“At this point in time, the lab has not come to an understanding of what the root cause is, so how do you go about designing a fix when you don’t know what the root cause is?” NTSB member Earl Weener asked of Boeing during the hearing.
Jerry Hulm, a Boeing systems engineer and associate technical fellow, answered that the company had developed a two-pronged approach focusing on protecting the battery and the aircraft. The FAA approved the fix on April 19.
“Not knowing the exact root cause, our detailed analysis highlighted some improvements that could be made that would improve the capability of the battery to avoid these types of events,” Hulm said of the additional insulation now added between the eight battery cells. “Not knowing the root cause, we had to preclude the smoke from getting into the aircraft,” he said of the new 1/8th-in.-thick stainless steel battery enclosure with a vent line assembly that dumps smoke overboard if a thermal runaway in one cell does occur.
In its continued search for the root cause, the NTSB says it is planning to “conduct teardown examinations as soon as possible” of several aircraft batteries similar to “one involved in an aircraft incident”, which is where the CT scans come in.
“To facilitate those examinations, CT scans of these batteries and their subcomponents are required to non-destructively determine as much information as possible about those components,” the NTSB says.