The Japanese probe into a battery problem that forced the emergency landing of a Boeing Dreamliner last year wrapped up Thursday with investigators saying they still haven't found the root cause.
The domestic All Nippon Airways (ANA) flight on January 16, 2013 was forced to make an emergency landing after pilots noticed a burning smell inside the cockpit that was traced to the plane's lithium-ion battery pack.
But neither Boeing nor battery manufacturer GS Yuasa were able to pinpoint what caused the battery to overheat and on Thursday the Japan Transport Safety Board (JTSB) issued its final report that suggested a short circuit might be responsible.
"The heating phenomenon that began in the number-six cell (of the main battery on board the plane) is believed to have been caused by an internal short-circuit. However, in the end, its developmental mechanism could not be identified," said the 115-page report.
The flight from Ube in Japan's far west to Tokyo marked the most serious case of battery overheating in the Dreamliner which features a composite fibre fuselage that reduces weight and boosts fuel efficiency.
ANA, the single biggest operator of 787s, and its domestic rival Japan Airlines (JAL) were among the carriers hit by the worldwide grounding of Boeing's plane following a series of battery problems that also led to a fire onboard an empty JAL plane parked at Boston's Logan Airport.
The Japanese report noted that specks of metal were found inside the battery pack and might be linked to the overheating, but GS Yuasa has rejected suggestions that the impurity was dropped into the packs during manufacturing.
Among the 137 passengers and crew members on board, four passengers suffered minor injuries when they came off an emergency slide, the probe said.
That incident prompted a still-ongoing investigation by US National Transportation Safety Board (NTSB).
Boeing admitted in April last year that, despite months of testing, it did not know the root cause of the battery problems, but it rolled out modifications to prevent a recurrence.
The Dreamliner has also been hit by a series of unrelated glitches, including a fault with an air pressure sensor and the brake system.
Despite the troubles, the aircraft remains popular. This week Ethiopian Airlines agreed to buy 20 Boeing 737 aircraft in a deal worth $2.1 billion.