The de Havilland Comet was the world's first commercial jet airliner to reach production. Developed and manufactured by de Havilland, it first flew in 1949 and was considered a landmark British aeronautical design. After a successful introduction into commercial service, early Comet models suffered from catastrophic metal fatigue, causing a string of well-publicised accidents.
The Comet was withdrawn temporarily and redesigned. The Comet 4 series subsequently enjoyed a long and productive career of over 30 years, although sales never fully recovered. The Hawker Siddeley Nimrod, the military derivative of the Comet airliner, is still in service. The original decades-old airframes are being rebuilt with new wings and engines to produce the Nimrod MRA 4, expected to serve with Britain's Royal Air Force until the 2020s, almost 70 years after the Comet's first flight.
Design work began in 1946 under Ronald Bishop, who had been responsible for the Mosquito fighter-bomber. Several configurations were considered, including twin booms and a swept-wing, tailless design, but a more conventional design was eventually chosen and announced as the Comet in December 1947. First deliveries were expected by 1952.
The first flight of a prototype DH.106 Comet took place on 27 July 1949, 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." The aircraft 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.
The clean, low-drag design featured many unique or innovative design elements, including a swept leading edge, integral wing fuel tanks and four-wheel bogie main undercarriage units designed by de Havilland. The Comet was also the first pressurised jet-propelled commercial aircraft. For emergencies, life rafts were stored in the wings near the engines and a life vest was stowed under each seat bottom.
Two pairs of de Havilland Ghost 50 Mk1 turbojet engines were buried in the wings close to the fuselage. British designers 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, a major problem for turbine engines. However, 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 Comet was originally intended to have two hydrogen peroxide powered de Havilland Sprite booster rockets for takeoff under hot and high conditions from airports such as Khartoum and Nairobi. These were tested on 30 flights, but the Ghosts were apparently powerful enough without them. The later Comet 4 was highly rated for its takeoff performance from high altitude locations such as Mexico City. Its newer AJ.65 Avon engines, low weight (compared to the Boeing 707 and Douglas DC-8), and exceptionally clean design all contributed to its high performance. Early-model Comets required about five or six man-hours of maintenance labour per flight hour, fewer than the propeller-driven planes it replaced.
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. For example, a water tank was used to test the entire forward fuselage section for metal fatigue by repeatedly pressurising to 2.75 psi overpressure (11 psi) 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, 4.75 psi above the normal service ceiling of 36,000' (10973 m). One window frame survived a massive 100 psi, about 1,250% over the maximum pressure it would encounter in service.
The first production aircraft (G-ALYP) flew in January 1951. On 22 January 1952, G-ALYS was the first Comet to receive a certificate of airworthiness. On 2 May, G-ALYP took off on the world's first all-jet flight with fare-paying passengers, beginning scheduled service to Johannesburg. The last plane from the initial order (G-ALYZ) began flying in September 1952, carrying freight along South American routes while simulating passenger schedules.
The Comet was a hit with passengers, and commercial success was widely anticipated. Queen Elizabeth The Queen Mother was an early passenger on a special flight, becoming the first member of the British Royal Family to fly by jet. The Comet flew about 50% faster than advanced piston-engined types like 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. The Ghost engine was smooth, relatively simple, fuel-efficient above 30,000 ft (9144 m), had low maintenance costs, little vibration, and enabled operations above weather the competition had to fly through. 30,000 passengers were carried during the first year of service and over 50 Comets were ordered.
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. The break-up began with a stabiliser and may have been exacerbated by over-manipulation of the fully powered flight controls. The Comet 1 and 1A have been criticised for a lack of "feel" in their controls. However test pilot John Cunningham contended that "it flew extremely smoothly and responded to the controls in the best way De Havilland aircraft usually did".
During this investigation, the Royal Navy conducted recovery operations, including the first use of underwater television cameras. 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 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.
Then 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). Winston Churchill tasked the Royal Navy with helping to locate and retrieve the wreckage so that the cause of the accident could be found.
Engineers subjected an identical airframe, G-ALYU ("Yoke Uncle"), to repeated re-pressurisation and over-pressurisation and after 3,057 flight cycles (1,221 actual and 1,836 simulated), Yoke Uncle failed due to metal fatigue near the front port-side escape hatch. Investigators began considering fatigue as the most likely cause of both accidents and initiated further research into measurable strain on the skin. Stress around the window corners was found to be much higher than expected, "probably over 40,000 psi," and stresses on the skin were generally more than previously expected or tested. This was due to stress concentration, a consequence of the window's square shape.
The 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.
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.
Before the Elba accident, G-ALYP had made 1,290 pressurised flights and at the time of the Naples accident, and G-ALYY had made 900 pressurised flights. Walker 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 1000 to 9000 cycles. By then, the RAE had reconstructed about two-thirds of G-ALYP at Farnborough and found fatigue crack growth from a rivet hole at the low-drag fiberglass forward "window" around the Automatic Direction Finder, which had caused a catastrophic breakup of the aircraft in high altitude flight.
The square windows of the Comet 1 were redesigned as oval for the Comet 2, which first flew in 1953. The skin sheeting was thickened slightly. The remaining Comet 1s and 1As were either scrapped or modified with oval window rip-stop doublers and 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. The Comet did not resume commercial airline service until 1958, when the much-improved Comet 4 was introduced and became the first jet airliner to enter transatlantic service. The Comet nose section was also used on the Sud Aviation Caravelle. As is often the case in aeronautical engineering, other aircraft manufacturers learned from and profited by de Havilland's hard-learned lessons. According to John Cunningham, representatives from American manufacturers such as Boeing and Douglas "admitted that if it hadn't been for our problems, it would have happened to one of them".
De Havilland also pushed into the new field of long-range missiles, developing the liquid-fueled Blue Streak. It did not enter military service but became the first stage of Europa, a launch vehicle for use in space flight. In flight tests, the Blue Streak performed well—but the upper stages, built in France and Germany, repeatedly failed. In 1973, the Europa program was canceled, with Blue Streak dying as well. The last of them wound up in the hands of a farmer who used its commodious fuel tanks to house his chickens.
De Havilland returned to the airline world in 1962 with a three-engine jetliner, the Trident. However, he designed it to fit the needs of one airline and one man: Lt-Col W. Sholto Douglas later Lord Douglas, chairman of British European Airways. Other airlines found it unattractive and turned to a rival tri-jet: the Boeing 727. De Havilland built only 117 Tridents, while Boeing went on to sell over 1,800 727s.
The Comet 4 was a further improvement on the stretched Comet 3 with even greater fuel capacity. This design had come a long way from the original Comet 1. The aircraft had grown by 5.64 m (18 ft 6 in) and typically seated 74 to 81 passengers compared to the Comet 1's 36 to 44. It had a longer range, higher cruising speed, and higher maximum takeoff weight. These improvements were possible largely because of Avons with twice the thrust of the Comet 1's Ghosts.
BOAC ordered 19 Comet 4s in March 1955 and a Comet 4 (G-APDA) first flew on 27 April 1958. Deliveries to BOAC began on 30 September 1958 with two aircraft. BOAC's G-APDC initiated the first 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 on 4 October 1958. Rival Pan Am's inaugural 707 service began three weeks later.
American operator Capital Airlines ordered four Comet 4s and 4As in July 1956. 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 British European Airways (BEA) as the Comet 4B, with a further fuselage stretch of 38 inches (96.5 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.
Thirteen aircraft were lost in fatal accidents and of these, five were considered to have been brought about by aircraft design or fatigue problems. The last fatal accident involving the Comet was at Tripoli, Libya on 2 January 1971, caused by pilot error.
A total of 76 Comet 4 family aircraft were 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 49 remaining airworthy civil Comets. In 1997, a Comet 4C which had been owned by the British government made the last documented Comet flight.
Although the Comet was the first jet airliner in scheduled passenger service, the damage done to the aircraft's reputation by the Comet 1 disasters contributed to Boeing's domination of the jetliner market. The first prototype 707 was flown in 1954 and Douglas launched the DC-8 program in 1955.
Twenty-four airlines flew the Comet and it remained in passenger service for almost three decades, until 1981. Designed over 50 years earlier at the beginning of the jet age, a variant of the Comet, the Nimrod, flying with modern avionics, is still in service with the Royal Air Force.