US government investigators say they still can't explain what caused a potentially catastrophic battery fire aboard a Boeing 787 that led to the grounding of Dreamliners around the world.
National Transportation Safety Board chairwoman Deborah Hersman said investigators were working "around the clock" to understand what sparked the January 7 fire aboard a parked Japan Airlines 787, as well as a smoking battery that forced the emergency landing of an All Nippon Airways 787 on January 16.
But she said that the systems designed to contain such an incident on the all-new 787 "did not work as intended."
"This is an unprecedented event. We are very concerned. We do not expect to see fire events on board aircraft," Hersman told reporters.
"The expectation in aviation is to never experience a fire on board an aircraft. In two weeks' time we saw two cases of battery failures on a 787 and the grounding of the entire fleet by the FAA. The significance of these events cannot be understated."
She said the NTSB had still not reached any conclusions on what caused the fire in the auxiliary power unit (APU) of the unoccupied plane at Logan airport in Boston.
When investigators first saw it after the fire was extinguished, she said, there was structural and component damage around the battery.
"The APU battery was spewing molten electrolyte, very hot material," she said.
Subsequent testing showed clear evidence of a short circuit, a thermal runaway - a rapid, uncontrolled increase in temperature - and a fire, but scientists still did not know the sequence of events.
Hersman said the battery's monitoring unit itself was extensively damaged, making it less useful to investigators.
"We are early in our investigation," she said. "We are working very hard to determine what happened and why it happened."
The Federal Aviation Administration and other major aviation regulators last week ordered all 50 Dreamliners in service worldwide grounded after the two battery incidents.
Boeing has since halted deliveries of the 787, introduced into service in October 2011 as an ambitious, energy-efficient aircraft designed with 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.
787 was certified safe
US and other authorities cleared the Dreamliner as a safe aircraft more than two years ago after extensive testing, making the battery problems even more of a mystery.
"What we have seen from these two events does not comport with any of the risk analysis that we would expect to see with respect to reliability or smoke or a fire event in these batteries," FTSB's Hersman said.
"These events should not happen as far as design of the aircraft. There are multiple systems to protect against a battery event like this. Those systems did not work as intended. We need to understand why."
Deepening the mystery, earlier Thursday in Japan investigators said the battery that forced the landing of the ANA 787 showed no signs of having caught fire or having experienced a sudden surge in voltage.
The battery pack's voltage had been at normal levels before it rapidly plunged just before the system alert that forced the emergency landing, a Japan Transport Safety Board official told AFP.
Boeing's shares climbed 1.4 percent to $75.32 on Thursday even as the questions remained over the ambitious 787 program.
The company said in a statement late Thursday that it was assisting the NTSB and Japanese investigators probing the 787 problems.
"The company has formed teams consisting of hundreds of engineering and technical experts who are working around the clock with the sole focus of resolving the issue and returning the 787 fleet to flight status."
"Boeing deeply regrets the impact that recent events have had on the operating schedules of our customers and their passengers."
United Continental Holdings chief executive Jeff Smisek said his company, the parent of United Airlines, still believes in the 787. United has six Dreamliners, the only US carrier operating the high-tech plane.
"We continue to have full confidence in the aircraft and in Boeing's ability to fix the issue."
However, he added, "I don't know when the airplane will be flying."