Twa Flight 800 -- Blast Sound Questioned -- Cockpit-Tape Noise Unlike Those Made By Other Bombs

WASHINGTON - The loud noise at the end of Trans World Airlines Flight 800's cockpit voice recorder is more consistent with a fuel explosion than with the sharp sound caused by the bomb that brought down Pan American Flight 103 over Lockerbie, Scotland, in 1988, sources close to the investigation said yesterday.

But specialists who analyzed the recording have reluctantly concluded it probably will not tell them why the Boeing 747 plunged into the ocean off Long Island last month, killing all 230 people aboard. The sound could have been a fuel-tank explosion, but it also could have been a bomb that was different from the Pan Am blast. It even could have been a rapid structural breakup, the sources said.

A comparison with numerous other voice recorders from past airplane bombings, fuel-tank explosions and structural disasters did not find an exact matching sound. And although the sound has the characteristics of a "fuel-air mixture" explosion, at least one other bombing has produced a somewhat similar sound, officials said.

Complicating the investigation further, a sound-spectrum analysis of the recording has failed to pinpoint the location of an explosion or to detect the characteristic signature of a "wave pulse" - or shock wave - from a bomb.

The wave pulses buried in the recording are too poorly defined, perhaps because of acoustics of the airplane's structure or the location of the explosion, the sources said.

When the so-called "black boxes" with the voice recorders were discovered in the weeks following the July 17 crash, it was hoped they might allow investigators to determine whether the crash was an accident or sabotage. Instead, the analysis, along with other findings, has left some perplexed, deepening the mystery of what happened aboard Flight 800.

"The recording didn't tell us what we needed to know," one senior law-enforcement official said. "This is very frustrating."

Like other evidence, the recording adds to a wealth of knowledge of what did and did not happen to the 747, but not how.

The FBI also has failed to find any explosive residue on aircraft parts, and metallurgical tests so far confirm a major explosion but not whether it was caused by a bomb or some cataclysmic mechanical failure.

Sound-spectrum analysis of cockpit voice recorders has proved to be one of the most effective means for spotting and describing explosive damage.

The Pan Am 103 bombing over Lockerbie left such a detailed explosive signature that the recording alone was evidence of a bomb.

Investigators compared the TWA voice recording with those from the Pan Am bombing and several other crashes that involved bombings, fuel-tank explosions and structural breakups. In addition, they compared it with explosive tests conducted by British aviation-safety authorities.

Even though the sound lasted only a short time before the doomed aircraft's electricity was cut off, specialists were able to determine how rapidly it grew, its intensity and other characteristics.

Unlike the rapid onset of the Lockerbie bombing sound, the TWA sound grew in intensity at a slower rate and lasted longer, officials said.

The sound was "not terribly inconsistent" with a center fuel tank explosion that destroyed a Philippine Boeing 737 in 1990, the officials said.

In that case, the source of ignition was never determined, but investigators suspected wiring had been installed inside the tank and perhaps caused a spark.

The TWA tape was also compared with recordings recovered from the bombings of an Air India plane in 1988, a French UTA DC-10 1989, an Avianca Colombian Boeing 727 in 1989 and a TWA 707 in 1974.

Investigators also used for comparison the explosive decompression of a United Airlines 747 when a cargo door blew off in 1989.