The US National Transportation Safety Board (NTSB) will publish “within two or three weeks” its final report on the 2013 lithium-ion battery fire aboard a Boeing 787 at Boston airport, according to one of its investigators.
The investigation began after the auxiliary power unit battery caught fire on a Japan Airlines Boeing 787 Dreamliner at Boston Logan International Airport on 7 January 2013. Regulators grounded the global fleet of Dreamliners for almost four months after a second battery incident days later on an All Nippon Airways flight in Japan prompted an emergency landing and evacuation.
Speaking at Knowledge Foundation’s Battery Safety conference in Washington, D.C. on 13 November, NTSB National Resource Specialist Robert Swaim refused to be drawn on the root cause of the internal short circuit in one of eight cells that led to the thermal runaway incident, but he appeared to rule out dendritic growth caused by charging at cold temperatures as the sole culprit.
“It’s not just dendrites related to temperature but other things that temperature can affect,” Swaim told BEST. “You will see this in the reports when they come out in a couple of weeks.”
Swaim said the report would include more recommendations to the Federal Aviation Administration, on top of the five released on May 22.
Another NTSB investigator, Mike Bauer, said the GS-Yuasa battery was tested at temperatures outside its actual operating regime aboard the Dreamliner. “[The original certification test] was for a floating configuration at nominal temperature, rather in an installed-type configuration,” he said.
“It was operating in a different temperature than it was designed for,” Bauer added.
Swaim said that NTSB observed minor oscillations of charge current. “We put the battery chargers on a testbed and noted a ‘rippling’ current into the battery as they charged,” adding the amount of current would be disclosed in the final report.
The lack of cell voltage data made it difficult to identify which cell suffered the short but the NTSB almost immediately ruled out overcharge and over-discharge, as well as the possibility of an external source of heat or mechanical damage.
Boeing has since redesigned the battery system to include a stainless steel containment chamber, and, in the event of a fire, smoke and particulates are vented via tubes overboard through the aircraft’s skin.