WASHINGTON -- U.S. government investigators said Thursday they could not yet explain what caused a potentially catastrophic battery fire aboard a Boeing 787 that forced the grounding of Dreamliners around the world.
National Transportation Safety Board Chairwoman Deborah Hersman said investigators were working "around the clock" to understand what led to the Jan. 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 Jan. 16.
But she said that the systems designed to contain such an event on the all-new Dreamliner "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."
Hersman said the NTSB had still not reached any conclusions on what caused the Jan. 7 battery fire aboard the empty 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, that specific plane's auxiliary power unit (APU).
"The APU battery was spewing molten electrolyte, very hot material," she said.
Sequence of Events a Mystery
Subsequent testing showed clear evidence of a short circuit, a thermal runaway, and a fire, but scientists still did not know what the sequence of events was.
"We are early in our investigation," she said. "We are working very hard to determine what happened and why it happened."
The NTSB and other major aviation regulators last week ordered all 50 Dreamliners in global service grounded after the two battery incidents.
Boeing (IW 500/16) 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.
Hersman said it was still a mystery why the systems designed to prevent such an event on the 787 failed, apparently in two cases, despite authorities having cleared the Dreamliner as a safe aircraft more two years ago.
"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," she 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."
Copyright Agence France-Presse, 2013