"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. 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 testing of some of the batteries that have been removed from the 787 fleet from the field," she said. Meanwhile in a separate joint statement, Lahood and Huerta said the FAA is looking at the certification process and specifically at the required tests and design of the aircraft's lithium ion battery. The FAA invited the NTSB to observe this FAA-led process," they said. Welcoming the progress reported by the NTSB, Boeing said the findings will provide the public with a better understanding of the nature of the investigation.