NINE YEARS AGO this month, a Boeing 747 jet exploded off the coast of Long Island on its way to Paris, killing all 230 people aboard. In the final report from its investigation into the TWA Flight 800 crash, the National Transportation Safety Board concluded that the explosion was probably caused by an electrical short circuit igniting vapors in the center wing fuel tank. In that report, handed down in August 2000, the NTSB repeated a recommendation that it had made to the Federal Aviation Administration not long after the initial accident: Modifications to reduce or eliminate flammable vapors in the tanks should be developed. The board further suggested that once developed, the technology should be required in new airplanes and, wherever feasible, existing airplanes.

By 2002, FAA engineers had created a device that was small enough to be retrofitted onto commercial aircraft and, when tested by Boeing, proved effective in reducing flammability. Although the technology has now been available for three years -- and a larger military version has existed for even longer -- there is still no rule requiring the devices in passenger planes. The elimination of the dangerous vapors remains one of the NTSB's five most-wanted aviation safety improvements.

To its credit, the FAA has issued at least 70 directives since the 1996 crash to reduce ignition sources near the center wing fuel tank -- a necessary step given the tank's proximity to the bulk of the airplane's wiring. But minimizing electric sparks, though important, does not eliminate the danger. To finish the job, the FAA must mandate the reduction of flammable tank vapors.

The FAA estimates that the airlines will have to pay more than $600 million to outfit the entire U.S. fleet of commercial planes with the devices. That's a substantial investment to minimize what is -- granted -- a small risk. But as TWA 800 proved, the risk is there, and it's not a risk most airline passengers would want to take. FAA officials say a flammability reduction rule will be on the books this year, and we hope that happens. The 10th anniversary of the TWA 800 disaster should not pass without this crucial rule in place.