Tuesday 1 October 2013

The comet falls to earth

A few days ago, I was on about metal-fatigue in early Liberty Ships.  The hogging and sagging that ships go through out upon the ocean requires material of extraordinary strength and flexibility.  One of the things that engineers understand is that structures that flex are often much stronger than those which are completely rigid.  Often the problem comes down to a compromise between flexi-strong buildings and inhabitants feeling too sea-sick to work in them.
The few catastrophic failures in the Liberty Ship fleet were due to metal fatigue propagating from stress points like hatch corners. The SS John P Gaines, for example, broke in half in 1943 and the stern section (containing the bridge, the crew's accommodation and 90% of the people aboard) stayed afloat for 33 hours which is a huge tribute to good design, redundant engineering, water-tight doors and excellent discipline.

Ten years after the end of WWII, there was a notorious spate of cases of fatal metal-fatigue, in a different medium - the atmosphere.  The plane involved was the de Havilland Comet which had a habit of breaking up in mid-air and coming rapidly to earth - or in the case of BOAC Flight 781, rapidly to the Mediterranean Sea in January 1954. Like the SS John P Gaines, which eventually finished on the ocean floor off the Aleutian Islands, there wasn't a lot material from the 781 crash for forensic engineers to work on. It took several crashes, including another one leaving Rome 3 months later, and several dozens of deaths before it was realised that metal fatigue was the common factor in all the Comet disasters.  The engineers had designed these rather swish square windows for the passengers to look out of.  The de Havilland engineers, not completely ignorant about metal fatigue, had designed all windows to be glued into place, but they were riveted into place by the manufacturing arm of the company. Another killer example of communication failure between different wings of the same project. The normal heaves, bends and shifts of the air-frame in flight propagated cracks from the rivets in the window frames until they reached the sharp corners of the windows whence they unzipped the whole aluminum skin. Instant depressurisation ruptured the lungs of everyone aboard and killed them before impact. That's why all modern aircraft windows have rounded corners.  The Comet also put the nail in the coffin of British commercial passenger aircraft design for 20 years until they took a half share in Concorde.

And while we are on plane safety, why do the seats on aeroplanes all face forward so that you can whiplash your head forward to contact the seat on front?  Survival through front-end impacts (very few aeroplanes get rear-ended) would be much greater if everyone sat with their backs to the engine – to use a railway metaphor.  Indeed why do they have windows in aeroplanes at all if they so weaken the airframe.
And while we're on railways, how come a) they don't fit seat-belts and b) allow people to travel standing up and sitting on suitcases in the corridor.  Is the railway company's face red when these standing passengers are killed when the train leaves the ails by accident?

I started down this rabbit-hole because I had been bloggin' about how Werner Herzog missed a flight which fell out of the sky.  It reminded me of a story about my father missing a flight from Rome when I was tiny and thus providentially escaping death.  Being a trained researcher, I Googled off to see what flight he might have missed and BOAC 781 is by far the most likely.  It would have made a big difference to the family dynamic if me and my twin sister had been born posthumous. So then I asked my mother again about something that happened when I was in short pants and she had no memory of anything like that happening to her husband. Neither did it ring any bells with my sister.

It's a funny thing is memory.  Next you'll be telling me that my clear memory of learning naked woodwork from my Uncle Jim in the workshop at the bottom of his garden never happened either!

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