The Japan Transport Safety Board (JTSB) has released findings from an investigation into the 16 January 2013 main battery incident aboard an All Nippon Airways Boeing 787-8 aircraft, which resulted in an emergency landing and contributed to a global grounding of the type.
While the 100-page investigation report does not identify a specific cause for the lithium ion battery failure aboard ANA flight 692, it provides probable causes and has recommendations for both the FAA and Boeing.
“Internal heat generation in cell six very likely developed into venting, making it the initiating cell, resulting in cell-to-cell propagation and subsequent failure of the main battery,” says the report.
“It is very likely that cell six internal heat generation and increased internal pressure caused it to swell, melt the surrounding insulation material and contact the brace bar creating a grounding path that allowed high currents to flow through the battery box. The currents generated arcing internal to the battery that contributed to cell-to-cell propagation consequently destroying the battery.”
It adds that the heat generation in cell six was “probably” due to a short circuit, but stresses that “the conclusive mechanism thereof was not identified.”
The ANA incident occurred while the aircraft bearing registration JA804A was operating the Yamaguchi Ube-Tokyo route. After the flight crew received several battery warnings and smelled fumes in the cockpit, they made an emergency landing at Takamatsu, where all 129 passengers and eight crew evacuated via the aircraft’s slides.
The ANA incident followed another lithium ion battery failure on 11 January 2013 aboard a Japan Airlines 787 parked at Boston’s Logan International Airport. The two incidents resulted in a four-month grounding of the global 787-8 fleet in the beginning of 2013.
The JTSB investigation commenced in 16 January 2013, with a site inspection of the aircraft at Takamatsu and interviews with personnel involved with the incident. In 2013, it conducted extensive research into the 787-8’s battery including experiments. During the course of the investigation it held progress meetings with Boeing and the US National Transportation Safety Board.
Echoing comments from both the NTSB and Boeing itself, the JTSB stated that the airframer’s original testing regimen for the battery was insufficient, mainly because it did not adequately simulate a realistic operational scenario.
“An internal short circuit test by nail penetration method under the simulated on-board configuration with the battery ground wire demonstrated a thermal runaway, while the test without the ground wire did not,” says the JTSB.
This test lead the JTSB to believe that when the battery was undergoing developmental testing, the battery box was not grounded with a ground wire.
“It is very likely that the engineering test conducted during the developmental phase did not develop into thermal runaway because the battery box was not grounded with the ground wire. This demonstrates that it was inappropriate to exclude the internal short circuit test from the safety assessment based on the test result which was not conducted simulating the actual airplane configuration.”
It recommends that the FAA instruct manufacturers to test equipment under conditions simulating actual airplane configurations, and review (and if necessary revise) technical standards for testing lithium ion batteries.
It also suggests that the FAA examine the estimated failure rates of lithium ion batteries during the 787’s certification campaign, and based on its findings review the safety assessment. It also recommends reviewing “the type certificate for its appropriateness on heat propagation risk.”
In its recommendations for Boeing, it notes that low temperatures when the aircraft was parked the night of 15 January could have contributed to the internal short circuit in the battery. It suggests that the “FAA should supervise Boeing” to continue studying ways to improve lithium ion battery safety, including short circuit mechanisms, and look at ways to improve battery charging systems and their contacts with the battery.
The JAL and ANA incidents saw Boeing redesign the 787-8’s lithium ion batteries by moving the cells farther apart. It also encased them in a stainless steel box and creating a vent to move smoke off the aircraft in the event of a battery fire. An NTSB report into the JAL incident is expected in the coming weeks.