TOKYO--After a 20-month investigation, Japan's transportation
safety authority said in a report Thursday it couldn't identify the
precise cause of a battery malfunction that prompted an emergency
grounding of a Boeing Co. 787 Dreamliner in Japan last year.
In a report, the authority said that instead of one single major
defect, a number of factors apparently contributed to a
short-circuiting that burned the main battery on an All Nippon
Airways-operated plane in 2013.
Investigators say that the severe damage to the battery made it
a difficult to analyze the accident.
The Dreamliner is the first commercial airliner to use lithium
ion batteries as its main power supply. The batteries were supplied
by Japan's GS Yuasa Corp.
The report requested that Boeing continue its probe into the
accident, and that the U.S. Federal Aviation Administration step up
safety checks on such batteries in the future.
The January 2013 incident occurred when the Dreamliner
experienced a malfunction of its main battery and was forced to
make an emergency landing. The battery was later found to be
severely damaged from overheating.
The trouble came just days after a Dreamliner operated by Japan
Airlines reported smoke from its battery while it was parked in
Boston. The incidents prompted a grounding of the global 787 fleet
for three and a half months.
Almost a year later, another battery overheating occurred on an
ANA plane parked in a Tokyo airport.
The fact that all three cases took place in January led Japanese
investigators to believe that cold temperatures likely played a
part in the malfunctioning.
The report speculated that a phenomenon called "deposition" was
behind the short-circuits. This happens when lithium metal builds
up in cold temperatures and forms needlelike objects in the battery
solution.
Deposition alone, however, is unlikely to cause such major
short-circuits, the report also said. It suspect other factors also
played a part, such as voltage spikes during a discharging of the
battery.
The incident was made worse after a failure in one cell cascaded
across the others inside the battery, the report said. As the cell
overheated, it became inflamed and came into contact with other
cells, creating even bigger short-circuits.
The possibility this happening wasn't properly recognized by
either the FAA or by the manufacturer, the report said.
Boeing Japan wasn't immediately available for comment.
Write to Mitsuru Obe at mitsuru.obe@wsj.com
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