From: Andy Toth (transfer_mechanism@hotmail.com)
Date: Thu Nov 15 2001 - 04:19:11 MST
November 15, 2001
Crash Inquiry Focuses on Tail Fin That Broke Off
By MATTHEW L. WALD and AL BAKER
Investigators are focusing on how the tail fin of American Airlines Flight
587 was ripped away from the spine of the aircraft in the moments after
takeoff Monday, and what role that played in the deadly crash.
Officials of the National Transportation Safety Board are not close to
declaring that the loss of the fin, a 27- foot-high structure that is
essential to flight, was the cause of the crash, or even the first part to
leave the jetliner, an Airbus A300, in its puzzling in-flight breakup.
But as a sign of how air safety experts are concentrating on the fin,
American Airlines decided yesterday to inspect that part on its 34 surviving
A300's. Officials with Airbus, the European manufacturer of large airplanes,
were discussing whether to advise other airlines to do the same. And federal
investigators called their top scientific experts on materials to the crash
site to study the part, which is made of a combination of plastic and carbon
fibers, not metal.
The tail was torn off, leaving the attachment points, which are made of the
same composite, still bolted to the plane's metal frame, investigators said.
They have not found any evidence that an explosion or contact with another
object in flight caused the damage.
The tail was found in Jamaica Bay, the first piece in a trail of debris,
though officials said they could not conclude it was the first part of the
plane to break off. Information being retrieved last night from the plane's
damaged flight data recorder may shed more light on what happened, officials
said.
Despite this new focus on the tail fin, the board has not ruled out any
cause for the crash, which killed 260 people on the plane and 5 on the
ground in Belle Harbor, Queens. Investigators still say that they have found
no evidence of criminal acts, including terrorism.
Loss of the tail fin would have made flight impossible, pilots said. Among
the questions for investigators are whether the fin separated because of
stresses that its design should have been able to handle or if the plane, in
the air less than two minutes on a day with light winds and clear air,
somehow encountered extreme air conditions.
The cockpit voice recorder captured the sound of the plane rattling, and the
two pilots said they believed they were encountering the turbulent wake of
an airplane ahead of them. Such encounters are routine, though investigators
have not confirmed that Flight 587 actually did pass through turbulence from
another plane.
It was following a Japan Airlines Boeing 747 as it departed Runway 31 at
Kennedy International Airport Monday morning. Rules require that planes be
separated by four miles on takeoff, and indications so far are that the
distance between the two planes was greater than that.
Marion C. Blakey, the chairwoman of the transportation safety board, said at
a briefing yesterday afternoon that the paths of the two planes and the
timing "would be consistent with a wake vortex encounter."
She added: "That said, I want to stress that we do not know the specific
facts here, beyond that. We do not know whether this really contributed in
any way to the actual accident, but we are looking at this very closely."
On Tuesday, the safety board took the unusual step of calling its chief
materials scientist from his laboratory to the crash site. Yesterday it
asked the Federal Aviation Administration to call in its top scientist with
that specialty. If the crash is attributed to the failure of a nonmetal
composite part, it would be the first time that had happened to a commercial
jetliner, experts said.
Airbus, far more than Boeing, its main rival, relies heavily on composite
materials, to save weight and to avoid corrosion. Experts say that composite
materials are stronger per pound than aluminum, and are less prone to
fatigue, in which repeated flexing weakens the material.
But composite materials also can weaken. Sometimes the reinforcing carbon
comes loose from the matrix that forms the bulk of the composite, or layers
come apart, a process called delamination.
Investigators seemed to be settling in yesterday for a long investigation,
although they have already begun clearing away the wreckage. Both engines
were trucked to Kennedy Airport for analysis. They will then be sent to an
American Airlines maintenance base in Tulsa.
"We won't have a eureka moment here," said a high-ranking investigator. He
added that he was confident that the safety board would find the cause,
although it might take months.
The crash, the first in the United States of an Airbus, is unusual because
big jets seldom break up in the air. But it is now usual for the
circumstances of crashes to be unusual, because the level of air safety is
so sophisticated. Once a problem is found to have caused one crash, it is
corrected on other airplanes and seldom given the opportunity to cause
another. Thus each crash tends to be from a new cause.
One transportation official described the investigation as filled with
"head-scratching and bafflement and puzzlement." George W. Black Jr., the
member of the safety board assigned to the crash, said the case was "weird."
At a briefing last night, Mr. Black gave several details that tended to
narrow the focus of the investigation:
¶Analysis of the sounds on the cockpit voice recorder showed that the
engines continued working beyond the point where the pilots seemed to lose
control of the plane.
¶According to the maintenance log recovered from the crash site, there had
been no major repairs recently, and no items awaited repair.
¶The fittings that hold the fin to the fuselage are visually inspected every
five years; the most recent inspection for the plane that crashed was in
December 1999.
¶Before the plane was delivered in 1988, the composite material in one of
the six fittings holding the tail in place was found to be delaminating,
meaning its layers were coming apart. It was fixed by adding layers and
inserting rivets, and the manufacturer decided that no extra inspections
were required afterward.
The investigators have many areas of inquiry still open. One is that seven
years ago, the plane hit turbulence in flight and 47 people on board were
injured. Whether this could have produced structural damage that went
undetected was not clear, although the plane itself was designed to
withstand a degree of turbulence that could result in injuries or deaths.
One new area for investigators is the composites, which are harder to read
for clues than metal parts. They have been used for years, but usually in
doors or panels, not in the plane's skeleton. American Airlines will use a
variety of inspection techniques, including a tap test, in which technicians
hit the part and listen for sounds that would indicate a void, said Thomas
Haueter, the board's deputy chief of aviation safety.
Advocates say carbon composites are better than metal. "It will outperform
aluminum in fatigue," said Stephen I. Miscencik, the chief of design
engineering at TPI Composites, a company in Warren, R.I., that makes bus
bodies, yachts and windmill blades out of composites. "It's a super material
to use in fatigue applications," he said in a telephone interview.
The carbon and plastic composites used by aircraft manufacturers tend to be
extremely strong and resistant to cracking, and were strenuously tested,
Robert T. Bocchieri said. Dr. Bocchieri is an expert in assessing failures
of composite materials and a senior engineer at Applied Research Associates,
an engineering firm in Sunnyvale, Calif.
Cracks develop in metals or plastics as they age, but cracks tend to be
stopped in these materials by the embedded fibers, he said. Still, "aircraft
makers have tended to be extremely conservative in their use of composites,"
he said.
Because the materials, when proposed for airliners, were intended for a new
domain in which hundreds of lives were at stake, they "were put through
massive numbers of cycles of full-scale testing," Dr. Bocchieri said. "They
had to simulate the entire life history of the aircraft."
Airbus has made progressively greater use of composites. It plans to expand
their use even further in its A380 superjumbo jet. According to Airbus, each
A300 uses 14,600 pounds of composites, and by doing so, saves 3,300 pounds
of weight.
The fin of the A300 is almost all composite. Mr. Black said yesterday that
when it was pulled from Jamaica Bay, the fin looked good.
"With the exception of the area where it separated, it is almost in new
condition," he said.
Both the fin and the rudder, the movable panel at the back of the fin, can
float, he said. Aviation experts said that both could have traveled a
significant distance from the spot at which they separated from the plane.
Thus, although they were found closer to the airport than the main wreckage,
investigators are not certain that they were the first to come loose.
The investigators displayed a poster-size chart of radar images, but some of
the images clustered like flocks of birds, which they probably were.
With further work, the investigators hope to determine which of the radar
echoes were actually airplane parts.
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