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"Question remains after NYC crash How jet's fin fell off puzzles investigators"



Tuesday, December 18, 2001

Question remains after NYC crash How jet's fin fell off puzzles
investigators 
By Alan Levin
USA TODAY


WASHINGTON -- As federal investigators prepare for what could become a
long probe into the crash last month of an Airbus A300 in New York, they
are asking one key question here, in aircraft factories in France and
Germany and at a NASA materials lab in Virginia.

How did the tail come off? 

American Airlines Flight 587 took off Nov. 12 in calm skies from John F.
Kennedy International Airport in New York. But about a minute after
liftoff, it wobbled, the 27-foot carbon-fiber fin flew off and the jet
spiraled into the ground.

Investigators quickly gathered a wealth of evidence. They know how the
jet flew. They collected all the key pieces of wreckage. But that has
not helped them uncover how and why the fin tore loose, according to
officials familiar with the investigation. 

The National Transportation Safety Board says the jet rocked twice,
possibly by a turbulent stream of air left in the wake of a larger jet
flying four miles ahead. Such encounters are routine near major
airports. After the second jolt, the jet's rudder -- a moveable panel at
the rear of the vertical fin -- made a series of sharp swings. 

The jet skidded and rolled violently for about 4.5 seconds, apparently
due to the rudder swings. Then the tail section snapped and the jet
plummeted. According to sources, investigators are focusing on several
areas: 

Test finds small defect

Flight 587 is the first commercial jet accident in which a non-metal
part broke off and apparently caused the crash. Officials here and in
France ordered inspections of A300s and A310s, which have the same tail,
as a precaution. More than 300 A300 and A310 jets have been visually
inspected. No significant problems with the carbon-fiber material have
been found. 

But last week, using a more sensitive ultrasonic test, United Airlines
found a small defect in the composite material in the tail of an Airbus
A320, a smaller jet that has a similar tail. United and Airbus say the
defect is minor and the jet is being put back into service. 

Federal sources say they don't believe the discovery by United indicates
a safety risk. But they are concerned that more ultrasonic testing might
be required to detect whether any jets have weakened tails.

Visual tests cannot find damage below the surface of a carbon-fiber
structure, says Paul Lagace, an expert at the Massachusetts Institute of
Technology. However, he cautioned that composites are designed to
withstand small flaws without weakening.

The key to understanding whether additional tests might be needed is the
examination of the composites on Flight 587. Because the NTSB has
investigated few crashes involving composites, it assembled a team of
experts to assist in the investigation. Earlier this month, the board
brought the tail fin and rudder to NASA's Langley Research Center in
Hampton, Va.

The NTSB will have to learn new techniques, but the detective work is
similar to any investigation.

''Yes, there are some technical differences from . . . dealing with
aluminum or titanium,'' says Vernon Ellingstad, director of the safety
board's office of research and engineering. ''But beyond that, the
general process is similar.''

Their findings could have an enormous impact on the aviation industry.
If the NTSB finds that composite materials weaken over time in ways that
had not previously been understood, it could prompt changes or repairs
on thousands of jets.

Computers lend some help

To help the NTSB understand what happened to the jet, analysts working
here and at Airbus plants in Europe are recreating the flight on
computers. In recent years, computers have made it easier to estimate
the forces that a jet encounters in flight. If the forces on the tail
were moderate, that could mean the tail was inadequately designed or the
structure was weakened. 

U.S. and European regulations require manufacturers to determine the
maximum strength required to keep the tail intact if the rudder moves
all the way to one side, and then build it 50% stronger. But
investigators want to know whether the tail can withstand the lashing
created by the rudder swinging back and forth. 

Sources familiar with the investigation say that other incidents on A300
jets are also being studied. For example, another American A300 went out
of control on May 12, 1997, over West Palm Beach, Fla. The jet slammed
back and forth with even greater force than did Flight 587, according to
NTSB data, yet the tail was not damaged. The pilots recovered after the
jet fell 3,000 feet.

Pilot training in question

Pilots rarely touch a jet's rudder in flight. However, something made
Flight 587's rudder move as far as possible, investigative sources say.
It is possible that one of the jet's pilots moved the rudder as a
reaction to the turbulent wake it struck. One reason for the computer
analysis is to determine whether it would be possible for a jet's tail
to break off while a pilot was rapidly moving the rudder.

Investigators also are looking at American Airlines' pilot training.
American is one of several airlines that teaches its pilots how to right
a jet after it goes out of control. One technique it teaches is to move
the rudder. Airbus and Boeing had warned American that using the rudder
in an emergency could lead to greater problems, possibly structural
damage. But American officials and pilots defend the training. Pilots
are told to use the rudder cautiously and only at slow speeds, they say.


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