US agencies clear Boeing 787 Dreamliner test flights: Report
"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.
Another condition considered by Boeing during certification was whether a failure that resulted in a single cell venting through its ruptured disk would create smoke
emission from the battery, she said.
Boeing assessed that the likelihood of a smoke emission event from a 787 battery will occur less than once in every 10 million flight hours.
The 787 fleet has accumulated less than 1,00,000 flight hours, yet there have now been two battery events resulting in smoke less than two weeks apart on two different aircraft, Hersman observed.
She said the investigation has demonstrated that a short circuit in a single cell can propagate to adjacent cells and result in smoke and fire.
"The assumptions used to certify the battery must be reconsidered. As we move forward, we will begin