Investigations of separate incidents involving Boeing 787s in the U.S. and Japan appear to concur that the batteries that burned in each case did not overcharge. But investigators continue to seek causes for the two incidents that led to the grounding of the worldwide 787 fleet. The probes remain focused on the eight-cell lithium-ion batteries manufactured by Japan’s GS Yuasa for Thales, which supplies the 787’s electrical power conversion system.
At a news conference in Tokyo on January 23, Norihiro Goto, chairman of the Japan Transport Safety Board (JTSB), said that evidence from the flight data recorder (FDR) of All Nippon Airways Flight 692, which needed to make an emergency landing at Takamatsu airport on January 16, indicates that a malfunctioning lithium-ion main battery did not exceed its maximum voltage of 32 volts. The pilots diverted the aircraft from its flight plan after receiving a cockpit warning of a battery failure. “On the surface, it appears there was no overcharging,” Goto said, according to Reuters.
On January 24, the U.S. National Transportation Safety Board (NTSB) provided a fourth update of its investigation into the APU battery fire on board a Japan Air Lines 787 parked at Boston Logan International Airport on January 7. Deborah Hersman, the Safety Board chairman, said the battery involved in that incident showed signs of electrical short circuiting and “thermal runaway,” an uncontrolled chemical reaction. “We do have some traces on the FDR regarding the battery,” she said. “We do not have any data from the FDR that shows an exceedance of 32 volts.” The NTSB gave reporters a tour of the materials lab at its Washington, D.C. headquarters, where its analysis of the incident battery continues. There the Safety Board displayed the burned battery case and two sets of electrodes unwound on a table–one charred electrode from a damaged battery cell and one from an undamaged cell.
Hersman said the NTSB and Japan’s JTSB work together closely and share information on the two incidents. She said it is too early to know whether or not the incidents share a common cause. “Once we have the opportunity to see what [the JTSB is] finding, we are very much looking forward to being able to compare and collaborate on whether or not we’re seeing similarities or differences with the batteries as we’re doing those teardowns,” she said.