Japan's transport safety agency said it is still unclear whether battery chemistry or an electrical issue caused a main battery on a Boeing Co 787 Dreamliner operated by All Nippon Airways to overheat last month, forcing it to make an emergency landing.
Investigators may widen their probe of the battery problems to other equipment on the technologically advanced aircraft, Japan Transport Safety Board (JTSB) chairman Norihiro Goto told a news conference.
All Boeing's 787s are out of action as investigators in Japan and the United States try to find the cause of two incidents with the plane's lithium-ion batteries - a battery fire on a Japan Airlines 787 at a U.S. airport and the emergency landing on a domestic ANA flight after battery problems triggered a smoke alarm.
Goto also said CT scans showed six of the main battery's eight cells on the ANA Dreamliner were badly damaged, charred and deformed.
On Monday, Boeing asked the Federal Aviation Administration (FAA) for permission to conduct Dreamliner test flights, suggesting it is making progress in finding a solution to the battery problems.
Goto said Boeing had not discussed its test flight request with the Japanese agency, and he did not know if the planemaker had found clues as to the cause of the battery problems. The test flights "could mean they have made progress," he said.
Launch customer ANA is the world's biggest Dreamliner operator with 17 of the jets. With local rival JAL owning seven, Japan accounts for almost half the 50 787s that have been grounded since Jan. 17.
U.S. officials on Friday said they are making progress in their investigation into the battery fire on JAL's jet, although they have yet to set any timetable for completing their work.
ANA said last week it lost around $15 million in revenue as a result of the Dreamliner grounding, while JAL said the halting of 787 flights would shave $7.6 million from its operating profit in the year to end-March. Both companies have said they will discuss compensation for the losses with Boeing.
GS Yuasa, the Japanese firm that makes batteries for the Boeing 787, said its April-December net profit fell 3.6 percent to 5.52 billion yen ($59.55 million) on revenue that slipped 4.6 percent to 196 billion yen.
The company forecast its full-year net profit would drop by almost a third to 8 billion yen, while sales would be up a touch at 288 billion yen.