TOKYO--Japanese aviation regulators remain stumped over the root
cause of the battery incident that led to the worldwide grounding
of the Boeing Co. (BA) 787 Dreamliner jet, echoing the apparent
lack of progress made by their U.S. counterparts several weeks into
their respective investigations.
The Japan Transport Safety Board said Tuesday that CT scans--a
kind of X-ray technology that creates three-dimensional
images--revealed the extent of damage to the powerful lithium-ion
battery. But the scans offered few useful clues as to what had
caused the battery to overheat. That has led investigators to widen
the range of possibilities of what went wrong, rather than narrow
it as the probe enters its third week.
Asked whether it was true the investigation had made little
progress, JTSB chairman Norihiro Goto said, "Unfortunately that may
be right."
Mr. Goto said it wasn't clear when the investigation would end.
But the chances of that being soon seemed to dim, as the JTSB said
the latest analysis means the probe will be broadened to include
related electrical systems and parts.
Mr. Goto said there were indications that a "thermal runaway"
had taken place--a hazardous state in which the temperature of a
battery cell jumps dramatically and the heat spreads quickly to
other cells inside the battery. But he said the sequence of events
that led some parts of the battery to incur greater damage than
others was unclear.
The U.S. National Transportation Safety Board has said the Japan
Airlines Co. (9201.TO) 787 jet parked in Boston on Jan. 7
experienced both a thermal runaway and short circuits.
Nine days later, warnings of a burning battery in mid-flight
prompted an All Nippon Airways Co. (9202.TO) Dreamliner to make an
emergency landing in Japan. The U.S. Federal Aviation
Administration subsequently ordered the grounding of all 787
aircraft. Other regulators around the world soon followed suit.
Images from CT scans taken by the Japan Aerospace Exploration
Agency two weeks ago, and released Tuesday, showed all eight
battery cells in two rows inside the pack had been damaged by
overheating.
Two cells sitting side-by-side in an inner row incurred the most
damage, appearing more charred than the others. A neighboring cell
had a small hole about two thirds of the way down its side, and in
six of the eight cells, thin plates dangling near the positive
electrodes had melted.
The JTSB said it sent two parts--the contactor and the battery
diode module--to France, where they were made, to be examined by
the French equivalent of the NTSB.
The contactor, which is located inside the battery pack and
which suffered some damage, acts as a switch that can cut the
battery's voltage if it exceeds the allowed amount. The battery
diode module sits outside the main battery and only allows the
current to flow in one direction.
Japan's aviation officials also said they sent another two
parts--the bus power control unit and the generator control
unit--to the manufacturing company in Phoenix, Arizona last week in
an effort to extract any memory data left on the boards. The JTSB
declined to disclose the name of the U.S. company.
Write to Yoree Koh at yoree.koh@wsj.com and Yoshio Takahashi at
yoshio.takahashi@dowjones.com
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