Past Talks : OCTOBER 2024

“The de Havilland Comet airliner – the 75th Anniversary of her maiden flight”, by Alistair Hodgson

The origins of the Comet date back to August 1942, when our then Prime Minister, Winston Churchill, travelled to Moscow for an Allied conference with Russian Foreign Minister, Vyachevslav Molotov and Averell Harriman, the US Ambassador to Russia. Looking at the map, it should have been a straightforward journey from London to Moscow. Instead the war necessitated a journey via Gibraltar, Cairo, Teheran, then Moscow. To cover the long distances necessitated a long range aircraft, a B24 Commando, which was a converted bomber. Churchill was aged 65 and he felt the intense discomfort and noise of the flight. On his return he was determined that  when the war was over, commercial passenger aircraft should not be makeshift, converted bombers. As a result, the Brabazon Committee was formed to determine the requirement, and for British industry to construct them, rather than buy American passenger aircraft.

Already four interim conversions were planned and ready for construction: The Avro York, based on Lancaster components; the Handley Page Hermes, based on the Halifax; Avro Tudor, based on the Lincoln and the Saunders Roe Sandringham, based on the Sunderland. Each entered service, but were no match for the US built Constellation, the Stratocruiser and the DC4, 6 & 7.

It was an era when air travel was regarded as a high cost luxury, resulting in the Brabazon Committee recommending the: 230ft wingspan, Bristol Brabazon (with just 100 seats); Airspeed Ambassador; Bristol Britannia; DH Comet and DH Dove. Each aircraft is a story in itself, however, on the 8 December 1944 the Top Secret DH106, or Brabazon 4, as it was also known found its place on the drawing board.  Early iterations included, in 1944, a chubby Venom lookalike, with 3 DH Ghost jet engines, then in 1945 a canard design, followed in 1946 by a design resembling the tailplane-less DH Swallow. Ultimately in 1947 the Comet emerged as we know it now, except that it had a 45º wing sweep. This was refined to a 20º swept wing to give a low wing loading and an excellent low speed performance to enable it to mix with current propeller driven aircraft. That way,  airfield operators would not have to spend a fortune lengthening runways – which they did anyway in anticipation of demand. It was a long distance aircraft, with plans for journeys to Singapore and beyond.

The original Comet design was 90ft long, with a cockpit crew of x2 Pilots, Flight Engineer and Radio operator. The cabin was pressurised and carried just 28 passengers, each with a generous 45” seat pitch. Much of the remaining space was dedicated to capacious, separate, well-fitted Gents & Ladies toilets.

The engines were buried in the wing roots to reduce asymmetry if an engine failed and the flight controls were fully powered.

On the 27 July 1949 the press were given a preview day, when they swarmed over the aircraft. After most of the press had gone home, it was still a lovely calm day so, almost as a second thought, it was decided to conduct the very first test flight, witnessed by just a few late, straggling press! It is interesting to note that the undercarriage of first aircraft used one enormous wheel under each wing. As another first for the Comet, each main wheel was soon replaced by the four-wheel bogies we now see on all large aircraft.

Everything went well and on the 2 May 1952, the Comet, in BOAC colours, flew from London to Johannesburg making it the world’s first pure-jet, fare paying passenger flight. The Comet was a great success and orders rolled in necessitating plans for a second factory, based in Northern Ireland, to cope with demand.

But …….. in October 1952, a Comet departing from Rome failed to get airborne and ran off the end of the runway, writing-off the aircraft but otherwise no serious injuries. This was followed in March 1953 by a Canadian Pacific Comet 1A on its delivery flight to Australia, which again failed to take-off at Karachi Airport, killing all 11 people on board. Three months later in May 1953, six minutes after take off from Calcutta Airport in a thunderstorm, a BOAC Comet 1 crashed, killing all 43 people on board. Initially each event was attributed to pilot error, but at that time the flight controls had no ‘feedback’ to intimate an impending stall. It soon became apparent that each aircraft crashed due to entering a deep stall as a result of the nose being pulled back too high.

Improvements were made to the wings, but in January 1954 a BOAC Comet departed from Rome Airport and broke up over the Mediterranean crashing into the sea near the island of Elba, killing all 35 people on board. Shortly after, in April 1954 another Comet departed Rome en-route to Cairo when it too crashed into the sea, near Naples.

The Comet was promptly grounded and a massive exercise took place to recover as much wreckage as possible for proper scientific investigation at Farnborough. Thorough fatigue testing was undertaken, simulating 3,060 flights, inside a purpose-built water tank, which under pressure does not explode, however the fuselage itself burst, revealing structural failure.

The findings revealed fatigue, but not just in the square windows, as we have all been led to believe. Much more was involved. During its original proof-testing, de Havilland tested the fuselage at pressures well above the regulation levels and for a significantly higher number of test cycles. De Havilland was very optimistic and pleased with the results. 16,000 cycles of testing were carried out in the first phase of testing and then a further series of test cycles were undertaken for a second phase. All of these tests were carried out on the same section of fuselage, which was acceptable practice at that time. However, as a result of the Farnborough investigation it became apparent that such a high number of cycles resulted in ‘cold working’ which affected the crystals in the metal. This was a new discovery from which industry benefitted worldwide. With this new knowledge, de Havilland learned that it should have used a different fuselage for both phases of testing.  

The skin thickness of the fuselage was about the same thickness as a credit card. The square shape of the Comet windows made air flow around it, whereas a round window allows the air to flow more easily. Additionally, the Comet windows should have been bonded and riveted, but during manufacture in the cold winter of 1949, the bonding would not set, so the windows were instead just riveted. Not only that, but when de Havilland carried out its own testing it did not even fit windows at all to the test fuselage. The window frames were instead tested separately. The intensive Farnborough testing revealed that not just the windows came under stress, but all openings in the fuselage were affected and ultimately, the aperture for the ADF aerial (Automatic Direction Finder) was where the fatigue developed and grew.

With hindsight, the reality was that this was new territory and no one actually knew how the fuselage would behave.  

As a result of the Farnborough investigation, de Havilland extensively redesigned the Comet, leading to the Comet 3, using stronger, reinforced structures, including a greater skin thickness, which had also been shown to be a weakness. However, it was too late, commercial sales for the aircraft died. Consequently the RAF took over newly constructed aircraft cancelled by airlines. At this point, Alistair spoke about covert operations conducted by 51 Squadron, RAF, whose business is intelligence gathering, but details of what their Comets indulged in are still very obscure.

In the interim, test pilot John Cunningham undertook a sales tour in the Comet by circumnavigating the globe, becoming the first person to do so by jet and winning the Harmon Trophy in the process (awarded for outstanding achievements).

All of these modifications resulted in the Comet 4 series with more powerful Avon engines replacing the earlier Ghost jet engines and the ability to carry many more passengers and to carry them greater distances, due to a much enhanced fuel capacity. The Comet was also able to operate from smaller airfields than its main competitors, due to its carefully considered wing design. Sales of the Comet 4 revived the fortunes of the aircraft and it continued to operate until the early 1980s, with Dan Air being a notable operator until its retirement from commercial service.

Later, the Nimrod maritime aircraft was developed for the RAF from the Comet and it was not only very successful in its long service, but by flying on just two of its four engines, it could loiter on station for hours on end. Nimrod was a very capable aircraft much missed when retired – prematurely?

Finally, Alistair presented his conclusions to the Comet story:

– It was a pioneering aircraft

– It was probably the last aircraft designed to meet the ‘British Empire Service’

– The early failures resulting in catastrophic losses were in fact due to lack of knowledge, it was far ahead of its time.

– De Havilland very successfully understood and overcame the structural problems

Thank you Alistair for a most interesting and enlightening evening to a full house in the auditorium and 30+ ZOOM attendees.