by Devesh Agarwal
Updated Feb 8, 2013, 0100 GMT, 0630 IST
Ms. Hersman's presentation and the video of her briefing the media is at the end of this story.
“U.S. airlines carry about two million people through the skies safely every day, which has been achieved in large part through design redundancy and layers of defense,” “Our task now is to see if enough – and appropriate – layers of defense and adequate checks were built into the design, certification and manufacturing of this battery.”After an exhaustive examination of the JAL lithium-ion battery, which was comprised of eight individual cells, investigators determined that the majority of evidence from the flight data recorder and both thermal and mechanical damage pointed to an initiating event in a single cell. That cell showed multiple signs of short circuiting, leading to a thermal runaway condition, which then cascaded to other cells. Charred battery components indicated that the temperature inside the battery case exceeded 500 degrees Fahrenheit.
As investigators work to find the cause of the initiating short circuit, they ruled out both mechanical impact damage to the battery and external short circuiting. It was determined that signs of deformation and electrical arcing on the battery case occurred as a result of the battery malfunction and were not related to its cause.
Chairman Hersman said that potential causes of the initiating short circuit currently being evaluated include battery charging, the design and construction of the battery, and the possibility of defects introduced during the manufacturing process.
During the 787 certification process, Boeing studied possible failures that could occur within the battery. Those assessments included the likelihood of particular types of failures occurring, as well as the effects they could have on the battery. In tests to validate these assessments, Boeing found no evidence of cell-to-cell propagation or fire, both of which occurred in the JAL event.
The NTSB learned that as part of the risk assessment Boeing conducted during the certification process, it determined that the likelihood of a smoke emission event from a 787 battery would occur less than once in every 10 million flight hours. Noting that there have been two critical battery events on the 787 fleet with fewer than 100,000 flight hours, Hersman said that “the failure rate was higher than predicted as part of the certification process and the possibility that a short circuit in a single cell could propagate to adjacent cells and result in smoke and fire must be reconsidered.”
As the investigation continues, which will include testing on some of the batteries that had been replaced after being in service in the 787 fleet, the NTSB will continue to share its findings in real time with the FAA, Boeing, the Japan Transport Safety Board, and the French investigative agency, the Bureau d'Enquêtes et d'Analyses (BEA).
“The decision to return the fleet to flight will be made by the FAA, which underscores the importance of cooperation and coordination between our agencies,” Hersman said.
She also announced that the NTSB would release an interim report of factual findings within 30 days.
NTSB Presentation. Download it here.
Video of briefing
FAA’s Special Conditions for the B-787 battery system. Download it here.
Airframer Boeing issued this statement
Boeing welcomes the progress reported by the U.S. National Transportation Safety Board (NTSB) in the 787 investigation, including that the NTSB has identified the origin of the event as having been within the battery. The findings discussed today demonstrated a narrowing of the focus of the investigation to short circuiting observed in the battery, while providing the public with a better understanding of the nature of the investigation.
The company remains committed to working with the NTSB, the U.S. Federal Aviation Administration (FAA) and our customers to maintain the high level of safety the traveling public expects and that the air transport system has delivered. We continue to provide support to the investigative groups as they work to further understand these events and as we work to prevent such incidents in the future. The safety of passengers and crew members who fly aboard Boeing airplanes is our highest priority.
The 787 was certified following a rigorous Boeing test program and an extensive certification program conducted by the FAA. We provided testing and analysis in support of the requirements of the FAA special conditions associated with the use of lithium ion batteries. We are working collaboratively to address questions about our testing and compliance with certification standards, and we will not hesitate to make changes that lead to improved testing processes and products.