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NOVA: WHY THE TOWERS FELL. Tonight at 8, PBS (WNET/Ch. 13).


here's a painful familiarity to many of the images, and even some of the faces and stories, in tonight's installment of the PBS series "Nova," called "Why the Towers Fell."

In this context, though, they serve a different purpose.

Engineers sift through the wreckage of the World Trade Center for clues.
"A disaster on this scale raises two crucial questions," intones the narrator. "Was the collapse of the building inevitable, and need so many people have died?"

Initially keyed to correspond with the release date of the report on the World Trade Center Towers collapse by the American Society of Civil Engineers, "Why the Towers Fell" now will release some of those findings (in the show's last six minutes, the portion not available for preview) a few days before the ASCE report, which was postponed.

Even in the body of the "Nova" hour, though, some aspects of the disaster are revealed that add to, or directly refute, common wisdom.

Jet fuel was not excessively flammable, as was widely suggested after the jetliners plowed into and ultimately collapsed the twin towers. Instead, engineers say, its main role was to ignite other combustibles, acting like lighter fluid.

Forensic engineers, the architectural equivalents of the puzzle-solving scientists portrayed on "CSI: Crime Scene Investigation," look at video of the towers burning and falling, and examine the wreckage afterward, to isolate the factors that led to the pancaking — or "progressive collapses" — of the two structures.

The ultimate culprit, or at least one of the most surprising ones, turns out to be drywall. A stronger structure around the interior stairwells may have helped keep the structure together longer, at least until more people above the impact points managed to escape and survive. (Only four did.)

Looking at the horrible images while thinking about bolts rather than bodies is a different experience. It's not necessarily a less emotional one, though; even the topnotch animation, which shows the structural damage caused by each plane's impact in a skeletal, see-through perspective, is horrific in its own way.

Since its goal is to make future building safer, though, this program is an hour to absorb and ponder, not to avoid.

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