Investigators Narrow Focus in Dreamliner Probe: WSJ

U.S. aviation safety investigators examining Boeing Co.'s  787 Dreamliner increasingly are focusing on manufacturing or design problems with the batteries as possible causes of overheating rather than on other parts of the jet's electrical system, the head of the National Transportation Safety Board said Monday.

While the safety board still hasn't identified the root cause of a lithium-ion battery fire in January on a parked Japan Airlines Co. 787, NTSB Chairman Deborah Hersman told The Wall Street Journal "we're looking at the [battery] manufacturing process," including searching for potential "internal defects" that could have touched off a short-circuit.

Taken together, Ms. Hersman's comments provide the strongest sign yet of the board's overall direction in the nearly two-month-old 787 probe. Investigators have said they believe a short circuit began a sequence of events that ended with a battery fire and heavy smoke inside the Japan Airlines jet. But they haven't been able to determine what caused the short circuit. In the past, Ms. Hersman and other NTSB officials placed equal emphasis on exploring causes that were internal and external to the battery.

Ms. Hersman indicated that above all, board experts now are interested in unraveling internal battery behavior such as "how each individual cell is charged" and "how those charges are monitored." In addition, she said the probe is building on "some steps that have been taken" recently by Boeing and the Federal Aviation Administration to reassess production and inspection practices at the GS Yuasa Corp. 6674.TO -3.69% factory in Japan that makes the batteries.

A Boeing spokesman said the company's team "has worked tirelessly in support of the NTSB to help develop an understanding of the event," adding that "until the investigation concludes, we can't speculate on what the results will be."

A spokesman for Yuasa wasn't immediately available for comment.

Last week, the board issued a preliminary report indicating, among other things, that investigators basically determined various electronic components connected to the battery—ranging from a charging unit to a sophisticated surge protector—were intact and didn't exhibit any failures or defects. Tests after the fire showed those components generally worked as expected, according to the report.

In the interview, Ms. Hersman reiterated "we feel very confident" that the short circuit started an uncontrollable overheating problem. But she seemed less positive than before about the outcome of the probe and stopped short of predicting that investigators eventually will find the precise answer to what caused the battery fire. "Nobody has a crystal ball," she said. "I have tremendous confidence in our investigators," she said, but then added "we will do what we can" and "rely on other people" such as government and industry experts to help search for the answer.

For now, though, other developments highlight the emphasis on searching for potential causes within the guts of the Dreamliner's 63-pound battery. Ms. Hersman said the safety board is currently testing a number of new, undamaged batteries in the laboratory and documenting their power fluctuations and other factors over a number of weeks. Such "monitoring over time" allows investigators to understand how the batteries operate at "high levels (and) low levels" of power, she said, providing a "very comprehensive look" at their performance.