NTSB: 787 Battery Shows Evidence of 'Thermal Runaway'
The damaged lithium-ion APU battery from the Japan Air Lines Boeing 787 that caught fire on January 7 while parked at Boston’s Logan Airport experienced an uncontrolled chemical reaction known as a “thermal runway” and short circuiting, but the cause and sequence of these events are still unknown, according to the U.S. National Transportation Safety Board (NTSB).
Seventeen days into its investigation of the incident, the first of two separate battery failures that led to the grounding of the worldwide 787 fleet, the NTSB conducted an update briefing on Thursday at its Washington, D.C., headquarters. “Today I can tell you what we know so far,” said Deborah Hersman, the board chairman. “We know that the lithium-ion battery experienced a thermal runaway. We know that there were short circuits, and we know that there was a fire. The work that we continue to do will tell us why these things happened.”
Hersman said “cells 5, 6 and 7” of the eight-cell battery unit manufactured by GS Yuasa of Japan “are of particular interest to our investigators because they appear to have experienced the most thermal damage,” based on a computed tomography (CT) scan of the battery assembly and examination of the individual cells.
The NTSB investigative team that responded to the scene reported that damage to surrounding structures and components was confined to a 20-inch radius around the APU battery rack. “The APU battery was spewing molten electrolytes, very hot material,” she said. “We did also see some damage involved with the extrication of the battery as well. The firefighting response teams (took) the battery out before NTSB investigators arrived, so we were examining the scene after the battery was removed.”
Hersman declined to estimate how long the NTSB’s investigation will take. She deferred to the FAA a question about whether the worldwide grounding of the 787 fleet was an overreaction, but said that her board “respects the FAA’s decision.” The JAL battery fire, followed on January 16 by a main battery fire that forced the emergency landing of an All Nippon Airways 787 in Japan, are serious and extraordinary events, she said. “The expectation in aviation is to never experience a fire on board an aircraft. In two weeks time, we saw two cases of battery failures on the 787 and the grounding of the entire fleet by the FAA. The significance of these events cannot be understated.”