The U.S. National Transportation Safety Board has determined that several assumptions used in the Federal Aviation Administration’s application of nine special conditions in the certification of the lithium-ion battery system on the Boeing 787 proved incorrect, NTSB chairman Deborah Hersmann revealed Thursday during a media briefing at the board’s headquarters in Washington, D.C. The revelation raises questions about the effectiveness and diligence of FAA oversight in the certification of the 787 following two separate cases of burned batteries aboard in-service Dreamliners and the subsequent grounding of the entire world fleet.
Hersmann reported that the flight data recorder from the Japan Air Lines 787 that caught fire at Boston Logan Airport on January 7 showed that the total voltage in its failed APU battery unexpectedly dropped from a full charge of 32 volts to 28 volts, suggesting failure of a single four-volt cell. Further evidence uncovered during the examination of the battery suggests that “thermal runaway” began with internal short circuits in one of the power pack’s eight cells and spread to adjacent cells, causing the fire.
However, during its testing in the certification process to meet the special conditions, Boeing found no evidence of what Hersmann termed cell-to-cell propagation or fire. Boeing also determined that a single-cell failure would result in smoke emission less than once in every 10 million flight hours. Hersmann reported that the 787 fleet had accumulated less than 100,000 flight hours before the second failure.
“This investigation demonstrated that a short circuit in a single cell can propagate to adjacent cells and result in smoke and fire,” said Hersmann. "The assumptions used to certify the battery must be reconsidered.”
Next, the NTSB plans to examine the safety certification process used by the FAA and Boeing for the battery design “and determine why the hazards identified in [the] investigation were not mitigated,” added Hersmann. The safety board expects to open the public docket for comments and produce an interim factual report within 30 days.