Closer inspection of a flight risk

May 06, 2002|By James H. Williams Jr.

CAMBRIDGE, Mass. - Much of the investigation of the crash of American Airlines Flight 587 on Nov. 12 has focused on the vertical stabilizer, or tail fin, of the Airbus Industrie's A300-600 jet, a state-of-the-art, complex structure about 28 feet tall that has been implicated in the crash.

Flight 587's vertical stabilizer snapped off - a unique occurrence in modern commercial aviation - and splashed into Jamaica Bay, away from the fuselage crash site in the Queens neighborhood of Belle Harbor.

The plane had taken off from JFK International Airport 103 seconds earlier. The crash killed all 251 passengers, the nine-member crew and five people on the ground.

For the past 30 years, the bulk of my research career has been dedicated to the mechanics, design, fabrication and nondestructive evaluation (NDE) of nonmetallic fiber-reinforced composite materials - the same type of materials of which the Airbus A300-600 vertical stabilizer is made.

In essence, Airbus' NDE policy for its A300-600 composite vertical stabilizer is that damage that cannot be seen with the unaided eye will not compromise its structural integrity.

This is a lamentably naive policy. It is analogous to assessing whether a woman has breast cancer by simply looking at her family portrait.

Although Airbus has tried to defend its NDE policy, the vertical stabilizer on at least one other A300-600, which displayed no damage to the unaided eye, was found to contain a significant structural flaw upon closer inspection. Airbus has stated that the aircraft was safe to fly even with such damage.

The Federal Aviation Administration (FAA), along with the National Transportation Safety Board (NTSB), appears to be moving toward a more rational NDE policy that would ground all A300-600s that undergo side-to-side forces exceeding 35 percent of the force of gravity.

Airbus' input would be sought regarding subsequent inspection of each such aircraft, though I believe Airbus' potential advice has been compromised by its dubious policies.

Dozens of American Airlines A300-600 pilots think they have been stonewalled in their requests for strong corporate and regulatory actions. Although they believe the inspection policies for this aircraft are inadequate, they continue to fly the plane as the investigation proceeds at an uncomfortably sluggish pace. Although the NTSB and the FAA have been slow in this investigation, it is my hope that, with NASA's technical support, they will ultimately fulfill their missions concerning Flight 587. Even so, that would be just the beginning.

There are several unexpressed issues that remain, including the aging and repair of nonmetallic fiber-reinforced composites.

Aging is the deterioration that occurs during long-term mechanical forces and environmental conditions experienced while a plane is in service. Composites will lose both strength and stiffness during this period. Further, studies of the long-term effects of exposure to aircraft environments of moisture, pressure and temperature remain to be conducted for many composite materials. Since such deterioration may be monitored nondestructively, this is another reason for long-term NDE monitoring.

Reportedly, the vertical stabilizer on Flight 587 had been repaired with metal rivets and additional materials. Such repairs of structural damage in composites are frequently unreliable, especially for joints and attachments involving primary (load-bearing) structures.

The rupture of the vertical stabilizer on Flight 587 occurred in the vicinity of repairs, adjacent to an attachment point. Therefore, the FAA must carefully establish and articulate a policy for the repair of primary composite structures.

Finally, Airbus' extensive design and testing programs for the A300-600 composite vertical stabilizer may be deficient if they were based on outmoded or flawed engineering assumptions or an inadequate certification process.

The causes of Flight 587's crash remain uncertain; uncommanded rudder inputs, wake turbulence and structural failure - perhaps in combination - are the prime candidates. There are several issues that are not under contention concerning primary composite aircraft structures, however. Chief among these are the requirements for more effective implementation of their NDE and a better understanding of their aging and repair.

With thousands of commercial aircraft containing structures made of composite materials, the safety of our families, friends and the dedicated crews who serve us demand that these issues be addressed. And, out of respect for those directly affected by Flight 587, the sooner the better.

James H. Williams Jr. is the School of Engineering Professor of Teaching Excellence, Charles F. Hopewell Faculty Fellow and professor of writing and humanistic studies at the Massachusetts Institute of Technology. He founded the Composite Materials and Nondestructive Evaluation Laboratory in MIT's Mechanical Engineering Department.

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