projected delivery schedule set for 1952.
During 1947–1948, de Havilland undertook an extensive research and development phase, utilising a number of stress test rigs at Hatfield for small components and large assemblies. Sections of the pressurised fuselage were subjected to the conditions of a flight at altitude in the company’s decompression chamber. The DH 108s were also modified to test the DH 106′s power controls.
The first flight of the first prototype DH 106 Comet (carrying Class B markings G-5-1) took place on 27 July 1949 from Hatfield, and lasted 31 minutes. The pilot was de Havilland Chief Test Pilot John Cunningham, a famous wartime night-fighter pilot, who later commented, "I assumed that it would change aviation, and so it has proved. It was a bit like Concorde. Also on board were co-pilot Harold Tubby Waters, engineers John Wilson (electrics) and Frank Reynolds (hydraulics), along with flight test observer Tony Fairbrother. Fairbrother commented, The world changed as our wheels left the ground.
G-5-1 was publicly displayed at the 1949 Farnborough Airshow before beginning flight trials. A year later, the second prototype made its maiden flight. On 2 April 1951, this aircraft was delivered to the BOAC Comet Unit at Hurn under the registration G-ALZK and carried out 500 flying hours of crew training and route proving.[22] Both prototypes were distinguished by large main wheel units that were replaced by four-wheeled bogies on each main leg for the subsequent production series.
The British Government considered the development of the Comet a highly ideological matter, as high-ranking officials perceived the need to meet foreign competition and surpass them when there was the opportunity to do so:
During the next few years, the UK has an opportunity, which may not recur, of developing aircraft manufacture as one of our main export industries. On whether we grasp this opportunity and so establish firmly an industry of the utmost strategic and economic importance, our future as a great nation may depend.
—Duncan Sandys, Minister of Supply, 1952.
Design
The Comet is an all-metal low-wing cantilever monoplane powered by four jet engines, approximately the length of a Boeing 737, carrying fewer people in a significantly more spacious environment. The earliest Comets had 11 rows of seats with four seats to a row in the 1A configuration used by Air France; BOAC used an even roomier arrangement of 36 seats on 45-inch (1,100 mm) centres. The Comet’s four-place cockpit held two pilots, a flight engineer, and a navigator. The cabin was quieter than those of propeller-driven airliners. Amenities included a galley that could serve hot and cold food and drinks, a bar, and separate men’s and women’s washrooms. For emergencies, life rafts were stored in the wings near the engines and life vests were stowed under each seat bottom.
The clean, low-drag design featured many unique or innovative design elements, including a swept-wing leading edge, integral wing fuel tanks, and four-wheel bogie main undercarriage units designed by de Havilland. Two pairs of de Havilland Ghost 50 Mk1 turbojet engines were buried in the wings close to the fuselage. Chief Designer Bishop chose this configuration because it avoided the drag of podded engines and allowed a smaller fin and rudder, since the hazards of asymmetric thrust were reduced. The engines’ higher mounting in the wings also reduced the risk of ingestion damage (foreign object damage [FOD]), a major problem for turbine engines. These benefits were compromised by increased structural weight and general complexity, including armour for the engine cells (in case of an engine explosion) and a more complicated wing structure. This arrangement also carried higher risk of catastrophic wing failure in case of an engine fire, cited as the main reason the Boeing Aircraft Company chose podded engines in their subsequent jet bomber and airliner designs. The fuel system incorporated underwing pressure refuelling, developed by Flight Refuelling Ltd, which allowed much faster refilling of fuel tanks than was possible previously.
The Comet was originally intended to have two hydrogen peroxide-powered de Havilland Sprite booster rockets for takeoff under hot and high altitude conditions from airports such as Khartoum and Nairobi. These were tested on 30 flights, but the Ghosts were considered powerful enough without them, although Sprite fittings were kept on production aircraft. The later Comet 4 was highly rated for its takeoff performance from high altitude locations such as Mexico City. Newer and more powerful AJ.65 Avon engines replaced the Ghosts on the Comet 2. High engine performance combined with a low weight (compared to the Boeing 707 and Douglas DC-8), and exceptionally clean design all contributed to its high performance. Early-model Comets had the advantage of requiring low maintenance, the de Havilland Ghost engines being a key contributing factor. Mounting the engines in a low-wing position combined with numerous service panels allowed for "easy" and efficient maintenance.
The Comet’s thin metal skin was composed of advanced new alloys (Directorate of Technical Development 564/L.73 and DTD 746C/L90) and was both chemically bonded using the adhesive Redux and riveted, which saved weight and reduced the risk of fatigue cracks spreading from the rivets. When it went into service with BOAC on 2 May 1952, the Comet was the most exhaustively tested airliner in history. After the Comet entered production, for safety reasons, and to limit the damage to the specimens, a water tank was used instead of the decompression chamber. The entire forward fuselage section was tested for metal fatigue by repeatedly pressurising to 2.75 pounds per square inch (19.0 kPa) overpressure and depressurising through more than 16,000 cycles, equivalent to about 40,000 hours of airline service. The windows were tested under a pressure of 12 psi (83 kPa), 4.75 psi (32.8 kPa) above the normal service ceiling of 36,000 ft (11,000 m). One window frame survived a massive 100 psi (690 kPa), about 1,250% over the maximum pressure it would encounter in service.
In 1953, Sud-Est’s design bureau, while working on the Sud Aviation Caravelle, licensed several design features from de Havilland, a company Sud had previously collaborated with on earlier licenced designs, including the DH 100 Vampire. [N 12] The entire nose and cockpit layout from the Comet 1 was grafted onto the Caravelle.
Operational history
Introduction
The first production aircraft (G-ALYP) flew on 9 January 1951 and subsequently was on loan to BOAC for development flying by the Comet Unit. On 22 January 1952, G-ALYS was the first Comet to receive a Certificate of Airworthiness, six months ahead of schedule. As part of the BOAC route proving trials, on 2 May, G-ALYP took off on the world’s first jetliner flight with fare-paying passengers, beginning scheduled service to Johannesburg. The last Comet from the initial order (G-ALYZ) began flying in September 1952, carrying cargo along South American routes while simulating passenger schedules.
The Comet was a hit with passengers including Queen Elizabeth, the Queen Mother and Princess Margaret, who were guests on a special flight on 30 June 1953 hosted by Sir Geoffrey and Lady de Havilland, and thus became the first members of the British Royal Family to fly by jet.[48] A total of 30,000 passengers was carried during the first year of service. For the travelling public, the Comet offered flights about 50% faster than advanced piston-engined types such as the Douglas DC-6 (490 mph for the Comet compared to 315 mph for the DC-6B), and its rate of climb was also far higher, which could cut flight times in half. In August 1953 BOAC scheduled the Comet London to Tokyo in 35 hours, compared to 85 hr 35 min for their Argonaut; Pan Am’s DC-6B flight 2 was scheduled 46 hr 45 min. Smooth, quiet jet flight was a new experience for passengers used to piston-engined airliners (although passengers of today would consider it noisy, particularly when seated aft of the wing). BOAC’s Comets featured the BOAC-designed slumberseat; a comfortable, reclining design, allowing for greater leg room in front and behind. The large picture window view and accommodations for a table setting for a row of passengers afforded a feel of comfort and luxury atypical of airliners of the period. One of the most striking aspects of flight on the Comet was the quiet, vibration-free flying touted by BOAC.
Commercial success was widely expected, with a profitable passenger load factor as low as 43%. The Ghost engine was smooth, relatively simple, fuel-efficient above 30,000 ft (9,144 m),[N had low maintenance costs, and enabled operations above weather the competition had to fly through. At the height of Comet’s early flying career, the BOAC Comet 1 fleet flew routes such as London-Singapore, London-Tokyo, and London-Johannesburg several times a week.
Early accidents and incidents
On 26 October 1952, a BOAC flight departing from Ciampino airport near Rome failed to become airborne and ran into rough ground at the end of the runway. Two passengers sustained only minor injuries, but the aircraft was a total loss. On 3 March 1953, a new Canadian Pacific Airlines Comet 1A (CF-CUN), known as "Empress of Hawaii," being delivered to Australia, also failed to become airborne on takeoff from Karachi, Pakistan. The aircraft plunged into a dry drainage canal and collided with an embankment, killing all five crew and six passengers on board, the first fatal crash of a passenger jet airliner.
Both of these accidents were originally attributed to pilot error as over-rotation had led to a loss of lift from the leading edge of the aircraft’s wing. It was later determined the wing profile led to a loss of lift at high angle of attack, and the engine inlets suffered from a lack of pressure recovery in these conditions as well. The wing leading edge was re-profiled with a pronounced droop and a wing fence was added to control spanwise flow. A fictionalised investigation into these takeoff accidents was the subject of the 1959 novel, Cone of Silence 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 1984 non-fiction work, Strange Encounters: Mysteries of the Air.
The next fatal accident involving passengers was on 2 May 1953, when a BOAC Comet 1 (G-ALYV) crashed in a severe tropical storm six minutes after taking off from Calcutta/Dum Dum (now Netaji Subhash Chandra Bose International Airport), India, killing all 43 on board. The crash was attributed to structural failure of the airframe with witnesses observing the wingless Comet on fire plunging into the Indian Ocean.
India Court of Inquiry
A court of inquiry was convened by the Central Government of India to examine the cause of the accident.[N 17] The conclusions of the inquiry focused on the extreme negative G forces encountered in the thundersquall. A large proportion of the aircraft was recovered and reassembled at Farnborough.[59] The break-up was found to have begun with a left-hand elevator spar failure in the stabiliser. The immediate focus was on the severe turbulence encountered that induced down-loading, which subsequently precipitated the loss of the wings. Examination of the cockpit controls led to a belief that the pilot may have inadvertently overstressed the aircraft when pulling out of a steep dive by over-manipulation of the fully powered flight controls.
Recommendations from the court revolved around the enforcement of stricter rough air speed limits. The tragedy led to two significant developments: all Comets were equipped with "weather radar" and the introduction of Q feel, a system that ensured that control column forces (invariably called "stick forces") would be proportional to control loads. The artificial feel was the first of its kind to be introduced in any aircraft. The Comet 1 and 1A had been criticised for a lack of feel in their controls, although test pilot John Cunningham contended, it flew extremely smoothly and responded to the controls in the best way de Havilland aircraft usually did.
DH.106 Comet 1 of BOAC at London Heathrow on 2 June 1953
[edit] Comet disasters of 1954
Main articles: BOAC Flight 781 and South African Airways Flight 201
Rome’s Ciampino airport, the site of the first Comet hull loss, was again the origin of more disastrous Comet flights just over a year later. On 10 January 1954, 20 minutes after taking off from Ciampino, Comet G-ALYP ("Yoke Peter"), BOAC Flight 781, broke up in flight 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 "sketchy" and incomplete radio transmissions left behind, there appeared to be no obvious reason for the crash. 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.
Abell Committee Court of Inquiry
Media attention centred upon sabotage; other speculation ranged from "clear-sky" turbulence to an explosion of vapour in an empty fuel tank. The committee soon focused on six potential aerodynamic and mechanical causes: control flutter (which had led to the loss of the de Havilland DH 108 Swallow 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 fire was the most likely cause of the problem, and a number of changes were made to the aircraft to protect the engines and wings from damage which might lead to another fire.
The cost of solving the Comet mystery must be reckoned neither in money nor in manpower.
During this investigation, the Royal Navy conducted recovery operations. The first wreckage was discovered on 12 January and the search continued until August, by which time, 70% of the main structure, 80% of the power section and 50% of the aircraft systems/equipment had been recovered. The forensic reconstruction effort was only lately underway when the Abell Committee reported their findings. On 4 April, 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." Comet flights resumed on 23 March 1954.
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 flight from London to Johannesburg), when it crashed in the waters near Naples. The fleet was immediately grounded once again and a large investigation board was formed under the direction of the Royal Aircraft Establishment (RAE). Prime Minister Winston Churchill tasked the Royal Navy with helping locate and retrieve the wreckage so that the cause of the accident could be found.[69] The type’s Certificate of Airworthiness was revoked and line production suspended at Hatfield while the BOAC fleet was grounded.
[edit] Cohen Committee Court of Inquiry
An illustration showing the recovered (shaded) parts of the wreckage of the de Havilland Comet 1 G-ALYP "Yoke Peter" and the forward ADF aerial window in the cabin roof where the initial fatigue failure occurred – after an illustration in Air Disasters (1989).
On 19 October 1954, a court of inquiry was set up under the chairmanship of Lord Cohen to examine the causes of the Comet crashes.[70] Investigators under the leadership of Sir Arnold Hall, Director of the RAE at Farnborough, began considering fatigue as the most likely cause of both accidents and initiated further research into measurable strain on the skin. With the recovery of large sections of G-ALYP from the Elba crash and G-ALYU, an extensive "water torture" test eventually provided conclusive results. Stress around the window corners was found to be much higher than expected, and stresses on the skin were generally more than previously expected or tested. This was due to stress concentration, a consequence of the windows’ square shape, the levels of stress at these corners could be two or three times that across the rest of the fuselage.
Before the Elba accident, G-ALYP had made 1,290 pressurised flights and at the time of the Naples accident, G-ALYY had made 900 pressurised flights. Dr. P.B. Walker, Head of the Structures Department (RAE) said he was not surprised by this, noting that the difference was about 3 to 1, and previous experience with metal fatigue suggested a total range of 9 to 1 between experiment and outcome in the field could result in failure. Thus, if the tank test result was "typical", aircraft failures could be expected at anywhere from 1,000 to 9,000 cycles. Engineers subjected an identical airframe, G-ALYU, to repeated re-pressurisation 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 the fuselage had ripped open at a corner of the forward port-side escape hatch cutout. A further test reproduced the same results. By then, the RAE had reconstructed about ⅔ of G-ALYP at Farnborough and found fatigue crack growth from a rivet hole at the low-drag fibreglass forward "window" around the Automatic Direction Finder, had caused a catastrophic breakup of the aircraft in high altitude flight.
The rivet problem was exacerbated by the punch rivet construction technique employed. The 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 may cause the start of fatigue cracks around the rivet. Hall, the principal investigator, concluded, "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.
The Cohen Court closed on 24 November 1954. Although the court found that the basic design of the Comet was sound, nonetheless, de Havilland began a refit programme that involved strengthening the fuselage and wing structure, employing thicker gauge skin and replacing all square windows and panels with rounded ones.
As is often the case in aeronautical engineering, other aircraft manufacturers learned from, and profited by, de Havilland’s hard-learned lessons. Although the Comet had been subjected to the most rigorous testing of any contemporary airliner, the "dynamic stresses" of pressurisation were not well known, and the Comet had pushed ‘the state-of-the-art’ beyond its limits. According to John Cunningham, representatives from American manufacturers such as Boeing and Douglas (privately) "admitted that if it hadn’t been for our [de Havilland’s] problems, it would have happened to one of them.
Resumption of service
The Comet did not resume commercial airline service until 1958.[78] Following the structural problems of the early series, all remaining Comets were withdrawn from service, with de Havilland launching a major effort to build a new version that would be both larger and stronger. The square windows of the Comet 1 were replaced by the oval versions used on the Comet 2, which first flew in 1953, and the skin sheeting was thickened slightly. The remaining Comet 1s and 1As were either scrapped or modified with oval window rip-stop doublers (a thick, structurally strong ring of material that prevents a crack from spreading further), but a program to produce a Comet 2 with more powerful Avons was delayed. All production Comet 2s were modified to alleviate the fatigue problems and most of these served with the RAF as the Comet C2. Development flying and route proving with the Comet 3 allowed BOAC to accelerate the certification of what was destined to be the most successful variant of the type. On 24 September 1958, the Comet 4 received a Certificate of Air Worthiness and, the next day, BOAC took delivery of its first two Comet 4s.
The Comet 4 enabled BOAC to inaugurate the first regular jet-powered transatlantic services to begin that same year, albeit the westward North Atlantic crossing still required a fuel stop at Gander International Airport, Newfoundland. BOAC got publicity by being the first across the Atlantic with the London to New York crossing on 4 October 1958, but by the end of the month Pan Am was flying the Boeing 707 along the same route and in 1959-60 the Douglas DC-8 would be ready. The American jets were larger, faster, longer-ranged, and more cost-effective to operate than the Comet. In analysing the route structure the Comet could fly effectively, BOAC reluctantly cast about for a successor and, by 1958, had entered into an agreement with Boeing to purchase the 707.
In 1959 BOAC began shifting its Comet operations from the Atlantic run to other routes and releasing the Comet to the associate companies, the moves resulting in the Comet 4′s ascendancy as a premier airliner being relatively brief. Besides the 707/DC-8 duo, the imminent introduction of the Vickers VC10 meant the Comet’s competitors assumed more of the role initially pioneered by the Comet, that of high-speed, long-range passenger service.[82] Orders from other air carriers were gradually falling off in the 1960s with a total of 76 of the Comet 4 family delivered from 1958 to 1964. BOAC retired its Comet 4s from revenue service in 1965, but other operators continued flying Comets in commercial passenger service until 1981. Dan-Air played a significant role in the fleet’s later history and, at one time, owned all 48 remaining airworthy civil Comets. On 14 March 1997, a Comet 4C (XS235) which had been acquired by the British Ministry of Technology and used for radio, radar and avionics trials, made the last documented Comet flight.
Variants
Comet 1
DH106 Comet 1 preserved in the colours of BOAC G-APAS, this aircraft also served with the RAF as XM823, now at RAF Cosford
The square-windowed 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 5,050 lbf (22.5 kN) Ghost 50 Mk 1 (later 5,700 lbf (25 kN) Ghost DGT3 series) engines were fitted. The span was 115 ft (35.05 m), length 93 ft (28.35 m), Maximum Takeoff Weight 105,000 lb (47.628 kg) with 36–48 passenger configurations.
An updated Comet 1A was offered with higher allowed weight and water-methanol injection; 10 were produced. In the wake of the 1954 disasters, all Comet 1s and 1As were brought back to Hatfield, first placed in a protective "cocoon" and retained for testing.[85] All were substantially damaged in stress testing or were scrapped entirely.
Comet 1X: Two RCAF Comet 1As were rebuilt with heavier-gauge skins to a Comet 2 standard for the fuselage, and renamed Comet 1X.
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.[86]
DH 111 Comet Bomber
Originally proposed in 1948 as the "PR Comet", a "high-level photo reconnaissance" adaptation of the Comet 1, de Havilland further developed a bomber variant to Air Ministry specification B35/46 as the DH 111 Comet Bomber with a submission to the Air Ministry on 27 May 1948. The substantially altered airframe powered by four 5,700 lbf (25 kN) Ghost DGT3 engines, was designed around the special bomb, and featured a narrowed fuselage along with a bulbous nose to accommodate the H2S Mk IX radar; the crew of four would be housed in a pressurised cockpit under a large bubble canopy. Additional fuel tanks carrying 2,400 imperial gallons (11,000 L) were built into the fuselage to attain a range of 3,350 miles (5,390 km). The DH 111 proposal was evaluated by the Royal Aircraft Establishment but serious concerns regarding weapons storage led to a negative RAE review. The capability of the proposed V bomber trio also made the DH 111 redundant and further development work at de Havilland was abandoned on 22 October 1948.
Comet 2
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; some of these changes had been made to make the aircraft more suitable for transatlantic operations.[88] 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.[N 21]
[89] A total of 12 of the 44-seat Comet 2s were ordered by BOAC for the South Atlantic route.[90] 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 with deliveries beginning in 1955. Modifications to the interiors allowed the Comet 2s to be used in a number of different roles. For VIP transport, the seating and accommodations were altered while provisions for carrying medical equipment including iron lungs was incorporated. Specialised ELINT and electronic survelillance capability was later added to some airframes.
Comet 2X: Limited to a single Comet Mk 1 powered by four Rolls-Royce Avon 502 turbojet engines and used as a development aircraft for the Comet 2.
Comet 2E: Two Comet 2 airliners were fitted with Avon 504s in the inner nacelles and Avon 524s in the outer ones. These aircraft were used by BOAC for proving flights during 1957–1958.
Comet T2: The first two of 10 Comet 2s for the RAF were fitted out as crew trainers, with the first aircraft (XK669) flying for the first time on 9 December 1955.
Comet C2: Eight Comet 2s originally destined for the civil market were completed for the RAF and assigned to No. 216 Squadron.
Comet 2R: Three Comet 2s were modified for use in radar and electronic systems development, initially assigned to No. 90 Group (later Signals Command) for the RAF. In service with No. 192 and No. 51 Squadrons, the 2R series were equipped to monitor Warsaw Pact signal traffic and operated in this role from 1958.
Comet 3
The Comet 3, which flew for the first time on 19 July 1954, was a lengthened Comet 2 powered by Avon M502 engines developing 10,000 lbf (44 kN) with greater capacity and range, including the addition of wing pinion tanks. 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.[95] Only two Comet 3s were constructed with G-ANLO, the only "flying" Comet 3, 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. Nine additional Comet 3 airframes were not completed and their construction was abandoned at Hatfield.[96] In BOAC colours, G-ANLO was flown by John Cunningham in a marathon round-the-world promotional tour in December 1955. As a flying testbed, it was later modified with Avon RA29 engines fitted, as well as replacing the original long-span wings with reduced span wings as the Comet 3B and demonstrated in British European Airways (BEA) livery at the Farnborough Airshow in September 1958. Assigned in 1961 to the Blind Landing Experimental Unit (BLEU) at RAE Bedford, the final testbed role played by G–ANLO was in "autoland" experiments. When retired in 1973, the airframe was used for foam arrester trials before the fuselage was salvaged at BAE Woodford, to serve as the mock-up for the Nimrod.
Comet 4
The Comet 4 was a further improvement on the stretched Comet 3 with even greater fuel capacity. The design had progressed significantly from the original Comet 1, growing by 18 ft 6 in (5.64 m) and typically seating 74 to 81 passengers compared to the Comet 1′s 36 to 44. 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.
BOAC ordered 19 Comet 4s in March 1955, with G-APDA first flying on 27 April 1958. Deliveries to the airline began on 30 September 1958 with two 48-seat aircraft.[99] BOAC’s G-APDC initiated transatlantic Comet 4 service and the first scheduled transatlantic passenger jet service in history, flying from London to New York with a stopover at Gander, Newfoundland on 4 October 1958. Rival Pan Am’s inaugural Boeing 707 service began later that month.
BEA’s Comets received a welcome response from crews and passengers but they were not so well liked by the baggage handlers. The baggage/cargo holds had doors directly underneath the aircraft, so each item of baggage or cargo had to be loaded upwards from the top of the cab of the baggage truck, through the little hole, then slid along the hold floor to be stacked inside. Likewise, the individual pieces of luggage and cargo had to be retrieved slowly with great effort on arrival.
American operator Capital Airlines ordered 14 Comet 4s in July 1956.[103] The Comet 4A was designed for short-range operations and had a stretched fuselage with short wings (lacking the pinion (outboard wing) fuel tanks of the Comet 4). This order was cancelled but the aircraft were built for BEA as the Comet 4B, with a further fuselage stretch of 38 in (97 cm) and seating for 99 passengers. The first Comet 4B (G-APMA) flew on 27 June 1959 and BEA aircraft G-APMB began Tel Aviv to London-Heathrow service on 1 April 1960.
The last Comet 4 variant was the Comet 4C with the same longer fuselage as the Comet 4B coupled with the longer wings and extra fuel tanks of the original Comet 4, which gave it a longer range than the 4B. The first Comet 4C flew on 31 October 1959 and Mexicana began scheduled Comet 4C flights in 1960. The last two Comet 4C fuselages were used to build prototypes of the Hawker Siddeley Nimrod maritime patrol aircraft. Comet 4C (SA-R-7) was ordered by Saudi Arabian Airlines with eventual disposition to the Saudi Royal Flight for the exclusive use of King Saud bin Abdul Aziz. Extensively modified at the factory, the aircraft included a VIP front cabin, a bed, special toilets with gold fittings and was distinguished by a resplendent green, gold and white colour scheme with polished wings and lower fuselage that was commissioned from aviation artist John Stroud. Following its first flight, the special order Comet 4C was described as "the world’s first executive jet.
Comet 5 design
The Comet 5 was proposed as an improvement over previous models, including a wider fuselage with five-abreast seating, a wing with greater sweep and podded Rolls-Royce Conway engines. Without support from the Ministry of Transport, the proposal languished as a "paper project" only.
Hawker Siddeley Nimrod
Main article: Hawker Siddeley Nimrod.
The last two Comet 4 fuselages produced were modified as prototypes to meet a British requirement for a maritime patrol aircraft for the Royal Air Force, designated Type HS 801. This variant became the Hawker Siddeley Nimrod and was 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.
Capital Airlines ordered the Comet 4 in July 1956 which were to be supplemented by 10 Comet 4As, a variant modified for Capital. Following financial problems and the takeover by United Airlines, the order was cancelled.