The NTSB on Thursday said the Boeing 787 Dreamliner battery that caught fire earlier this month in Boston shows evidence of thermal runaway, a serious condition in which an increase in temperature causes progressively hotter temperatures.
Still unclear is the question of which occurred first, the thermal runaway or a short circuit in the battery that was also found, said NTSB Chairman Deborah Hersman at a news briefing on the board's investigation into the incident.
The fire took place aboard a Japan Airlines 787 as it sat at the gate at Boston Logan Airport. A cleaning crew noticed smoke in the cabin 26 minutes after the airplane arrived. It took firefighters nearly 40 minutes to put out the battery fire in the 787 rear auxiliary power unit.
Investigators are still going over the charred insides of the lithium-ion battery at the NTSB’s lab in Washington as they try to piece together what exactly caused the fire. The focus of their work is a search for flaws in the battery design, Hersman said. The FAA gave Boeing special approval to use the battery, which has four layers of protection built in to prevent overcharging.
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 cockpit and cabin.
“The expectation in aviation is to never experience a fire on board an aircraft,” Hersman said. "We have to understand why this battery resulted in a fire when there were so many protections that were to be designed into the system."
The FAA ordered a review of the 787 after the Boston battery fire. On January 16, after the second battery incident, the agency grounded the six 787s operated by United Airlines and urged other airlines to follow suit. Currently, all 50 Dreamliners that had been in service are grounded pending the outcome of investigations by the FAA, the NTSB and Boeing.