The mystery of Flight 587? What caused the crash of Flight 587? Investigators sift through wreckage for clues. (Physical Science Forces/Flight).

WHEN AMERICAN AIRLINES FLIGHT 587 departed from New York's JFK airport on November 12, 2001, it sailed smoothly into a clear blue morning sky. For 251 passengers and 9 crew on board, it was an ordinary takeoff. But in less than two minutes, the tail fin of the 300,000-pound jumbo jetliner--an Airbus A300--jerked violently from side to side and then ripped apart from the plane's body. Within seconds, Flight 587 nose-dived into a residential neighborhood in Queens, killing everyone aboard as well as five people on the ground.

What doomed Flight 587? Investigators from the National Transportation Safety Board (NTSB) are determined to find out. Last year, the NTSB investigated 34 major airplane crashes--the fewest such disasters to have occurred in any year since World War II. Now among the questions the NTSB seeks to answer in the crash of Flight 587: What role did turbulence, or blustery air currents, play? Was there a fatal flaw in the space-age "composite" material used to craft the tail? Is there evidence of sabotage? "It could take us another 24 months before we finally piece the puzzle together," said NTSB spokesman Ted Lopatkiewicz in January. Read on to learn what investigators have learned so far about three possible culprits.

SUSPECT #1: WAKE TURBULENCE

? ACCORDING TO an NTSB crash report of December 18, one possible factor in Flight 587's demise: wake turbulence, blasting air currents caused by two tornado-strength streams of air (vortices) that spiral off a plane's wingtips. In this case, the vortices formed in the wake of a 42-ton Japan Airlines Boeing 747 that took off 2 minutes 20 seconds before Flight 587 (see diagram, above).

How does a plane produce a vortex? As air blasts over the top of a plane's curved wing--a shape called an airfoil--it speeds up to create low pressure above the wing and high pressure beneath it. The resulting pressure difference creates lift, the upward force beneath the wing that keeps a plane in flight. But it also spawns vortices that spin off wingtips at speeds up to 350 kilometers per hour (217 miles per hour).

A vortex is most dangerous to planes directly after takeoff. "A plane is still close to the ground and has very little room to recover from the impact," says engineer Miroslav Krstic at the University of California at San Diego. That's why the Federal Aviation Administration (FAA) requires at least four miles of space between flights. The Japan Airlines jet was flying farther than that distance ahead of Flight 587 at the time of the crash, claims the NTSB.

Scientists doubt wake turbulence alone knocked Flight 587 out of the sky. "Commercial jets are built to withstand forces up to 2.5 times that of gravity [G-forces]," says UC San Diego engineer John Kosmatka. The recovered flight-data recorder, or "black box," reveals that turbulence hit the plane with one tenth of a G-force--barely strong enough to disturb passengers.

SUSPECT #2: TAIL-FIN FLAW

? WITHIN HOURS of the crash, investigators fished out of Jamaica Bay their biggest clue yet: the 8.2-meter (27-foot)-high tail fin. They discovered six sheared attachment points, or tongues, that connect the tail fin to the fuselage, the plane's central body (see photo, above). Investigators now wonder if a structural flaw in the tongues could have doomed the plane.

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One major concern is the material used to craft newer planes: high-strength plastic reinforced with carbon fibers and glue. Called a "composite," the material has become increasingly popular in the construction of plane parts, because it's stronger, lighter, and less corrosive than aluminum and steel--the most common plane-part materials.

One downside, say experts: Composites lack ductility, or flexibility. An accidental blow during construction, for example, might easily weaken the fibers (see diagram, below), making a part vulnerable to breakage under stress, such as that caused by turbulence or sharp plane movements. In 1988, inspectors of the doomed Airbus A300 noticed that some carbon-fiber layers in one of its tail's tongues had separated, a flaw called delamination. But the NTSB says engineers repaired the problem before the plane was ever launched into flight.

New High-tech Materials--Suddenly
Suspect?

Today, plane parts are often made with
composites, a mixture of plastic, carbon
fibers, and glue. Stronger and lighter than
metal, they can be brittle. Below are four
kinds of forces they must withstand:

1 TENSION

The ability of composites to
withstand stretching depends
on the tensile or flexible
strength of the fibers.

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2 COMPRESSION

The glue must hold
the fibers as straight
columns to prevent
them from buckling.

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3 SHEAR

The composites may have to bear uneven weight loads
and must resist deforming. The glue must be strong and
adhere well to the fiber.

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4 BENDING

Bending is caused by a combination of
tensile, compression, and shear loads.

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SUSPECT #3: SABOTAGE?

? "AT THIS POINT we've found no physical evidence to suggest terrorist activity," says Lopatkiewicz. Absent from the plane's structure are soot patterns--telltale evidence of an in-flight explosion: "The black smoke stains typically run backward from the point of explosion along the exterior of the aircraft." Both engines fell to the ground largely intact; technicians who inspected the engines have found "no evidence of engine failure, rupture, or foreign object damage," Lopatkiewicz insists.

ANSWERS: UP IN THE AIR

PERHAPS THE MOST convincing theory to date lays blame on a combination of material failure in the tail and air turbulence. Exactly how the two factors worked together to cause the crash of Flight 587 is yet to be determined.

For the latest information, check out the NTSB Web site: www.ntsb.gov

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Cross-Curricular Connection

History: Research and report on a past transportation disaster, and present reasons and theories why the disaster occurred.

Did You Know?

* New "composite" materials used to build plane parts are up to 25 percent lighter than aluminum and retain their shape even with extreme temperature changes.

* The Airbus 300 is designed to fly for up to 26 years. The plane used for Flight 587 was 13 years old when it crashed. The jet has a wingspan of 45 meters (147 feet) and can reach speeds up to 853 kilometers (530 miles) per hour.

* Wingtip vortices trailing a commercial jet can stretch up to 12.9 km (8 mi) long.

Directions: Match the word in the left column with the correct phrase or word in the right column.

-- 1. turbulence    a. tornado-like air streams

-- 2. airfoil       b. a plane-building material

-- 3. fuselage      c. shape of the top of a plane's curved wing

-- 4. composite     d. plane's central body

-- 5. ductility     e. flexibility

-- 6. vortices      f. blasting air currents

-- 7. tongues       g. attachment points on planes

ANSWERS

1. f 2. c 3. d 4. b 5. e 6. a 7. G

National Science Education Standards

Grades 5-8: motions and forces * understandings about scientific inquiry * understandings about science and technology

Grades 9-12: motions and forces * understandings about scientific inquiry * understandings about science and technology

Resources

"Storm Upset Jet on Ground in 1987" by Don Phillips, The Washington Post, January 2, 2002,

"Aircraft Safety and the Case of the Plastic Tail" by Matthew L. Wald, The New York Times, November 27, 2001.

For more on aerospace engineering, visit the NASA Langley Research Center Web Site: www.larc.nasa.gov