Deborah Hersman, chairwoman of the National Transportation Safety Board,… (Andrew Harrer, Bloomberg )
Though a month of investigating the grounded Boeing Co. Dreamliner 787 passenger jet and its fire-prone batteries has turned up clues, federal officials reported that the cause of the problem remains a mystery.
Meanwhile, airlines have made other arrangements for passengers for the foreseeable future as its 787s are grounded and the battery issue is investigated.
Speaking to reporters Thursday from Washington, National Transportation Safety Board Chairwoman Deborah Hersman said investigators found a short-circuit in a lithium-ion battery on one aircraft and even traced it to a specific cell, but they didn't yet determine the root cause.
Also on Thursday, the Federal Aviation Administration permitted Boeing to conduct test flights of the 787 for the first time in three weeks to gather additional data to return the aircraft to service.
The announcements were the latest updates of the safety investigation into the 787 after the FAA grounded the aircraft Jan. 16 following two fires within two weeks involving the batteries. Since then, investigators have probed the plane's battery design and certification process.
In her comments, Hersman said that some assumptions made by federal regulators and Boeing about the 787 were "faulty."
"Our task now is to see if enough — and appropriate — layers of defense and adequate checks were built into the design, certification and manufacturing of this battery," she said. "The failure rate was higher than predicted as part of the certification process, and the possibility that a short-circuit in a single cell could propagate to adjacent cells and result in smoke and fire must be reconsidered."
When the plane was being certified for flight operations, Boeing determined that the likelihood of smoke or fire from a 787 battery would occur less than once in every 10 million flight hours, she said. But already, there have been two critical battery events on the 787 fleet with fewer than 100,000 flight hours.
The 787's battery system, which is made in Japan by Kyoto-based GS Yuasa Corp., contains a cluster of eight individual cells packaged together in one box. In a 787 fire that occurred Jan. 6 at Boston Logan Airport, Hersman said investigators determined that all the recent mechanical damage to the cells and the battery case occurred after the short-circuiting in cell No. 6.
The battery then experienced "thermal runaway," a chain reaction in which heat spreads rapidly from cell to cell. The heat cascaded to other cells in the battery, as is evidenced by the damage to those adjacent cells, Hersman said.
Potential causes of the initiating short-circuit that are being evaluated include battery charging, the design and construction of the battery, and the possibility of defects introduced during the manufacturing process, she said.
The NTSB is also working with officials from Japan — where the second fire occurred on Jan 16. — as well as Boeing, the FAA, the Navy and investigators in France.
"The decision to return the fleet to flight will be made by the FAA," she said, "which underscores the importance of cooperation and coordination between our agencies."
The FAA has been aware of the potential flammability of lithium-ion batteries for years. Still, when the agency was certifying the Dreamliner for flight operation, it issued special conditions in 2007 for lithium-ion battery installations on the Dreamliner because its regulations didn't cover the technology.
The FAA issued a statement Thursday on behalf of Transportation Secretary Ray LaHood and FAA chief Michael Huerta that said the FAA was in the midst of a comprehensive review of the 787's critical systems, including the aircraft's design, manufacture and assembly.
The principal purpose of the agency's upcoming test flights will be to collect data about the battery and electrical systems while the aircraft is airborne.
"Based on what information our experts find, the FAA will take any action necessary to further ensure safety," the statement said. "We must finish this work before reaching conclusions about what changes or improvements the FAA should make going forward. The leading experts in this field are working to understand what happened and how we can safely get these aircraft back into service."
Boeing has delivered 50 787s to eight airlines worldwide. Six are owned by United Airlines — the only U.S. carrier that has 787s in its fleet.
The airlines have had to find new planes for passengers while its 787s sit idle.
Japanese carrier All Nippon Airways, which has 17 of the new planes, said it expecting 126,220 passengers to be affected by 1,887 flight cancellations through March 30. United has planned for its 787s to be out of service through February.