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July 08, 2009

Is the Dreamliner in Real Trouble?

No Highway in the Sky starring Jimmy Stewart and Marlene Dietrich

by Clive Irving

Here's a riddle: What connects Jimmy Stewart, Marlene Dietrich and the Boeing 787 Dreamliner? 

It's a 1951 movie called No Highway in The Sky. Stewart plays Theodore Honey, an aeronautical engineer, who is investigating the crash of a new plane. (Dietrich's role is ornamental). Honey suspects that metal fatigue, a barely known problem at the time, caused the crash. Authorities ignore his warnings but when the tail falls off an airplane just before takeoff Honey is vindicated. 

A pretty clunky drama it was--until, three years after it appeared, the world's first jetliner, the de Havilland Comet, suffered a series of mysterious crashes. Eventually, structural failure was proved to be the cause and metal fatigue was a factor.

That lesson was learned. More than fifty years of experience has ensured against structural failure in modern jets. But Boeing decided to build the 787 in an entirely new way, virtually eliminating metal and using, instead composites--where layers of fibers are bonded and molded into shape.

This technology was pioneered in military airplanes. Composites are of great appeal because they are both stronger and lighter than metal. An airliner, however, is very different. It is not subject to the stresses of combat, but it flies far more frequently and for far longer. For the first time, the 787 uses composites in parts that carry the greatest strains, critically where the wings meet the fuselage.

When Boeing suddenly announced, toward the end of June, that the 787's first flight was being called off, it cited problems at that very point. Thirty six ruptures had occurred at the core of the wings, 18 on each side of the fuselage. This structural failure showed up late in what are called static tests, where a complete airplane is stressed to breaking point--well beyond what the 787 would be expected to encounter in service. But there are some failures, like this one, which reveal themselves only in real time--the process cannot be sped up--and are not to be ignored because they expose unsuspected physical characteristics. This brings us back to Theodore Honey. He realized that metal could fail simply by aging. He predicted how long it would be before the airliner in the movie would be stricken by such a failure.

When composites fail, they do so in ways that cannot be detected by the maintenance checks used to detect metal fatigue. The FAA recommends inspecting composites with the use of X-rays and ultrasound scans, rather than the visual checks used for metal, where hairline cracks are often harbingers.

The test failure of the 787 indicates that Boeing has yet to fully understand the behavior of the materials it is using.

The company confessed that the weakness in the wing had not shown up on its computer models. This may well mean that aging remains an unpredictable element in the strength of composites and that only time exposes danger areas.  Two Japanese subcontractors, Fuji and Mitsubishi, are responsible for the critical wing components. Neither they, nor Boeing, have accumulated any experience of how composites of this size and carrying the greatest stresses of flight will stand up to prolonged use in airline service. No passengers would ever be put at risk--Boeing's own engineers and the FAA's certification system would ensure that. But until these new concerns are answered, the 787 program, already two years behind schedule, is likely suffer more delays.

Further reading:

* Ultrasound Camera Makes Internal Composite Damage Easy to Find (ReinforcedPlastics.com)
* What is Wrong with the Dreamliner? (Daily Traveler on CNT)
* Boeing's Dreamliner Debacle (Daily Traveler on CNT)
* On the Fly: The Daily Traveler on the airline industry

Comments

It was some time ago, and I'm sure things have changed somewhat, but a Boeing engineer said to a friend of mine (at that time the Editor of Air & Space Smithsonian) "The more we learn about composites, the better aluminum looks."

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