Comet

The Photo
One often reproduced photograph of Chester shows a Pickford Scammell towing a DeHaviland Comet around the corner of Eastgate Street and Upper Bridge Street at the High Cross. It is often written or said that this was a frequent sight in the City. In fact, this was an almost unique event which only occurred on two closely timed occasions as a result of desparate circumstances following a series of air crashes.

The Comet
The de Havilland DH.106 Comet was the world's first commercial jet airliner. Developed and manufactured by de Havilland at its Hatfield Aerodrome in Hertfordshire, United Kingdom, the Comet 1 prototype first flew in 1949. It featured an aerodynamically clean design with four de Havilland Ghost turbojet engines buried in the wing roots, a pressurised cabin, and large square windows. For the era, it offered a relatively quiet, comfortable passenger cabin and was commercially promising at its debut in 1952.

Design work on the Coment was kicked off at a meeting of the March 1943 Brabazon Committee, which was tasked with determining the UK's airliner needs after the conclusion of the Second World War. It was only that March that the Gloster Meteor, the first Allied jet fighter, made its first flight, but it had already become clear that the war was likely to be won. One of its recommendations was for the development and production of a pressurised, transatlantic mailplane that could carry non-stop, one ton of payload at a cruising speed of 400 mph (640 km/h). A development and production contract to de Havilland under the designation Type 106 was awarded in February 1945. The type and design were to be so advanced that De Havilland had to undertake the design and development of both the airframe and the engines. This was because in 1945 no turbojet engine manufacturer in the world was drawing up a design specification for an engine with the thrust and specific fuel consumption, that could power an aircraft at the proposed cruising altitude (40 thousand feet), speed, and transatlantic range as was called for.

As the Comet represented a new category of passenger aircraft, more rigorous testing was a development priority.From 1947 to 1948, de Havilland conducted an extensive research and development phase, including the use of several stress test rigs at Hatfield Aerodrome for small components and large assemblies alike. Sections of pressurised fuselage were subjected to high-altitude flight conditions via a large decompression chamber on-site, and tested to failure. However, tracing fuselage failure points proved difficult with this method, and de Havilland ultimately switched to conducting structural tests with a water tank that could be safely configured to increase pressures gradually. 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.

Within a year of entering airline service, problems started to emerge, with three Comets lost within twelve months in highly publicised accidents, after suffering catastrophic in-flight break-ups. Two of these were found to be caused by structural failure resulting from metal fatigue in the airframe, a phenomenon not fully understood at the time; the other was due to overstressing of the airframe during flight through severe weather. The Comet was withdrawn from service and extensively tested. Design and construction flaws, including improper riveting and dangerous concentrations of stress around some of the square windows, were ultimately identified. As a result, the Comet was extensively redesigned, with oval windows, structural reinforcements and other changes.

The Crashes
Overall, the Comet was involved in 26 hull-loss accidents, including 13 fatal crashes which resulted in 426 fatalities. Three early accidents are the best known, these were BOAC Flight 783 on 2 May 1953, BOAC Flight 781 on 10 January 1954, and South African Airways Flight 201 on 8 April 1954, led to the grounding of the entire Comet fleet. After design modifications were implemented, Comet services resumed in 1958.

On 2 May 1953, BOAC Flight 783, de Havilland Comet G-ALYV, broke up mid-air and crashed after encountering a severe squall, shortly after taking off from Calcutta (now Kolkata), India. All 43 passengers and crew on board were killed. The inquiry concluded that the aircraft had encountered extreme negative G forces during takeoff; severe turbulence generated by adverse weather was determined to have induced down-loading, leading to the loss of the wings. Examination of the cockpit controls suggested that the pilot may have inadvertently over-stressed the aircraft when pulling out of a steep dive by over-manipulation of the fully powered flight controls.

On 10 January 1954, 20 minutes after taking off from Ciampino (Rome), 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. Media attention centred on potential sabotage, but it was concluded that the likely problem was fire, possibly caused by turbine blades being shed. No apparent fault in the aircraft was found, and the British government decided against opening a further public inquiry into the accident. The prestigious nature of the Comet project, particularly for the British aerospace industry, and the financial impact of the aircraft's grounding on BOAC's operations, both served to pressure the inquiry to end without further investigation. 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 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. The Comet fleet was immediately grounded once again and a large investigation board was formed. The inquiry closed on 24 November 1954, having "found that the basic design of the Comet was sound", and made no observations or recommendations regarding the shape of the windows. De Havilland nonetheless began a refit programme to strengthen the fuselage and wing structure, employing thicker gauge skin and replacing the square windows and panels with rounded versions.

The Cause
The image of the Comet in Chester is of one of the two comets made by Short and Harland in Belfast and taken to Boughton in October/November 1955 for special testing after the three crashes of comets a few years earlier.

The planes were shipped to Preston and because of low bridges had to travel by a long route through Manchester, Stockport and Chester to get to Boughton.

While one result of the crashes was the adoption of square windows, the problems which led to the actual accidents were associated with a small aperture in the cabin roof which housed a direction finder beneath a fibre-glass panel, this had rounded corners but had been "punch riveted" rather than glued as in the specifications. Had this short-cut not been taken, the accidents might well not have happened. Even drilling the holes might have saved the planes, as the uneven pressure of the punch process led to the "starter" cracks for the fatigue fragmentation. The accident investigation also found that square windows had the potential to cause problems, but the cracks which caused the actual crashes started with a round hole.