NTSB Issues Interim Report on JAL 787 Battery Incident
The U.S. National Transportation Safety Board (NTSB) continues to focus on the design, certification and manufacturing processes of the lithium-ion battery system used on the grounded Boeing 787.
On March 7, the board published an “interim factual report” of its investigation into the January 7 battery fire discovered on a Japan Airlines (JAL) 787-8 parked at Boston’s Logan Airport. The lithium-ion battery used to start the aircraft’s APU, manufactured by GS Yuasa of Japan, remains central to the investigation. The JAL battery fire, followed on January 16 by a main battery incident that forced the emergency landing of an All Nippon Airways 787 in Japan, led to the worldwide grounding of the 787 fleet.
The APU battery consists of eight lithium-ion cells, assembled in two rows of four cells each. Each cell contains a flammable electrolyte liquid, and has a nominal voltage of 3.7 volts. When the JAL incident battery was disassembled, cells 5 through 8 on the right side of the battery showed the most thermal and mechanical damage, according to the NTSB. “Thermal damage was the most severe near cell 6,” the interim report states. “Continuity measurements using a digital volt meter indicated that all of the cells were found to be electrically short circuited except for cell 8.” “Vent discs,” plates that rupture when the internal pressure in a cell reaches a predetermined level, were found to be opened slightly on cells 1 through 3, intact on cell 4 and “opened more completely, leaving a ruptured appearance” on cells 5 through 8, the board said.
The aircraft’s flight data recorder (FDR) showed that the APU was started while the 787 was being taxied to the gate after arriving in Boston. Smoke was later detected in the aft cabin by cleaning personnel; at about the same time, a maintenance manager in the cockpit observed that the APU had shut down automatically. “The FDR data also showed that, about 36 seconds before the APU shut down…the voltage of the APU battery began fluctuating, dropping from a full charge of 32 volts to 28 volts about seven seconds before the (APU) shutdown,” the NTSB said.
The U.S. Federal Aviation Administration determined that applicable airworthiness standards did not address lithium-ion batteries, so it issued nine special conditions for their use on the 787. During the 787 certification process, Boeing performed a safety assessment to determine the potential hazards that could result from various electrical system component failures. The airframer decided that “the probability that a battery could vent was once in every 10 million flight hours,” the NTSB said. As of January 16, the 787 in-service fleet had accumulated less than 52,000 flight hours, and had experienced two separate incidents involving smoke emissions from a lithium-ion battery, the board noted.