THE TINIEST TERRORS

By DAVID VAN BIEMA

Could there possibly be a tinier or more innocent-seeming measurement than the millijoule? The unit of energy denotes roughly the wallop packed by a dime dropped on a table from a height of 2 in. But as the National Transportation Safety Board revealed in hearings held in Baltimore last week, minuscule can mean sinister. Calmly, patiently, safety-board explosion expert Merritt Birky explained that a spark carrying one-quarter millijoule of energy was all that was necessary to ignite the contents of the 12,890-gal. central fuel tank of TWA Flight 800 in 1996 off Long Island--a tank that then exploded, destroying the plane and killing all 230 people aboard.

It didn't take a missile. It didn't take a meteorite, a bullet or even a bird. In fact, the safety board still doesn't know the exact source of the spark that presumably ignited Flight 800's mostly empty central fuel tank, a container similar to those sloshing just below passengers' feet in many commercial carriers. But the trigger's precise identity may be disturbingly moot. In a week of technical testimony considerably more alarming than had been expected, safety-board chairman James Hall made it clear that the fuel, transformed from a stable liquid state to volatile vapors by the exhaust heat from air conditioners cooling the plane on a hot July evening, was so combustible that almost anything could have touched it off; that 970 other currently active 747s may be at some risk for the same catastrophe, especially when the air conditioning is overworked; and that, in Hall's opinion, the industry has been remiss in checking those planes for danger and researching ways to fix the tanks. According to papers released by the Federal Aviation Administration, the fuel tanks of 26 planes--13 civilian and 13 military--have blown up since 1959. By week's end frequent flyer and Senate majority leader Trent Lott pronounced himself "very nervous" and promised hearings on the topic.

Lott may have been playing to the millions of Americans who will soon participate in the second busiest travel week of the year. James McIntyre speaks only for himself. An air-accident investigator, McIntyre observed the hearings as an ex-pilot who flew TWA's New York-Paris route repeatedly with a nearly empty central tank. (Drained to reduce weight, it was filled only for longer flights.) Asked if he'd try it again after the week's disclosures, he replied carefully, "I wouldn't have a problem--in the wintertime."

No one had foreseen that kind of jumpiness. Three weeks ago, James Kallstrom, who led the FBI's dogged investigation into possible missile and bomb attacks on Flight 800, ruled out those possibilities, and last Tuesday he announced his retirement to private life. The disaster was now officially an accident, and the major drama at the safety-board hearings was expected to be the reactions of some 100 relatives of victims, invited as observers. Indeed, after bravely perusing transcripts from the plane's cockpit voice recorder (the captain at one point noted the plane was climbing especially fast, like a "homesick angel") and toughing out excruciatingly detailed computer-generated crash simulations and a slide marked "Chart 4.7--Body Fragmentation," several dozen relatives marched from the hall to protest the unapologetic testimony of medical examiner Dr. Charles Wetli, whose work they still think was slow and insensitive.

Yet by Tuesday evening it was clear that the hearings' emotional center was not the past but Hall's fears for the future. His primary frustration involved a long-held industry custom. Designers have always--"since the first airplane," noted Daniel Cheney, an FAA manager--understood the dangers inherent in cramming electricity into a narrow airborne hull with the flammable vapors that can result when a tank is hot and mostly empty, but they have addressed the problem primarily by isolating or eliminating the sources of possible sparks. Their assumption that further precautions involving the fuel tanks were unnecessary has historically been supported by the FAA, whose sometimes contradictory mandate requires it to tend both the airline industry's safety and its financial health. Thus a year ago, when the safety board recommended four short-term protective measures focused on the fuel tanks, the agency politely ignored them.

The huge surprise of last week's testimony was the determination that when a mostly empty fuel tank gets as hot as TWA 800's was (about 145[degrees] at takeoff, because of the 400[degree] exhaust thrown off by air-conditioning units a foot away), the electrical charge necessary to detonate the resulting fumes is roughly a quarter of the smallest spark you feel when scuffing your foot on a rug. Said safety-board officer Peter Goelz: "We had no idea how little energy it took to cause an explosion." Hall remarked, "I for one don't see how every ignition source can ever be eliminated." The obvious conclusion: instead of trying to snuff every minuscule fuse, designers should disarm the fuel-tank bomb.

There was some discussion of possible igniters: power surges and wires whose worn insulation could have turned them into spark plugs. Even spontaneous combustion couldn't be ruled out. Hall's main attention and considerable scorn, however, were trained on the neglected science of fuel-tank security. "I think I reflect to some degree the concern the American traveling public has in this issue," he said in a deceptively soft drawl. "In this country, we look to the FAA for regulations on safety." Incensed that in the months since the crash, industry inspectors have checked the fuel-tank safety of only 52 of the 970 Boeing 747s in operation, Hall asked Boeing officials whether the 52 included Air Force One. Receiving the predictable affirmative answer, he harrumphed, "Every airline passenger has as much right to safety as the President."

By the time Hall was done, both his targets were claiming to be on his side. The FAA's Cheney announced that his agency had decided to study the safety board's recommendations. Insisting that "we are seriously embracing attacking this problem on the flammable-vapor level," he pledged to "take action" on fuel-tank maintenance programs for all U.S. aircraft. Boeing declared that it had been studying fuel-tank improvements since the crash and is considering more thorough insulation and "sweeping" out accumulated fumes during flight.

In fairness, it must be noted that the various fuel-tank solutions are fraught with complications. Two seemingly simple answers--distance the air-conditioning units from the tank or fly with it full of fuel--would boost ticket prices. So would "inerting," injecting a nonexplosive gas to decrease the fuel's volatility, although the manufacturer of the $1.5 million inerting units used in some military planes claims that simpler civilian versions would cost just $80,000 per plane. Some inerting gases, however, are potentially lethal: they reduce one danger to passengers but increase another. Cautioned Boeing's chief fuel-system engineer, Ivor Thomas: "We would much prefer to be slow and careful and correct than to rush into something where we find six months later it was the wrong thing to do."

Said Hall: "I don't think the board wants you all to rush into anything that is unsafe. We do want you to rush into looking at the problem." In this he was supported by the sober-looking crowd at the front of the ballroom. "You look at that," said Barbara Johns, whose 18-year-old daughter Courtney was the first victim identified after the crash, "and you wonder, If these are known safety problems, why aren't they correcting them before people die? That's the answer we'd like--because people we love are still flying."

--Reported by Melissa August and Jerry Hannifin/Baltimore and Mark Thompson/Washington

With reporting by MELISSA AUGUST AND JERRY HANNIFIN/BALTIMORE AND MARK THOMPSON/WASHINGTON