* NTSB will provide some details of probe on Thursday
* Agency thoroughly testing battery that caught fire on jet
* Boeing wants to conduct new test flights
By Doug Palmer
WASHINGTON, Feb 6 (Reuters) - The National Transportation Safety Board is "probably weeks away" from completing its probe into battery problems on the Boeing 787 Dreamliner, but will share its latest information on the jet on Thursday, agency head Deborah Hersman said.
Hersman, who is considered a top candidate to be the next U.S. transportation secretary, said the agency was doing an exhaustive examination of a lithium-ion battery that caught fire in one of the planes and led to the grounding of the Dreamliner.
"We're running through the macro level to the microscopic level on this battery," she told reporters at a Wednesday breakfast briefing. "We are going to have some information tomorrow, but I think we are probably weeks away from being able to tell people here's what exactly happened and what needs to change."
All 50 Dreamliners in service have been grounded since Jan. 16 while the NTSB, U.S. Federal Aviation Administration and other aviation regulators around the world investigate the battery failures. No root cause has been identified.
The NTSB is looking at both the possible causes of the fire and the FAA certification process for the Dreamliner.
Hersman said that at Thursday's briefing, "We will talk about special conditions that were put into effect at the time when the Dreamliner was certified."
Fire risk on planes has always been a major concern, especially given the amount of fuel they carry and the heat generated by jet engines. U.S. aviation standards require planes to have numerous on-board fire-suppression systems.
The FAA in 2007 granted the Dreamliner special conditions and said its contain-and-vent system was sufficient to control the build-up of explosive or toxic gases, except in situations considered "extremely remote."
That decision has come under scrutiny after the lithium-ion batteries in two 787 planes failed within days of each other, sparking a fire in one jet in Boston and generating warnings and an acrid smell that prompted the pilots of the second plane to make an emergency landing in Japan.
REQUEST TO CONDUCT TEST FLIGHTS
The NTSB is conducting the U.S. probe with help from Boeing, battery maker GS Yuasa Corp of Japan, the FAA and battery experts from other U.S. federal agencies. None of the agencies have identified what caused the battery failures on the 250-passenger airliner.
Boeing this week asked the FAA for permission to conduct new test flights of the 787, suggesting it is making progress in finding a solution to the battery problems, but the government agency has not yet announced a decision.
"In essence what happens when an aircraft is certified, it basically gets locked into the standards that are in existence at the time. So the question ... is whether or not as time goes on through the life of the aircraft, do they fly with new standards?" Hersman said.
Hersman declined to comment on a report that she was the White House's top choice to be the next transportation secretary, saying she was focused on her current job.
Asked what was the most worrisome thing the NTSB had uncovered so far in its investigation, Hersman referred back to a finding made public two weeks ago.
"There were short circuits in cells of the battery and there was thermal runaway in the battery, multiple cells where we saw uncontrolled chemical chain reactions," she said. "Those features are not what we expected to see in a brand new battery and a brand new airplane."
Hersman said the NTSB has been looking at the risks of lithium-ion batteries for some time and has recommended strategies to reduce potential hazards.
There will always be advances in technology, but the safety side of that is "to make sure you've done the right risk assessment, that you understand what the failure modes are and that you've mitigated any potential risks," she said.
"I would not want to categorically say that these batteries are not safe," Hersman said. "Any new technology, any new design, there are going to be some inherent risks. The important thing is to mitigate them."