The Boeing 787 Dreamliner battery that caught fire earlier this month in Boston shows evidence of short-circuiting and a chemical reaction known as “thermal runaway,” in which an increase in temperature causes progressively hotter temperatures, federal accident investigators said Thursday.
It’s not clear to investigators which came first, the short-circuiting or the thermal runaway, National Transportation Safety Board chairman Deborah Hersman said. Nor is it clear yet what caused either of them, she said during a news briefing on the boardinvestigation.
The fire took place aboard a Japan Airlines 787 shortly after it landed at Logan International Airport on Jan. 7. All the passengers had left the craft, but a cleaning crew noticed smoke in the cabin 26 minutes after the plane arrived at its gate. It took firefighters nearly 40 minutes to put out a battery fire in the aircraft’s rear auxiliary power unit.
Investigators are still dissecting the charred insides of the battery at the board’s laboratory in Washington in an effort to piece together clues to the cause of the fire. The focus of their painstaking work is a search for flaws in the battery that may have caused either the short-circuiting or thermal runaway.
The battery monitoring unit that might have provided answers was severely damaged in the blaze, Hersman said.
Investigators also tested the battery charger and another component related to charging. They found minor failures in both, but that would be expected given the fire damage, officials said.
“We’re still trying to determine the significance of those findings,” Hersman said.
The Dreamliner, Boeing’s newest and most technologically advanced airliner, was designed with safeguards aimed at preventing its two lithium ion batteries from catching fire and containing a fire should one occur.
A little over a week after the fire in Boston, another 787 battery failure led to an emergency landing in Japan. There were no flames in that incident, but there was smoke in the cabin, Hersman said.
“The expectation in aviation is to never experience a fire on board an aircraft,” yet there were two battery failures on the 787 within two weeks, Hersman said. “We have to understand why this battery resulted in a fire when there were so many protections that were designed into the system.”
After the battery fire in Boston, the Federal Aviation Administration ordered a review of the design, manufacture and assembly of the 787. On Jan. 16, after the second battery incident, the agency grounded the six 787s operated by U.S. carriers, all by United Airlines.