COMET Chapter Six Measuring Disaster

 

I invite you to witness the midnight blue and white de Havilland Comet settle onto the runway at Hatfield, Hertfordshire, England (above). It was Saturday, 10 April, 1954. The prodigal child had returned to the place of its birth.  

Across the tarmac, in the giant factory built to assemble the world's first jet powered passenger aircraft, the production lines were crowded with aluminum frames of the second generation, the Comet 2, being fitted with more powerful Rolls-Royce engines and a redesigned wing. 

But without paying homage to its offspring Comet G-ALYU – Yoke Uncle - taxied directly to the testing hanger.

The first generation was in sorry shape. After three years of service, of the nine Comet's leased to British Overseas Airways Corporation, only four were still flying. Yoke Yoke had disappeared near Stromboli just two days earlier. Yoke Victor had been lost north of Calcutta, India. Yoke Peter had exploded in mid-air near the island of Elba, Italy. Yoke Zebra had failed to get airborne off the runway at Rome (above) and was a total loss. And Yoke Uncle had been chosen to be the sacrificial lamb.

Workers who had assembled Yoke Uncle three years earlier presided over her demise. They removed the interior fittings – seats, carpeting, wall coverings – stripping Yoke Uncle to it's bare metal skin. To replace the seats and tables, heavy duty water plumbing was installed. 

And then on Friday, 7 May, 1954, Yoke Uncle took her last flight of sixty miles, from Hatfield to the Royal Aircraft Establishment (above)  at Farnborough. The fuel tanks were pumped dry. And, in a final indignation, Yoke Uncle's tail was sliced off. The engines were removed, to suffer their own autopsy, and the husk of Yoke Uncle was towed to the prepared site.

The engineers and scientists at the R.A.E. had always harbored suspicions about the de Havilland design. And the first pieces of Yoke Peter, dredged up from the bottom of the Tyrryenian Sea, and brought to Farnborough (above) had strengthened those concerns.

Six weeks ago, a month before Comet Yoke Yoke disappeared off of Naples, the engineers had begun preparing the site of Yoke Uncle's Calvary. A level concrete foundation had been poured. Atop this a series of precast concrete forms (Jersey Barriers) were placed at regular intervals. Atop these were laid and welded 4' by 4' steel plates to form a flat surface. 

Yoke Uncle was then towed until it's tricycle gear straddled the construction stand. More plates were welded together until they fully enclosed Yoke Uncle, forming a water tight tank 112 feet long, by 20 feet wide by 16 feet high.

When the tank was finished, 250,000 gallons of water was pumped into shell of Yoke Uncle and the surrounding tank until a pressure of 8.25 pounds per square inch was achieved in both. This was the “test pressure” the Comet had been designed to withstand, the difference between the outside air pressure at 40,000 feet, and the internal “pressurized” cabin, set to replicate an altitude of 8,500 feet. 

Over two and a half minutes an addition 100 gallons of water was pumped into Yoke Uncle. Then the 100 gallons was pumped out again, completing a 5 minute cycle, which would represent a 3 hour flight, such as between London and Rome. This was repeated for 999 cycles, and then repeated again and again.  Every 1,000 cycles the internal pressure inside Yoke Uncle was increased an even further 33%, to 11 pounds per square inch pushing against the 8.25 lbs of the surrounding tank. 

At the same time the wings were being constantly flexed, because that was where Farnborough was convinced the problem with the Comet's skin would reveal itself. 

They were aging the Comet 40 times faster than she was expected to age while in service.

While still flying, Yoke Uncle had experienced 1,221 of these pressurization cycles. In the tank, it now suffered a further 1,836 cycles – for a total of 3,057 – when a minor fluctuation in the pressure gauges alerted the engineers. Performing due scientific diligence, the tank was drained and the skin of the Comet inspected. A tiny crack 2 millimeters long had formed near a rivet hole attaching the forward port escape hatch to the aluminum skin of Yoke Uncle. But expecting a failure in the wings, the engineers patched the crack, refilled the tank, and resumed testing.

But as the summer progressed, the testing was repeatedly delayed as more tiny cracks appeared, radiating out from the windows. In their turn, they were all repaired. But with each new crack it became obvious the problem with the Comet was not in the wings, but in the thin aluminum composite skin of the pressure hull. And then, after 5,546 cycles there was an loud thud from within the tank, and the pressure gauges inside the shell of Yoke Uncle dropped abruptly. The pumps were turned off, and the tank drained again.

What the engineers and scientists found, startled them.  A 4.5 meter section – almost 15 feet long - of the cabin wall near number 7 window on the port side, had exploded (above). Had the cabin been pressurized with air, the effect would have been the equivalent of a 500 pound bomb going off. The water in the outer tank had suppressed the explosion, leaving the evidence intact. 

Had Yoke Uncle been still in service, the engineers calculated, this massive failure would have occurred after about 9,000 hours of flight.

Based on the metal fatigue on the Comet prototype, such a failure of the hull was not expected to occur until 16,000 cycles. And as the design life of a Comet was to be only 10,000 cycles, design engineers had seen no cause for concern. But in service the hulls were failing after only 3,000 + cycles. Why? 

 Arnold Hall (above), head of the R.A.E. noticed that on a test bed the Comet hull prototype had been initially pushed to 2.5 times the anticipated internal pressure. And it had passed. He now suggested the de Havilland engineers had thus accidentally “fatigue proofed” the prototype's hull, locking the rivets and Redux adhesive together, strengthening them as in a trial by fire. The production line Comet's had not been subjected to this extreme pressurization, and thus  “cold” hulls had failed at 3,000 + cycles.

The separate  and independent examination of the recovered wreckage of Yoke Peter told the story. The first crack had ripped apart the front of the passenger cabin, and within less than a second killed everyone on board.  The rear of the fuselage and tail broke downwards. The nose and galley section were spun off, and gravitational forces broke the the outer lengths of the wings downwards. The center fuselage with the stubs of the wings caught fire as they corkscrewed down, but this quickly burned out.  Or so went the theories. 

To prove both of these theories, Eric Lewis Ripley, of the R.A.E., built 50 1/36 scale Comet models, 5 feet long with 3 foot wingspans, but designed to come apart just as Yoke Peter Comet was assumed to have. By tracking the fall of these debris on a hanger floor, the hired Italian fishing boats on Elba returned to the crash site, searching for a particular piece of the puzzle on a particular section of the ocean floor. After just a couple of hours searching, the fishermen found it; Yoke Peter's Automatic Direction Finder windows (above).

The ADF windows were atop the aircraft (above), just forward of the wings. And they showed the “unmistakable fingerprint of fatigue..." 

 It fact, it was determined this had been the original crack, the exact spot where the pressurized cabin of Yoke Peter had first broken, by a joining of cracks from the aft lower corner of the forward escape hatch with one from the right-hand aft corner of the ADF window. Eric Ripley would later write a technical article on the issue, entitled, “Fractures talk their own language.”

https://www.youtube.com/watch?v=aHbyOqE0q3o

The evidence was now lining up. Yoke Uncle had first failed at 3,060 cycles, Yoke Peter at 1,290 cycles, but with repairs. Yoke Yoke after only 900 cycles. Yoke Victor at 3,050 cycles – all far below the 16,000 assumed by de Havilland's calculations. The cracks had all begun at the the windows because they were square. The right angles intensified the pressures at the corners to 450 pounds per square inch, far above the pressures designed to be accommodated. But it wasn't just the pressure. It was repeated stretching of the cabin, exacerbated by the short range of the Comet, requiring so many pressurizations and de-presseurizations.  

So there it was. The Comet's Achilles Heel was the square windows and the square Plexiglas ADF aperture,  antagonized by the aircraft's short range.  The only question left, was “What now?”

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COMET Chapter Five - A Stunning Blow

 

I invite you to watch as the sleek midnight blue and white aircraft designated “Yoke Yoke” (above) glides in for a landing on  runway 15/33. The four de Havilland Ghost turbojet engines cradled within the wings are throttled back to idle, as 38 year old South African Airways Captain William Mostert gently sets the 100,000 pounds of aluminum alloy, wires, rubber tubing, kerosene ambitions and 21 souls onto the asphalt. It is 4:36 pm on the cool evening of Wednesday, 7 April, 1954. As the twin four-wheeled bogies of “Yoke Yoke”s landing gear absorb the impact, the crew and passengers of Flight 201 have less than 25 hours and 30 minutes to live. (https://www.youtube.com/watch?time_continue=320&v=vkMHJmkx6E8&feature=emb_logo)

There were warning signs before the de Havilland Comet Yoke Yoke landed at Rome's Ciampino Airport,  completing the first leg of a flight ultimately headed for Johannesburg, South Africa. And although the Comet bore the colors and logo of the British Overseas Aircraft Corporation, the flight and crew were a South African Airways operation. 

The strain of the shut down after the crash of Yoke Peter, and the 60 recommended repairs, were forcing de Havilland to miss production commitments. There was no Comet 1 available for this scheduled SAA flight. And since the Comets leased to South Africa, Air Canada and Air France produced needed foreign currency, de Havilland and the government convinced BOAC to charter Yoke Yoke to SAA for the flight. It was a jury rigged, temporary fix to the Comet's problems.

But the problems ran deeper. SAA flight 201 was supposed to be on the ground in Rome for an hour -just long enough to refuel. However no matter how much fuel they put in Yoke Yoke's center fuel tank, the gauge refused to move. The flight was delayed while the engineers searched for the source of the problem. 

After draining the central tank, by elimination, it was assumed the co-axial cable running from the wing tank to the gauge was faulty. Which is when it was discovered that neither SAA nor BOAC Ciampino had a replacement cable in stock. A new cable would have to flown out from London, requiring Yoke Yoke to remain overnight in Rome.

That was mildly embarrassing, especially since de Havilland and BOAC were proudly declaring their primary advantage over piston engines was that the turbines of a jet required far less maintenance. In fact the entire industry was already moving from “maintaining” an aircraft at each stop, toward merely providing “service” - meaning only major transportation hubs would keep a collection of parts and mechanics. And the fuel indicator cable had not been considered to be essential. Oh, well. Part of the learning curve. So the passengers were put up in a hotel, and while waiting for the replacement cable it was decided to drain and inspect all the fuel tanks, just in case some thing else was the real problem.

And that was when they found, rolling around in the empty left wing tank (above) of Comet Yoke-Yoke,  22 quarter inch bolts. A panicked phone call to London eventually produced an answer to the mystery of what they were doing there. Nine days earlier, 52 bolts had been removed during a routine check of the tank. For some reason only 30 of those had been restored before resealing the tank. People were going to be fired over that mistake. But it got worse. While double checking all of the bolts, it was discovered that even the remaining 30 had not been fully tightened. For nine days the port wing tank had flown with less than half the structural support it was designed to have. The only thing which saved Yoke Yoke on all the flights during those nine days had been the redundancy de Havilland designed into the Comet. It had been a damn close call.

So, 25 hours late, on Thursday, 8 April, 1954, Comet Yoke Yoke left Ciampino airport bound for Heliopolis Airport, just east of Cairo, with a crew of 7 and 14 passengers. Five minutes after take off, at 5:37pm Central European Time, Yoke Yoke reported passing over the Ostia coastal beacon, 30 miles west southwest of Rome, at 7,000 feet. 

At 5:49pm, the Comet reported passing over the island of Ponza, 75 air miles from Rome, and climbing through 11,600 feet. Twenty-five minutes after take off, at 5:57pm, Yoke Yoke reported they were abeam with Naples, having covered 118 miles by air. They were now approaching their cruising altitude of 35,000 feet. At 6:07pm CET, Yoke Yoke contacted Heliopolis and reported their Estimated Time of Arrival in Alexandria would be 9:02pm. Some time after that broadcast, Comet Yoke Yoke simply disappeared. No one saw or heard an explosion. No one saw the debris fall from the sky. The plane simply vanished.

Such a loss was not uncommon at the time, even with a reliable workhorse like the Douglas Commercial dash 4, of which over 1,200 were built between 1943 and 1949.  On Thursday, 26 January, 1950 a U.S. military DC-4  (above) disappeared over the Yukon Territory, Canada, with 44 souls aboard. Despite a 2 month long search involving 85 Canadian and American aircraft and 7,000 personnel, no wreckage has ever been found, and no cause ever determined. 

On the morning of 23 June 1950, Northwest Airways flight 2501, with 58 souls on board, disappeared over Lake Michigan. After a six week investigation, the cause was listed as “unknown”.

On 21 July, 1951 a Canadian Pacific Airlines DC-4  (above)  carrying 27 souls,  disappeared south of Anchorage, Alaska. No wreckage has ever been found. The cause is listed as “...not determined." It was not that pilots and engineers lacked curiosity, but rather that no one had yet designed a recording device which could survive a crash violent enough to kill passengers and crew. But it was coming.

Independently in 1953 both David Ronald de Mey Warren with Defense Science and Technology Organization, in Melbourne, Australian,...

...and Professor James Jay "Crash" Ryan, at the University of Minnesota, U.S. -  who also invented the automobile retractable seat belt - ...

...designed wire recording systems which would note engine temperatures, fuel flow, aircraft velocity, altitude, control surfaces positions, and rate of descent, and could survive a crash. From the beginning it was decided to put these “black boxes”, aka Flight Data Recorders,  in the tail, since, as one designer put it, “I never saw an airplane back into a mountain.”

Ryan got a U.S. Patent in 1953, but Warren's work got quicker attention from the industry, specifically in 1958 from Sir Robert Hardingham, then Secretary of the British Air Registration Board. Hardingham was interested, in large part, because of the Comet crashes.

Then, on 4 June 1967 a British Midland DC-4 (above), with 84 people on board, crashed in a suburb of Manchester, England. The captain was one of the 12 survivors, but he had no memory of the events just preceding the crash. 

Luckily, however, the now required “black box” did remember. All DC-4's had 8 fuel tanks and a complicated plumbing systems to keep the aircraft balanced during flight. This crash – and perhaps earlier ones - had been caused because the cockpit fuel line controls were complicated and difficult to operate. In effect, the crew had chosen an almost empty fuel tank for the 2 right side engines, and during landing approach they both abruptly lost fuel and power. The aircraft flipped over and spun into the ground, 

However the box also showed the desperate pilot retained just enough control to prevent the plane from slamming into homes. No one on the ground was killed.

But on Friday, 9 April, 1954, the search planes could find nothing left from Comet Yoke Yoke except an oil slick and five bodies floating in the Mediterranean. By the time ships arrived, even these were gone. 

The next day the New York Times would report, “ “Britain today weighed the cost of a stunning blow to her proudest pioneer industry – jet civil aviation – as the crash of another Comet airliner was confirmed. Twenty-one persons, including three Americans, were believed to have died when the plane was lost in the Mediterranean...Tonight the Minister of Transport, A. T. Lennox-Boyd withdrew from all Comets the certificate of airworthiness... Sir Miles Thomas, chairman of the airline, said the new crash was 'a very great tragedy and a major setback for British civil aviation.'”:

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COMET - Chapter Four - Denial and Anger

 

I hope the grieving families of the 15 dead recovered from the ocean found some comfort in knowing that while awaiting burial their loved ones rested in peace in the dark little chapel of the fortress Chiesa di San Giacomo Maggiore, treated with reverence and honor. Except, for the sake of the living,  they were not left at peace. Each evening a handful of bodies were removed and carried to the hospital in Portoferrai on Elba's north shore.

Awaiting them there, scalpel in hand, was Antonio Fornari, the Professor of Pathology at Pisa University. It was Professor Fornari's job to learn from the dead, to discover what exactly had killed those privileged enough to have flown on the wonder of the modern world, the de Havilland Comet.

What Professor Fornari learned from the autopsies was that none of the bodies recovered from the sea had drown. None had either water nor smoke in their lungs. None had suffered shrapnel wounds. All 15 had ruptured lungs. Most also had suffered skull fractures, concurrent with their deaths. Professor Fornari was certain the combination of these two injuries were the primary cause of death in all cases. And he suspected the cause was an explosive decompression. But aviation medicine was beyond his training.

However, he did observe that all the victims had also suffered broken arms and legs. One body even had a leg entirely ripped off. But those injuries had been suffered postmortem. More, all of the victims had their clothing disturbed. Many were almost naked. One had only lost a shoe. And although none showed any signs of having been exposed to a fire, most bore marks similar to scalding on their backs. Three also had scalding marks on their face. One corpse had scalding marks on their chest and abdomen as well. In the end the Professor was unclear what this all meant.  

But after each autopsy, the body was made as presentable as possible and placed in a coffin before being returned to the chapel, for eventual delivery by British Overseas Airways Corporation and then to their families.

The next question was how to get the 70 to 80% of the Comet Yoke Peter, presently sitting on a barge in Porto Azzurro back to the Royal Aircraft Establishment at Farborough - about 40 miles west southwest of London. By ship it might take months. 

However there was one aircraft in 1954 which was capable of carrying the wreckage from Italy directly to England; the American built Fairchild C-119 (above),  popularly known as the “Flying Box Car”.  It's distinctive twin boom tail allowed easy loading and off loading from its 10 foot by 8 foot cargo hold. And it could carry a ton and a half of weight. The United States Air Force readily agreed to provide a “Box Car” for the transport flights back to England.

In the meantime, Arnold Hall, Director of the Royal Aircraft Establishment, wanted to confirm Professor Fornari's suspicion about the rapid decompression. A model of the Comet's fuselage was constructed at Farnborough. Dummies sat in for the passengers. 

Then, as film camera's rolled, the fuselage was pumped full of air until the container burst. In the explosive decompression the dummies were thrown forward into the back of the seat in front of them, or slammed into the ceiling. This confirmed Professor Fornari 's findings.  First the lungs had ruptured, and then the physical injuries had occured.

As the bits and pieces of Yoke Peter arrived in Farnborough (above), some new discoveries were made. 

Shreds of the cabin carpet were found tangled in the interior of Yoke Peter's tail (above).  A rear fuselage panel bore the distinct imprint of a coin from some passenger's pocket. And paint smears on the rear fuselage had been left by the passenger seats. These were all more evidence of an explosive decompression. But what had caused that? A bomb? Or had one of the Ghost engines exploded, ripping apart the pressurized cabin? 

Clearly the fuselage had come apart in mid-air, but where had the destruction begun? Workers at Farnborough began to reassembled the skin of Yoke Peter on a wire frame. It was a technique which would be used on many future aircraft losses.

Those most closely associated with the Comet were stunned by the accidents, and the groundings – or at least they later claimed to be. With their future careers clearly tied to the success of the Comet, BOAC Captain, R. Clifford “Cliff” Alabaster would later admit, “Initially, we didn’t think it could be mechanical breakup. We had every confidence in the airplane.” And Captain Ernest “Rod” Rodley agreed. “It was a perfect airplane as far as we were concerned. We were absolutely puzzled by the problems.” And a generation of pilots matured in the shadow of RAF Bomber Command, who endured a 46% death rate from 1939 through 1945, would reflexively follow orders even when common sense said to do so was suicide.

The Ministry of Transport and Civil Aviation, Mr A. T. Lennox-Boyd (above, left), had so far resisted pulling the Comet's Air worthiness certificate, in part because de Havilland and BOAC as well as Air France and the South African Air Service ordered all Comets returned to London without passengers, and at reduced speeds and altitudes.  But Lennox-Boyd did seek the opinion of the Air Registration Board. This group had been formed in 1937, and was made up of 4 representatives of the airlines 4 representatives of insurance firms, one professional pilot and a representative of the general public, the last two appointed by the Secretary of State. 

The pilot on the board was Peter Duffrey (above),  and as he recalled, “We had a series of meetings at Heathrow. They were quite sure they'd covered what was going on.”

The board considered the possibility of sabotage, and recommended increasing security around the jets. In case one of the Ghost engine's turbines had exploded, they proposed putting a sheet of steel between it and the pressurized cabin. They recommended installing weather radar in future Comets, so thunderstorms could be avoided. 

They recommended modifications to the wings, to increase lift at low speeds. They even considered the possibility of a failure of the pressurized cabin, but the extensive mathematical calculations by the de Havilland engineers, and the generous margins for safety used, convinced the board to reject this as a possibility. In all, 60 modifications were recommended.

The financial pressures after 10 weeks of grounding were tremendous. Both de Havilland and BOAC were hemorrhaging money. Salaries of pilots, mechanics, ground crews and support staff continued, while payments on loans taken out to build the Comet continued to pile up.  And British technical capabilities were now being openly questioned around the world. Because of the bad publicity, future sales were slipping away. 

As the editor of one British aviation magazine showed, it was a situation ripe for justification. “The positive cause of the accident may never be found," the author reasoned, "though every effort will be made to do so. The results of the public inquiry will undoubtedly be interesting, but they are not likely to be conclusive...To this extent one agrees with the more vigorous shouts from the Press. Now that we have done everything that the technicians consider necessary to forestall trouble in the future – Let us get the Comet flying again!”

Pilot Peter Duffrey (above) explained, “I voted against, because I felt the modifications had not adequately discovered the reason for the explosion.”  It did not matter. By a single vote the board recommend to release the Comets. 

So, on 5 March, 1954 - 54 days after the sudden and violent deaths of the passengers and crew of Yoke Peter - Air Chief Marshal Sir Frederick Bowhill (above), writing as if the board vote had been unanimous, told his Minster that while, “.... no cause has yet been found that would satisfactorily account for the Elba disaster...everything humanly possible has been done to ensure that the desired standard of safety shall be maintained. This being so. the Board....recommends that Comet aircraft should return to normal operational use...”  And so did he.

All recommended modifications were made as quickly as possible. Perhaps too quickly. And three weeks later, on 23 March 1954, the first Comet, with a full load of enthusiastic paying passengers, lifted off from London.  Sir Miles Thomas, Chairman of the British Overseas Airways Corporation, explained, "We obviously wouldn't be flying the Comet with passengers if we weren't satisfied conditions were suitable." 

And two weeks later, on Thursday, 8 April, 1954, Comet Yoke Yoke, carrying 14 passengers and 7 crew members, disappeared 25 minutes after taking off from Rome.  Decades later, Peter Duffrey would admit, “I still do not forgive those who restarted the services...”  The board members had been gambling with the lives of the crew and passengers. And as Duffrey noted, "I was one of the chits" they were gambling with.

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