The National Transportation Safety Board faulted both in a final report for not anticipating how the power packs might fail, and cited battery maker GS Yuasa for poor manufacturing.
Boeing said the NTSB's report had advanced the understanding of this incident.
"We concur with the report's probable cause finding - a short circuit within one battery cell led to venting and cell-to-cell propagation that caused the battery failure,'' a spokesperson said.
"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.''
The spokesperson said it was working on reliability improvements on the 787 and will do so throughout the span of the programme.
Read also:
• As dreamliner catches fire at Heathrow, will Boeing ground them all?
• Air NZ up with play on latest Dreamliner fire
• Dreamliner safety review could be cause of concern for Air NZ
Monday's findings bring to a close an almost two-year probe into events that triggered the longest grounding of a large commercial aircraft by US regulators since jets were introduced in the 1950s.
It also prompted a re-examination of the dangers of lithium-ion power packs that have helped drive advances in personal electronic devices and electric cars.
The fire occurred on January 7, 2013, while a Japan Airlines 787 Dreamliner sat at Boston's Logan International Airport. Boeing uses two lithium-ion batteries in the Dreamliner to power electronics and other equipment. It was a short circuit in one of the battery's eight cells that triggered a runaway failure that engulfed the entire power pack, the NTSB said.
Design failure
"The incident resulted from Boeing's failure to incorporate design requirements to mitigate the most severe effects of an internal short circuit" and the inability of FAA inspectors to recognise those deficiencies, the NTSB concluded.
The NTSB issued 16 new recommendations calling on the FAA to tighten its watch over new technology and improve guidance to its inspectors. It also asked Boeing to improve oversight of subcontractors and revise how it conducts safety assessments.
Boeing didn't immediately have a response to the report, Doug Alder, a company spokesman, said in an interview.
GS Yuasa didn't respond to an email request for comment.
Boeing redesign
Boeing has already redesigned the battery to include more protection around the cells to contain overheating, a steel case to prevent any fire from spreading and a tube that vents fumes outside the fuselage. The company said in March that it believes the fixes improved battery safety.
Even with those measures, the NTSB said its testing found the large lithium-ion batteries were vulnerable to failure. Cells may overheat when large amounts of power are being drawn and better protections should be installed, the NTSB said.
Japanese investigators reached similar conclusions as the NTSB while probing another battery incident that occurred during a flight and forced an emergency landing at Takamatsu Airport on January 16, 2013. The Japan Transport Safety Board found in a September 25 report an internal short-circuit "was probably" at fault though it was impossible to say what prompted it.
Plagued development
The 2013 battery failures came as Boeing struggled to move past the design and production miscues that plagued development of the Dreamliner, the Chicago-based planemaker's first all-new jet of the century. The 787 entered the market 3 1/2 years behind schedule in 2011, slowed by issues ranging from an in-flight electrical fire to shortages of titanium fasteners.
The negative publicity has died away this year as Boeing reached a steadier production tempo. Boeing had delivered 207 Dreamliners to 23 customers as of November 19, its website shows.
The Dreamliner was the first commercial jetliner built with a carbon-fibre air frame instead of aluminum, using more electricity than earlier models to produce efficiency gains. A division of France's Thales was contracted by Boeing to design the electrical system.
Potentially dangerous
As part of that design, Boeing installed two lithium-ion batteries, which hold more energy and last longer than older technology. Those factors also make them potentially more dangerous because they are made with flammable chemicals and contain enough energy to self-ignite if they malfunction.
They have been linked to other aviation incidents and accidents. A smaller lithium-ion battery used to power an emergency-locator beacon caught fire on a 787 on the ground in London on July 12, 2013.
Boeing had estimated that the chances of a single cell on one of its 787 batteries failing and venting flammable chemicals was one in 10 million. When the second failure occurred in Japan, the aircraft had flown just 52,000 hours, according to the NTSB.
This miscalculation was part of a cascading series of failures in the design and certification process, the safety board concluded.
Different batteries
The battery tested for possible failure by GS Yuasa wasn't the same as the ones installed on the Dreamliner fleet and the tests didn't anticipate the most severe conditions seen in service, the investigation found.
An inspection of GS Yuasa's manufacturing plant by the NTSB found evidence that foreign debris was allowed to contaminate batteries, "which could lead to internal short circuiting."
The company's inspections also couldn't detect other internal defects capable of producing short circuits. The report didn't blame those issues for the fire.
Boeing also failed to anticipate the battery's risks, the NTSB said. The company's engineers didn't even consider the potential for a single cell overheating and igniting adjoining cells, according to the report.
The FAA didn't give its inspectors sufficient guidance on overseeing the battery design and the agency lacked expertise, according to the NTSB.
The safety board has no regulatory authority and must rely on non-binding recommendations to improve safety.
In response to earlier recommendations, FAA Administrator Michael Huerta told the NTSB that the agency is studying additional battery test requirements, according to an August 19 letter. The agency is working with RTCA, a Washington-based nonprofit that advises the FAA on technology.
- Grant Bradley / Bloomberg
