The National Transportation Safety Board has pinpointed the start of the 787 Dreamliner battery fire on a parked Japan Airlines jet a month ago as a short circuit inside a single cell.
The agency still hasn’t identified the cause of the initial short circuit but has narrowed down the suspects.
Details provided by the NTSB make clear that Boeing will have to redesign the battery for a long-term fix.
In addition, the NTSB pointed to failures in the airplane certification process conducted by Boeing and the Federal Aviation Administration, which failed to identify the hazards revealed by this incident.
“The assumptions used to certify the battery must be reconsidered,” NTSB chief Deborah Hersman said in a detailed media briefing. “Our task now is to see if appropriate layers of defense and checks were built into the design, certification and manufacturing process.”
The overheating that started in cell 6 of the eight-cell battery spread to all the others and caused the fire, Hersman said.
She said Boeing assessed the chances of a single cell short circuit and the impact that such an event might have on the battery in various tests conducted during the certification process.
Boeing concluded that these tests “showed no evidence of cell-to-cell propagation or fire in the battery.”
But in the fire on the Japan Airlines jet, that’s exactly what happened.
In another certification test, Hersman said, Boeing studied the possibility that a failure in a single cell would result in smoke emission from the battery and estimated that this would happen “less than once in every 10 million flight hours.”
“The 787 fleet has accumulated less than 100,000 flight hours,” Hersman said. “Yet there have now been two battery events resulting in smoke less than two weeks apart on two different aircraft.”
The second event, several days after the fire on the Japan Airlines plane, forced an All Nippon Airways 787 flight in Japan to make an emergency landing.
Hersman identified three potential root causes for the short circuit:
But whichever of these is found to have started the overheating, it’s clear from Hersman’s remarks that the battery safety features failed to cope with the initial failure and Boeing will have to revisit the design.
“We’re looking at the total design of the battery, including the physical separation of cells, their electrical interconnections and their thermal isolation from each other,” Hersman said.
In a statement, Boeing said it remained committed to working with the NTSB, the FAA and its customers.
“We continue to provide support to the investigative groups as they work to further understand these events and as we work to prevent such incidents in the future,” Boeing said. “The safety of passengers and crew members who fly aboard Boeing airplanes is our highest priority.”
Boeing’s 787 was certified following a rigorous Boeing test program and an extensive certification program conducted by the FAA,” the company said. “We provided testing and analysis in support of the requirements of the FAA special conditions associated with the use of lithium ion batteries. We are working collaboratively to address questions about our testing and compliance with certification standards, and we will not hesitate to make changes that lead to improved testing processes and products.”
Contributing: Molly McMillin of The Eagle