US investigators Monday said the three-month grounding of the Boeing 787 in 2013 was due to a defect in the battery that was poorly addressed by manufacturers and regulators.
The National Transportation Safety Board's report divided blame for a pair of battery fire incidents in January 2013 that prompted a global grounding of the 787 Dreamliners. The planes were cleared to fly three months later when Boeing won approval for a battery fix.
The NTSB concluded that the probable cause of fires on planes in Boston and Japan was an internal short-circuit in the battery, which led to a "thermal runaway" that resulted in smoke and fire.
The report said Boeing's management of contractors Thales Avionics, which designed the electrical conversion system, and GS Yuasa, which made the batteries, "did not ensure" that manufacturing was "consistent with established industry practices."
Also, Boeing incorrectly assumed that the impact of a short circuit in the battery would be limited. The aerospace giant did not consider the most severe effects of a short-circut, or how to mitigate such a risk, the report said.
At the same time, the NTSB said the US Federal Aviation Administration also did not recognize the potential impact of an internal short circuit and, as a result, did not require adequate testing for it.
Boeing endorsed the NTSB's finding on what went wrong with the batteries.
"We remain confident in the comprehensive improvements made to the 787 battery system following this event, and in the overall performance of the battery system and the safety of the airplane," Boeing said.