US agencies clear Boeing 787 Dreamliner test flights: Report
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"At this time, we've been able to rule out two possible causes. We have ruled out the mechanical impact damage to the battery. All mechanical damage to the cells and the
battery case occurred after the initiating the short-circuiting in cell number six," she said.
The board, in its probe, has also ruled out external short-circuiting of the cells or battery, she said, adding several potential causes for the short-circuiting in cell
number six are being examined.
"We are looking for evidence of contamination, electro folds, wrinkles and pinches in the assembly of cells and the battery. And because the 787 battery is really a collection of eight individual cells packaged together in a box, we're looking at the total design of the battery", she said.
The NTSB also blamed the FAA certification process for having given approval for the battery
"Because the 787 incorporated a number of what the FAA calls 'novel or unusual design features', the applicable airworthiness regulations at the time did not address those new features."
"So as a result, in 2007, the FAA issued nine special conditions for the 787 with regard to the use of lithium ion batteries. These special conditions contained additional safety requirements that the FAA deemed necessary to ensure an
equivalent level of safety," Hersman said.
"Our investigators found that during the certification process, Boeing studied possible failures that could occur within the battery."
"They, number one, assessed the likelihood of the failure occurring, and two, they assessed the effects that that failure would have on the battery," she said.
"To do this, various tests were performed on battery cells. One test was characterised as abusive, and it was intended to short-circuit the induce short-circuiting and cell
venting," she said.
"Boeing has indicated that these tests that were conducted prior to certification showed no evidence of cell-to-cell propagation or fire in the battery. However, our investigative findings with respect to the event battery show that when a short circuit did occur, it resulted in cell-to-cell propagation in a cascading manner and a fire," Hersman said.