Why that assumption was wrong, and why federal aviation officials signed off on the 787's lithium-ion battery system design, are the subject of a two-day hearing before the National Transportation Safety Board in Washington, D.C.
"We are looking for lessons learned, not just for the design and certification of the failed battery but also for knowledge that can be applied to emerging technologies going forward," Deborah Hersman, chairman of the NTSB, said at the beginning of Tuesday's session. That said, Hersman and other NTSB officials were pointed in their questions of Boeing and the FAA.
The hearing continues Wednesday and comes as 300 Boeing employees around the world work to retrofit 787s with a redesigned battery system. The Federal Aviation Administration signed off on that new system Friday, essentially clearing the 787 to return to passenger service after a three-month grounding.
But while the FAA approved Boeing's redesigned lithium-ion battery system, NTSB investigators have yet to find the cause of a Jan. 7 battery failure aboard a 787 parked in Boston. NTSB investigators have said the battery short-circuited, causing it to smoke and burn.
When the 787 system was first conceived, Boeing and battery-maker GS Yuasa were less concerned with short-circuiting than with overcharging. They deemed short-circuiting unlikely over the life of the 787 fleet.
"Our belief ... was that any form of internal short circuit could lead to venting of that cell and release of the electrolyte but nothing more than that," testified Mike Sinnett, Boeing's chief engineer for the 787.
Boeing calculated that the odds of a battery spilling flammable fluid, which occurred in the Boston incident, was one in 1 billion. The odds of a less-severe failure in which a battery overheated and emitted smoke or gas was one in 10 million, Boeing determined.
The model had less than 52,000 hours of fleet-wide flying time when the Jan. 7 short circuit took place. That event was followed by a Jan. 16 incident in Japan involving a 787 operated by All Nippon Airways.
Steve Boyd, manager of the FAA's airplane and flight crew interface branch, declined to speculate how dangerous the Jan. 7 battery failure would have been to the aircraft had it been in flight and not parked.
"I don't know how to make that judgment," Boyd said. "It's difficult for us to extrapolate from that event."
Boyd voiced confidence in Boeing's test process, in which FAA specialists were involved. That testing included shooting a nail into a lithium-ion battery to simulate a short circuit. The battery didn't burn in that test.
"We believed Boeing's approach was a reasonable approach to show compliance," Boyd said.
A 2009 fire at a company that made components for the lithium-ion batteries was caused by an internal short circuit, the same circumstance that triggered a January fire that led to the plane's grounding.
Details of thate fire at a plant in Arizona were contained in records released Tuesday as the NTSB opened the hearing.
After the 2009 fire at Securaplane Technologies, a division of Christchurch, England-based Meggitt, Boeing made several design changes to the battery to ensure it couldn't be recharged too quickly, according to the NTSB documents. The aircraft manufacturer didn't alter its assumption that an internal short-circuit couldn't trigger a fire.
Boeing conducted "state-of-the-art" testing of the Dreamliner and its battery system, Sinnett said.
In light of the two battery failures, "I think we may apply tighter test criteria," he said.
Other certification issues that might arise at the hearing include the battery's standards, set by the FAA, and Boeing's use of its own employees to sign off on some of the initial FAA certification tests. The FAA allows engineers from aircraft manufacturers such as Boeing to sign off on designs because the agency lacks the resources and expertise. The agency retains the final authority to approve designs.
A panel of government and industry experts, including FAA and Boeing representatives, devised more rigorous testing standards for lithium-ion batteries in 2008, according to safety board documents. Those standards weren't applied to the 787 because the FAA had already set conditions on its batteries, according to the documents.
The FAA didn't update battery requirements after the new standards were written because it didn't believe that the 787's battery was unsafe, Ali Bahrami, manager of the agency's Transport Airplane Directorate, said at the hearing.
Boeing has more than 800 orders for the mostly composite 787. Company officials said last week they still plan to boost production on the 787 to a pace of 10 jets monthly by the end of 2013. Boeing reports first-quarter earnings on Wednesday.
Bloomberg News contributed to this report.