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US air safety investigators have identified how a battery fire on a Boeing 787 Dreamliner occurred last month but have not discovered the cause, the head of the NTSB said Thursday.
National Transportation Safety Board chairwoman Deborah Hersman also faulted in the battery's certification process under the Federal Aviation Administration, which approved the all-new 787 for flight.
Hersman said evidence pointed to a single cell on the eight-cell lithium-ion battery on a parked Japanese Airlines 787 at Boston's Logan airport.
There were multiple signs of short-circuiting in the cell, which led to an uncontrollable rise in temperatures, or thermal runaway, to adjacent cells, she said.
"We are now working to identify the cause of the short circuit on cell six," she said at a news conference to update her agency's probe of the January 7 incident.
"We have not reached any conclusions at this point."
The battery fire, and a burned battery that forced an All Nippon Airways 787 flight into an emergency landing on January 16, resulted in the global grounding of all 50 787s in service until the problem is fixed.
As the 787 grounding entered a fourth week, Hersman said investigators had ruled out mechanical impact damage to the battery, and were now focused on the cells' charge, and the manufacturing and design of the battery.
The FTSB chief criticized the battery's certification under the Federal Aviation Administration (FAA).
"The assumptions used to certify the battery must be reconsidered," she said, adding that her agency was conducting battery tests to determine "why hazards were not mitigated."
As part of the normal certification process, which focuses solely on safety, the FAA relied heavily on information provided by Boeing, she said.
Boeing estimated that a battery smoke event would occur less than once in 10 million flight hours, while it has happened twice in less than 100,000 flight hours, she said.
"We have seen two events on two aircraft less than two weeks apart," she said.
Hersman said the NTSB had completed disassembly of the burned battery and was wrapping up the "microscopic" investigation, while continuing to work closely with Japanese and French counterparts.
French firm Thales designed the Dreamliner's electrical system and commissioned Japanese firm GS Yuasa to produce the batteries.
"We're continuing to drill down a bit more," she added.
A detailed report on the two-track probe -- investigation of the fire and a review of the battery certification and testing -- will be issued within 30 days, she said.
Boeing has asked the FAA to allow it to conduct test flights of the 787 in its own probe of what caused the two events.
Since the global grounding, the US aerospace giant has halted deliveries of the 787, which begin service in October 2011 as an energy-efficient aircraft making extensive use of lightweight composite materials and pioneering electrical systems.