Crashes raise questions about composite parts in planes

The timing of the crash, the New York location, and the fact that it was an American Airlines jet - one of two carriers involved in the Sept. 11, 2001 attacks - raised immediate concerns about terrorism. Those fears were put to rest when investigators attributed the crash to pilot error.

Now, in the midst of a mystery over the disappearance of an Air France jet over the Atlantic Ocean on May 31, a less-scrutinized aspect of the New York crash is on the minds of some aviation experts.

The American Airlines plane - an Airbus A300, one of the larger models made by the European aircraft consortium - left the runway less than two minutes after an even larger Japan Airlines Boeing 747 took off, its four engines at maximum thrust creating a powerful wake.

As the A300 was buffeted by the wake, its first officer tried to compensate by aggressively shifting the tail rudder - too aggressively, it turned out. The force of the rudder movements put pressure on the tail, and it essentially fell apart.

The National Transportation Safety Board claimed the first officer's actions were improper, and attributed the crash to pilot error. Most of America breathed a sigh of relief that the crash was, at least, not a terrorist attack.

But there was another factor in the crash of American Airlines Flight 587 that was less noted in the weeks and months afterward, but that some aviation specialists are mentioning in the aftermath of the Air France disaster, which claimed 228 lives.

Investigators examining the broken tail fin from the New York crash discovered that six lugs made of composite materials - a lighter alternative to metal - had snapped under the pressure; six similar aluminum lugs did not.

Though the finding raised questions about the strength of the composite materials, the NTSB did not blame the use of composites for the crash. It stated that the rudder movements initiated by the co-pilot stressed the tail beyond the design capacity, and that, if not for the human error, the plane was safe.

But now the Air France plane - another Airbus model that also makes extensive use of composite materials - seems also to have broken apart in flight, perhaps while under unusual pressures.

Investigators emphasize that any speculation is premature. The most significant evidence so far has come from last-second signals sent by the plane's computers to Air France's maintenance headquarters. They indicate a rudder problem, among others, but no one can tell if it caused the crash or was merely a symptom, the computer having recorded the unusual rudder movements while the plane plunged for some other reason.

Some analysts have speculated that the plane's external airspeed indicator may have frozen as powerful thunderstorms rocked the plane. That could have caused either the pilot or the plane's computers to make adjustments that overstressed the rudder or some other part of the plane, perhaps causing it to break apart in a manner similar to the crash in Queens.

Airbus pioneered the use of composite materials as a fuel-efficient alternative to aluminum, which is heavier and limits the planes' flying range. But Boeing, too, has a lot invested in composites. Its new 787 Dreamliner uses more composites than any current commercial jet, and even advertises its improved cabin atmosphere, with more humidity to combat jet lag - possible only because composites don't rust like aluminum.

Generally, both Boeing and Airbus have had admirable safety records. Because of mechanical improvements, computerized safeguards, and backup systems, flying has never been safer, and pilots train on realistic simulators that mimic potentially catastrophic conditions.

But the Air France mystery raises at least a kernel of concern about cutting-edge materials used in aircraft construction, and is prompting fresh scrutiny of the NTSB's findings in the New York crash.

Both of the world's leading plane makers - and all of its air travelers - can only wait for answers.

Peter S. Canellos is the Globe's Washington bureau chief. National Perspective is his weekly analysis of events in the capital and beyond. He can be reached at canellos@globe.com.

Correction: Because of an editing error, yesterday’s National Perspective column about the use of composite parts on planes incorrectly characterized aluminum, which does not rust.