de Havilland Comet

DH.106 Comet

Comet 3 G-ANLO in BOAC-style markings at Farnborough in September 1954
Role	Airliner
Manufacturer	de Havilland
First flight	27 July 1949
Introduced	2 May 1952 with BOAC
Status	In military service only
Primary users	BOAC See Operators
Number built	114 (including prototypes)^[1]^[2]
Unit cost	£250,000 in 1952
Variants	Hawker Siddeley Nimrod

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The Comet was withdrawn temporarily and redesigned.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.

Development

During the Second World War, the Brabazon Committee studied Britain's postwar airliner needs. Sir Geoffrey de Havilland, head of the de Havilland company, was a committee member and used his influence and the company's expertise with jets to include mention of the need for a transatlantic jet mailplane. The Committee accepted the proposal, calling it the "Type IV" (of five designs), and awarded the production contract to de Havilland's DH.106. British Overseas Airways Corporation (BOAC) found the Type IV's specifications attractive and in December 1945 agreed to buy ten aircraft.

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.

Design

The Comet is an all-metal low-wing cantilever monoplane powered by four jet engines, approximately the length of a Boeing 737 but carrying fewer people in greater comfort. 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 (each with its own ashtray). The galley could serve hot and cold food and drinks and there was a bar. Other amenities included separate men's and women's washrooms. The passenger cabin was quieter than those of propeller-driven airliners. The Comet's four-man cockpit held two pilots, a flight engineer, and a navigator.

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.

Olympic Airways Comet 4B at Manchester in 1966, showing engines buried in wing and revised round windows of all Comet 4s

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 ft (10,973 m). One window frame survived a massive 100 psi, about 1,250% over the maximum pressure it would encounter in service.

Operational history

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 (9,144 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.

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. The following March, a new Canadian Pacific Airlines Comet 1A (CF-CUN), 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-ever fatal crash of a jet airliner. Both of these accidents were originally attributed to pilot error: over-rotation had led to a loss of lift from the leading edge of the plane's wing. However, it was later determined that 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, and a wing fence was added to control spanwise flow. A fictionalised investigation into these take-off accidents is a subject of the 1959 novel Cone of Silence by 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 take-off 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. 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".

Comet disasters of 1954

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. There was no obvious reason for the crash, and the fleet was grounded while the Abell Committee met to determine potential causes of the crash. The committee focused on six potential problems: control flutter (which had led to the loss of the de Havilland Swallow), 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.

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 ⅔ 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".

Loss of competitive advantage

Following the structural problems, 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. This one, the Comet 4, enabled De Havilland to return to the skies in 1958. By then, the United States had its Boeing 707 jetliner along with the Douglas DC-8, both of which were faster and less costly to operate. Orders for the Comet dried up, with the last one delivered in 1964.

De Havilland returned to the commercial airliner market in 1962 with a three-engine jetliner, the Trident. However, it was designed it to fit the needs of one airline and one individual: Lieutenant Colonel 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.

De Havilland also entered 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 spaceflight.

Variants

Comet 1

The square-windowed Comet 1 was the first model produced. An updated Comet 1A was offered and in the wake of the 1954 disasters, some of these were modified as Comet 1XBs with strengthened fuselages and oval windows. A bomber variant was proposed in May 1948 to Air Ministry specification B35/46 as the DH.111, but this was never developed further.

Comet 2

Comet C.2 XK715 of No. 216 Squadron Royal Air Force at Filton Bristol in 1964

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 openings. 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 the South Atlantic routes, their range was still not suitable for the North Atlantic. All but four Comet 2s were allocated to the RAF. Eight Comet C2 transport aircraft and two Comet T2 crew trainers were delivered to the RAF beginning in 1955.

Comet 3

The Comet 3 was a lengthened Comet 2 with greater capacity and range, which flew for the first time on 19 July 1954. It was demonstrated at the Farnborough SBAC Show in September. After the fatigue accidents, orders dwindled and only two Comet 3s were constructed. G-ANLO was the only flying Comet 3, and took part in a marathon round-the-world promotional tour in December 1955, flown by John Cunningham. It was modified with reduced span wings as the Comet 3B and was displayed at Farnborough in September 1958. The other Comet 3 was used for structural and technology testing during development of the similarly sized Comet 4. Nine further airframes were not completed and their construction was abandoned at Hatfield.

Comet 4

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 18 ft 6 in (5.64 m) 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 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 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, none were ever built. The MoT subsequently backed BOAC's order of Conway-powered Boeing 707s.

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 HS.801. This became the Hawker Siddeley Nimrod and was built at the Hawker Siddeley factory at Woodford Aerodrome. Entering service in 1969, five Nimrod variants have been produced, with two still in service, while the highly modified Nimrod MRA 4 is now due to enter service in 2010.

Production and service summary

The Comet was built at two different de Havilland factories: Hatfield and Hawarden. A total of 114 aircraft were completed and flown: 12 Comet 1s, 10 Comet 1As, 15 Comet 2s, one Comet 3, 74 Comet 4, and two HS.801s.

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.

Development

Design

Operational history

Early accidents and incidents

Comet disasters of 1954

Loss of competitive advantage

Variants

Comet 1

Comet 2

Comet 3

Comet 4

Comet 5 design

Hawker Siddeley Nimrod

Production and service summary

Aircraft on display

Operators

Civilian operators

Military operators

Cancelled orders

Specifications (Comet 4)

See also

Bibliography

External links