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Boeing: Cause of 787 battery fires remains mystery

The cause of the battery problems that grounded Boeing Co.’s 787 Dreamliners may never be known, the engineering leader for the jet, Richard J. Horigan, said Thursday. What caused innovative lithium-ion batteries to smolder on two 787s may never be known because heat destroyed the evidence, he said.

The Federal Aviation Administration says airlines can resume flying Dreamliners as soon as they replace the batteries with a revamped battery system. In the United States, only United Air Lines uses 787s , six of them. The FAA estimated repair costs for those planes at $2.8 million.

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Japan Airlines started flying the 787 between Boston and Tokyo a year ago. It is fixing the batteries on its seven Dreamliners, including one that has been grounded in Boston since the battery caught on fire at Logan International Airport in January. The carrier needs approval from Japan’s Civil Aviation Bureau before it can resume flying. The airline has been flying four flights a week between Tokyo and Boston on a Boeing 777 since the Dreamliners were taken out of service.

A smoking battery on a second 787 forced an emergency landing in Japan.

There are 50 of the planes in service worldwide, and Boeing has orders for 840 more.

Horigan said that all potential causes of the battery fire were eliminated in the new system.

‘‘So it’s not uncommon where you have events like this where you don’t get a definitive root cause but you have sufficient confidence in your design solution to know . . . you’ve identified it and addressed it,’’ he said.

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