The results of a probe into the Boeing 787 Dreamliner will likely not be known for weeks, investigators said Wednesday, meaning the fleet will remain grounded for at least that long.
The National Transportation Safety Board's chair Deborah Hersman declined to provide an update on the Japan Airlines probe findings ahead of an NTSB news conference set for Thursday at 11:00 am (1600 GMT).
"We're probably weeks away from being able to tell people what happened and what needs to be changed," Hersman told reporters.
Hersman said investigators were "proceeding with a lot of care" in probing the cause of a January 7 lithium-ion battery fire on a JAL 787 that occurred as the unoccupied plane sat on the tarmac at Boston's Logan airport.
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," Hersman said.
The battery problem on the JAL 787, and another battery incident on an All Nippon Airways 787 on January 16, led to the global grounding last month of all 50 Dreamliners in service until the issue is fixed.
However, a Boeing 787 will conduct a one-time ferry flight Thursday from Texas, where it was being painted, back to the Boeing plant in the northwestern state of Washington on Thursday, the Federal Aviation Administration said.
The plane will carry no passengers, only the crew needed to operate the flight, and special attention will be paid to the battery before and during the flight, the FAA said.
The Japan Transport Safety Board, probing the battery failure that forced an emergency landing of the ANA 787 in Japan, said Tuesday there was evidence of thermal runaway but that the cause was still unknown.
The NTSB is working with the FAA and Japanese regulators, the US Defense Department, NASA, Boeing and other suppliers on the investigation.
"This is a priority for the NTSB," Hersman said.
Hersman noted that the NTSB had been looking at lithium-ion batteries for a long time and had recommended measures to limit their risks.
The fire on the 787 in Boston "shows us there were some risks that were not mitigated, were not addressed," she said.
Boeing has asked the FAA to allow it to conduct 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 batteries.
Both companies are participating in the investigation.
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.
The prolonged investigations and grounding of the 787 have raised concerns about Boeing, which continues to produce five 787s a month.
"This will become a growing problem for Boeing as the planes can't be delivered" and payments will be delayed, Loren Thompson of the Lexington Institute told AFP.
But airlines are not canceling orders "because it is not clear that there is a major problem with the plane," he said.
"Orders will not be canceled unless the problem can't be fixed."
Boeing spokesman Marc Birtel said the company did not plan to change its planned 787 production rates and still assumed there would be "no significant financial impact" from the FAA grounding.
Boeing shares were up 0.2 percent at $76.01 in afternoon trade in New York.