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It looks like Boeing’s 787 Dreamliner fleet, grounded worldwide for the past week, won’t be airborne soon.

The recent fire in a lithium-ion battery aboard a parked Japan Airlines (JAL) 787 in Boston “is a very serious air-safety event,” National Transportation Safety Board (NTSB) Chairwoman Deborah Hersman said Thursday in a briefing on the agency’s investigation.

“We do not expect to see fire events on board aircraft,” she told reporters at the agency’s Washington, D.C., offices.

But the inquiry is far from determining the cause of the fire. “We have not yet ruled anything out,” Hersman said. “There’s a lot more work to be done.”

Air-safety experts said the Federal Aviation Administration isn’t likely to quickly lift its order grounding the 787, given the NTSB’s lack of progress toward finding the root cause and Hersman’s blunt warning.

Former NTSB member John Goglia said Boeing is in limbo until the board can pinpoint the cause of both the Jan. 7 Japan Airlines fire and the overheated battery that forced an emergency landing of an All Nippon Airways (ANA) 787 in Japan a week later.

“They’ve got to find the smoking gun,” said Goglia. “If there’s no definitive cause, this airplane is going to be down for a while.”

“Months, not weeks”

Jim Hall, a former head of the NTSB, concurred: “I think you are looking at months, not weeks.”

Hersman said that although Boeing built multiple and redundant safety features into the battery system, “those systems did not work as intended. … We need to understand why.”

The NTSB has concluded that the battery short-circuited and suffered a “ thermal runaway,” an uncontrolled overheating that spreads from cell to cell, she said.

But investigators have not established the sequence of those events, or even whether the short-circuit and the thermal runaway were causes or symptoms of what went wrong.

Neither has the NTSB determined for sure whether the battery overcharged or if there could be internal manufacturing defects.

The JAL fire broke out a half-hour after the plane had ended a 12-hour flight from Tokyo to Boston.

All 183 passengers and 11 crew members had left the plane when a mechanic doing routine maintenance checks detected smoke in the cabin, then saw flames from the battery.

The jet was almost new, delivered to JAL only in December.

Hersman said the plane was not plugged into a ground charger before the fire broke out.

She said that when NTSB investigators got to the plane, the battery had already been ripped out by firefighters.

Her team noted structural and component damage in the electronics bay within about a 20-inch radius of the battery.

The battery is used to start the auxiliary power unit, or APU, a small turbine in the tail of the jet.

“The APU battery was spewing molten electrolytes, very hot material,” Hersman said.

The methodical, high-profile investigation is unfolding inside the modest forensic laboratory at NTSB’s headquarters in southwest Washington, D.C.

In a fluorescent-lit room on the fifth floor holding a half-dozen investigators, reporters Thursday paraded by the burned battery from the Dreamliner.

The battery’s cobalt-blue casing sat splayed atop a wheeled cart. One of the battery’s eight cells was left inside for display. Investigators are focusing most on cells five, six and seven, the most damaged.

On an adjacent long table, two strips of 33-foot-long foil windings made of copper and aluminum — the innards of the battery cells — lay unspooled for examination. One of the windings was more heavily damaged, much of it charred and blackened.

The NTSB is working its investigators in two daily shifts in Washington, with more staff deployed to Japan, where the battery is made, and to Tucson, Ariz., where the battery charger is made.

In a statement Thursday afternoon, Boeing said it’s working closely with the regulators to analyze what happened.

“We are working this issue tirelessly,” it said.

Maintenance data

Two electronic devices that recorded maintenance data on the JAL plane are being downloaded at Boeing in Seattle to obtain information recorded after the airplane’s electrical power was interrupted.

In addition to the detailed forensic examination of the plane’s electrical system, Hersman said the NTSB is reviewing manufacturing records and gathering information collected in supplier audits at battery maker GS Yuasa in Japan and the maker of the charging system, Securaplane Technologies. in Tucson.

It is also examining whether the FAA’s certification standards were adhered to and if those standards were adequate, Hersman said.

Four days after the Boston fire, the FAA ordered a sweeping review of the 787’s safety, focused on the electrical systems and including a review of both the design and manufacturing processes.

At that stage, however, the Dreamliners were still flying.

But last week, when the pilot of the ANA plane received instrument warnings of an overheated battery and smelled a burning odor in the cockpit — prompting the emergency landing — the Japanese airlines grounded their airplanes and the FAA followed suit.

In that incident, all 137 people on board evacuated down the emergency slides.

This time, the battery involved was in the forward electronics bay, behind and below the cockpit.

This main battery is “the final power source, should all other electrical generation fail,” Hersman said.

No fire was found, but again hot chemicals had sprayed out of the battery, leaving a trail of dark residue across the compartment.

Although the aircraft was an older jet, delivered to ANA a year earlier, the battery had been installed as a replacement in late October.

How long might the NTSB investigation take to come to a conclusion on the cause of the Boston fire?

“It’s really very hard to tell at this point,” Hersman said. “We have all hands on deck.”

That leaves Boeing in a very tough spot.

The company said it has “teams consisting of hundreds of engineering and technical experts who are working around the clock” to solve the problem.

Former NTSB member Goglia said he wouldn’t be surprised if Boeing already has a team working on a plan to replace the lithium-ion battery with a more conventional one, something that would require an FAA recertification of that part of the electrical system.

“That could take a couple of months,” said Goglia.

Dominic Gates: (206) 464-2963 or