Winnowing Down the Theories

A Boeing 747 that screamed down the runway at New York's Kennedy International Airport on Monday carried with it high hopes that one of the most perplexing crashes in aviation history will yet be solved.

Federal investigators are using the plane to try to re- create conditions of a fatal evening last July, when TWA Flight 800 exploded in midair, killing all 230 people on board. The tests, to be continued into next week, signal how one year later aviation experts are still looking for answers - and how determined they are to find them.

Investigators have known for some time that vapors in the plane's nearly empty center fuel tank exploded, ripping apart the 25-year-old jumbo jet off the coast of New York. But the biggest, costliest crash investigation to date has yielded no physical evidence to support any of the three original theories of what brought down the plane: a bomb, a missile, or a mechanical failure.

The crash, however, has prompted some changes in airplane-safety and airport-security practices. The National Transportation Safety Board (NTSB), which investigates air disasters, found several problems with 747 fuel systems during this probe and recommended safety measures to prevent center fuel tanks from exploding, no matter the source of ignition. But the Federal Aviation Administration (FAA) has so far refused to order these changes, saying more tests are needed.

Even so, suspicion that the Flight 800 crash was the work of terrorists led four airports to add high-tech explosive-detection devices - and 50 more devices will be installed at other airports in coming months. A few airports, too, now use a passenger-profiling system to help identify suspected terrorists, and the FAA is testing a system to "match" passengers with luggage.

Now, in the absence of forensic evidence, it's looking less likely that a terrorist missile or bomb destroyed the plane. But scientists have narrowed possible mechanical ignition sources (see box, right), and they are designing more tests to determine if any could produce a blast powerful enough to rip apart a 170-ton airplane.

"We have made a great deal of progress," says NTSB chairman Jim Hall. "We have been able to narrow the investigation down to five or six different scenarios, and I am confident we will ... finally determine the ignition source."

In this week's tests at Kennedy Airport, a big Boeing 747 sat for a few hours on the tarmac, revving its engines and running its air-conditioning units. Then it barreled down the runway, miming the precise conditions under which Flight 800 took off.

The tests performed to date and those being set up are time-consuming and can lead to explosions, says Mr. Hall. He hopes the tests will end by December, when the NTSB plans to hold public hearings in Baltimore. It may be another year before investigators reach a final conclusion, he says.

The TWA probe is the first time the NTSB and FBI have collaborated. Both agencies claim the process has been smooth, but critics question whether having two investigative agencies - one intent solely on determining if a crime occurred - hampered the investigation at the outset.

Aviation experts say solving the crash is vital because 1,000 of the venerable 747s - first produced in 1969 - are still flying. "It is critically important that the investigation be thorough and that the conclusions be supportable beyond any doubt for the benefit of aviation safety," says Aaron Gellman, director of the Transportation Center at Northwestern University in Evanston, Ill.

To be sure, the scope of this joint investigation has been exhaustive. For months, Navy divers scoured the ocean floor near Long Island, fishing thousands of parts from the depths. Trawlers dragged a 40-square-mile section of the ocean bottom. Investigators built a 94-foot mock-up of the plane from the center fuel tank to the cockpit. Hundreds of investigators swarmed over parts, searching for clues. Laboratories across the US and in England have run chemical and metallurgical tests.

The FBI, which took the initial lead in the investigation, is now near to pulling out. Its intelligence sources - human and electronic - have turned up no evidence of foul play. No forensic evidence has been collected to support either the bomb or missile theories.

LATEST IDEAS ON WHY IT CRASHED

The NTSB has found causes for all but four crashes it's investigated. Here are the six theories for the TWA crash.

* Scavenge pump. The pump wasn't found, but its manufacturer is testing whether overheating can create a spark.

* Static electricity. Scientists are testing whether any fuel-line complications could have produced a static charge.

* Fuel probes. Seven probes measure the fuel level in the center tank. The manufacturer is testing to see if a probe could have been ignited by a spark outside the tank.

* Fuel tank in the wing. The FAA suspects a spark in this tank may have sent a flame through vent tubes linked to the center tank. Flight tests will explore the theory.

* Small explosive charge. Investigators are putting explosives on a 747 in England to see if they can ignite the fuel tank. Effects of the test-charges will be compared with the damage to Flight 800.

* High-speed particle penetration. Flight 800 might have been hit by a high-speed fragment from a nearby exploding missile, a meteorite, or space debris.