The National Transportation Safety Board has determined where the fire in the Boeing 787 in Boston started, but it's no closer to finding out what started it.
NTSB investigators believe the fire started in cell-6 in the 8-cell lithium-ion battery in the All Nippon Airways Dreamliner as it sat empty at the gate. The fire then spread to the seven other cells inside the battery. That's something NTSB head Deborah Hersman said Boeing didn't believe could happen based on its testing and the FAA certification.
"Boeing has indicated that these tests, that were conducted prior to certification, showed no evidence of cell to cell propagation or fire in the battery," Hersman said. "Our investigative findings with respect to the event battery show that when a short circuit did occur it resulted in cell to cell propagation in a cascading manner and a fire."
Hersman said Boeing also found the likelihood of smoke in the plane after a potential battery problem was small.
"Boeing assessed that the likelihood of a smoke emission event from a 787 battery would occur less than once in every 10 million flight hours," she said. "The 787 fleet has accumulated less than 100,000 flight hours."
That the problems Boeing didn't find in testing are now happening on the planes in service tells Hersman one thing. "The assumptions used to certify the battery must be reconsidered."
As for finding the root cause of this battery fire, "design, certification, manufacturing, these are all still on the table, and we have a lot of work to do."
The 787 is the first airliner to make extensive use of lithium batteries. Aircraft makers view the batteries, which are lighter, recharge faster and can store more energy than other types of batteries of an equivalent size, as an important way to save on fuel costs.
But lithium batteries in general are more likely to short-circuit and start a fire than other batteries if they are damaged, if there is a manufacturing flaw or if they are exposed to excessive heat.
Hersman said she will have another investigation update in 30 days. Meanwhile, the entire 787 fleet remains grounded.
The Associated Press contributed to this report.