Research shows the success of a bacterial community depends on its shape.
A version of this article by Professor James H. Williams Jr. appeared in the Baltimore Sun on Monday, May 6. Williams is the School of Engineering Professor of Teaching Excellence, the Charles F. Hopewell Faculty Fellow, and Professor of Writing and Humanistic Studies at MIT.
Like many of my generation, I recall where I was and what I was doing when I heard the news of the assassinations of J.F.K., M.L.K. and R.F.K. Unlike many of my generation, I also recall where I was and what I was doing when I heard the news from Orly, France in March 1974, Tenerife, Spain in March 1977, Chicago in May 1979, Mount Otsuka, Japan in Aug. 1985, Lockerbie, U.K. in Dec. 1988 and New York in July 1996: all major crashes of commercial airliners.
On Nov. 12, 2001, as I worked in front of a muted TV set tuned to cable news, I witnessed early broadcasts of the crash of American Airlines Flight 587, an Airbus Industrie's A300-600, in Belle Harbor, Queens, about 103 seconds after takeoff from J.F.K. International Airport. All 251 passengers, a crew of nine and five persons on the ground were killed.
Much of the investigation of the accident has focused on the aircraft's vertical stabilizer, a state-of-the-art, approximately 28-foot-tall complex structure that has been implicated in the crash. The vertical stabilizer on Flight 587 snapped off--a unique occurrence in modern commercial aviation--and landed in Jamaica Bay, away from the fuselage crash site.
For 30 years, the bulk of my research has been dedicated to the mechanics, design, fabrication and nondestructive evaluation (NDE) of nonmetallic fiber-reinforced composite materials, the type used to make the airbus A300-600 vertical stabilizer. I have thought about Flight 587 every day since the crash.
In essence, Airbus's 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 policy is lamentably naï¿½ï¿½ï¿½ï¿½ï¿½ï¿½ve. It is analogous to assessing whether a woman has breast cancer by looking at her family portrait.
Although Airbus has attempted to defend its NDE policy, the vertical stabilizer on at least one other A300-600, which displayed no damage to the unaided eye, upon closer inspection contained a significant structural flaw. Airbus has stated that, even with such damage, the aircraft was safe to fly.
The Federal Aviation Administration (FAA), in conjunction with the National Transportation Safety Board (NTSB), appears to be moving towards a more rational NDE policy that would ground all A300-600s that experience lateral forces exceeding 35 percent of the force of gravity. Lesser requirements would be imposed on aircraft experiencing lesser, though high, lateral forces. Airbus's input would be sought regarding subsequent inspection of each such aircraft, though I believe Airbus's potential advice has been compromised by its dubious policies.
Dozens of American Airlines A300-600 pilots feel they have been stonewalled in their requests for strong corporate and regulatory actions, and although they believe the inspection policies for this aircraft are inadequate, they continue to fly, as the investigation proceeds at a 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. Several unvoiced issues remain, two of which are the aging and the repair of nonmetallic fiber-reinforced composites.
By aging, I mean the deterioration that occurs during long-term in-service mechanical and environmental loading, in the absence of an overloading or catastrophic event. When subjected to the loading histories of aircraft, composites lose both strength and stiffness. Furthermore, 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, we observe another rationale 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's 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. No amount of analysis can overcome faulty assumptions or insufficient requirements.
The specific cause(s) of the crash of Flight 587 remains uncertain; uncommanded rudder inputs, wake turbulence and structural failure--perhaps in combination--are the prime candidates. Several issues concerning primary composite aircraft structures are not under contention; paramount 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 and comfort of our families, friends and the dedicated crews who serve us demand that these issues be addressed. Out of respect for those directly affected by Flight 587, the sooner the better.
A version of this article appeared in MIT Tech Talk on May 8, 2002.