The battery involved in a fire onboard a Boeing 787 Dreamliner showed signs of short circuiting, the National Transportation Safety Board said.
It also had indications of a “thermal runaway,” a situation in which a significant temperature increase can initiate a dangerous chain reaction, the agency said.
NTSB chairman Deborah Hersman expressed concerns about the adequacy of the systems to prevent a fire from occurring.
“The investigation will include an evaluation of how a fault that resulted in a battery fire could have defeated the safeguards in place to guard against that,” Hersman said in a news conference. “As we learn more in this investigation, we will make recommendations for needed improvements to prevent a recurrence.”
The NTSB has not ruled anything out as a potential factor in the battery fire, she said.
“There are still many questions to be answered,” Hersman said, according to a news release issued following the news conference.
All Boeing’s 787s have been grounded pending the results of investigations into problems with the plane, most notably two reported fires.
On Jan. 7, cleaning and maintenance crews noticed smoke in the cabin after a 787 landed in Boston from Narita, Japan. Passengers already had left the plane when a mechanic noticed the fire, apparently coming from the APU battery in the aft electronics bay.
The airport’s fire and rescue personnel arrived on the scene and reported it was “controlled.”
On Jan. 16, another possible fire was reported in a battery in a plane that landed in Japan.
After officials acted to ground the planes, Boeing said it was halting 787 deliveries.
“One of these events alone is serious,” Hersman said, according to the release. “Two of them in close proximity, especially in an airplane model with only about 100,000 flight hours, underscores the importance of getting to the root cause of these incidents.”
In its investigation, investigators are looking for signs of thermal runaway, signs of electrical short circuiting, manufacturing defects and “anything unusual,” the NTSB said.
The batteries were manufactured by FS Yuasa and are unique to the 787, the agency said. The same battery model is used for the main airplane battery and for the battery that is used to start the auxiliary power unit, which is the one that caught fire in Boston.
Radiographic examinations of the battery involved in an incident and a second battery were conducted at an independent test facility.
The lab is examining the battery elements to identify possible contaminants or defects, the NTSB said.
Investigators also traveled to Securaplane in Tuscson, Ariz., and to UTC Aerospace Systems in Phoenix this week to look at the APU battery charging unit and the APU controller.
The battery charging unit passed all significant tests and no anomalies were detected, the NTSB said.
NTSB investigators are working with Boeing teams in Seattle to analyze activities related to the design and manufacture of the electrical battery system.
In Japan, investigators traveled to Kanto Aircraft Instrument Co. in Fujisawa, Kanagawa, to examine the APU battery monitoring unit involved in the Jan. 7 incident.
The team cleaned and examined the two battery monitoring unit circuit boards housed in the APU battery case.
The circuit boards were damaged, the NTSB said. That limited the information that could be obtained from testing, it said.
The next steps are to complete in-house laboratory examinations, the examination and testing of “exemplar” batteries, and synthesizing lab findings with fire forensics and the aviation systems investigation, the agency said.
No time frame has been established for releasing the Dreamliners for flight.