The results of the National Transportation Safety Board's probe of a battery fire on a Boeing 787 Dreamliner are expected in the coming weeks, NTSB chair Deborah Hersman said Wednesday.
"We're probably weeks away from being able to tell people what happened and what needs to be changed," Hersman said at a news conference.
The NTSB chief said investigators were "proceeding with a lot of care" in probing the cause of a January 7 lithium-ion battery fire on a Japan Airlines (JAL) 787 that occurred as the unoccupied plane sat on the tarmac at Boston's Logan airport.
Hersman said the most concerning issues uncovered in the probe so far were short circuits and thermal runaway, a chemical reaction that produces uncontrollably rising temperatures.
"These factors are not what we expected to see in a brand-new battery," she said.
The battery problem on the JAL 787, and another on an All Nippon Airways 787 June 16, led to a global grounding in mid-January of all 50 Dreamliners in service until the issue is fixed.
Hersman declined to reveal the latest findings of the JAL investigation, saying she will update the public at a news conference Thursday at 11:00 am (1600 GMT).
The NTSB is working with US and Japanese regulators, Boeing and other suppliers on the investigation and has staffed multiple shifts, she said.
"This is a priority for the NTSB," she added.
The NTSB has been looking at lithium-ion batteries for a long time and recommended measures to limit their risks, Hersman said.
The fire on the 787 in Boston "shows us there were some risks that were not mitigated, were not addressed," she said.
Boeing asked US aviation authorities Monday to allow the company to carry out 787 Dreamliner test flights.
Since the global grounding, the US aerospace giant has halted deliveries of the 787, introduced into service in October 2011 as an energy-efficient aircraft that makes extensive use of lightweight composite materials and pioneering electrical systems.
Unlike earlier aircraft, Boeing sourced many of the parts for the 787 from subcontractors around the world.
French firm Thales designed the Dreamliner's electrical system and commissioned Japanese firm GS Yuasa to produce the lithium-ion batteries.
Both companies are participating in the investigation.
The auxiliary power unit battery, manufactured by GS Yuasa, was the original battery delivered with the 787 on December 20, 2012, the FTSB said.
"NTSB investigators were made aware of reports of prior battery replacements on aircraft in the 787 fleet, early in the investigation," the agency said last Friday.
Both JAL and ANA have reported they were forced to replace a number of batteries in their Dreamliners last year after experiencing problems, well before the fire and smoke incidents.