Friday, March 18, 2005

 

Trouble With Airbus Composites?

In November 2001, with the attacks of Sept. 11th still on nearly everone's minds, an Airbus A300 lost its rudder and vertical stabilizer assembly after encountering wingtip vortex from another airliner. The official NTSB report placed the blame on the pilot's use of the rudder instead of alierons to correct the aircraft's path - supposedly, the initial rudder input was excessive and caused an over-correction, which then lead to a second and also excessive correction, and eventually resulted in a combination of yaw and opposite rudder movement that caused aerodynamic overloading and failure of the vertical stabilizer.

But in the past weeks, news of another rudder seperation have surfaced:

At 35 000 feet above the Caribbean, Air Transat flight 961 was heading home to Quebec with 270 passengers and crew. At 3.45pm last Sunday, the pilot noticed something very unusual. His Airbus A310's rudder -- a structure over 8m high -- had fallen off and tumbled into the sea. In the world of aviation, the shock waves have yet to subside.

Mercifully, the crew was able to turn the plane around, and by steering it with their wing and tail flaps managed to land at their point of departure in Varadero, Cuba, without loss of life.

As they say, "not good". Thoughts persist surrounding the design and inspection techniques used for composites:

Airbus, together with aviation authorities on both sides of the Atlantic, insists that any deterioration of a composite part can be detected by external, visual inspection, a regular feature of Airbus maintenance programmes, but other experts disagree.

In an article published after the flight 587 crash, Professor James Williams of the Massachusetts Institute of Technology, one of the world's leading authorities in this field, said that to rely on visual inspection was "a lamentably naive policy. It is analogous to assessing whether a woman has breast cancer by simply looking at her family portrait."

This brings up a good point. While metal failures will typically start at the surface and migrate inward (except perhaps in the case of material impurities), the same perhaps is false for composites. And if that's the case, then techniques such as ultrasound and X-ray are undoubtably required to provide for a safe and thorough inspection.

Is all of this damning of composites? Certainly not. For starters, the strength-to-weight ratio, ability to alter stiffness and strength along different directions, and the immense increase in fabrication "flexibility" are all huge advantages over metallic materials. And let's not forget the nasty fatigue problems with aluminum - there is no minimal level of strain that will not fatigue aluminum (keep that in mind the next time you watch wings flex during take-off), and stress corrosion is a dirty little secret of many alloys - check out the saga of the T-34 for proof of aluminum's peculiar characteristics. But it would seem that the helicopter business has been dealing with composite rotor blade cracks for a while now, and I'm thinking that teething problems might be a fact of life for the next few design cycles using composite technology.

Here's some good musings on the problems facing future spaceflight, and how composites play into that whole equation.

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