Even before two battery failures led to the grounding of all Boeing 787 jets this month, the lithium-ion batteries used on the aircraft had experienced multiple problems that raised questions about their reliability.
Officials at Japan's All Nippon Airways, the jets' biggest operator, said in an interview Tuesday that it had replaced 10 of the batteries in the months before fire and smoke in two cases caused regulators around the world to ground the jets.
The airline said it told Boeing of the replacements as they occurred but was not required to report them to safety regulators because no flights were canceled. Officials with the U.S. National Transportation Safety Board said Tuesday that the battery replacements were now part of their inquiry.
The airline also explained the extent of the previous problems, which underscore the volatile nature of the batteries and add to concerns about whether Boeing and other plane manufacturers will be able to use them safely.
In five of the 10 replacements, All Nippon said that the main battery showed an unexpectedly low charge. The airline said that in three instances, the main battery failed to start normally and had to be replaced along with the charger. In other cases, one battery showed an error reading and another, used to start the auxiliary power unit, failed. All the events occurred from May to December of last year. And all the batteries were returned to their maker, GS Yuasa of Japan.
Kelly Nantel, a spokeswoman for the National Transportation Safety Board, said investigators had only recently heard that there had been "numerous issues with the use of these batteries" on 787s.
Boeing, based in Chicago, has said repeatedly that any problems with the batteries can be contained without threatening the planes and their passengers.
But in response to All Nippon's disclosures, Boeing officials said the airline's replacement of the batteries also suggested that safeguards to prevent dangerous overheating might have kicked in.
A GS Yuasa official, Tsutomu Nishijima, said battery exchanges are part of the normal operations of a plane but would not comment further.
Plagued by production delays, the 787 Dreamliner went into service last year as Boeing's next-generation, state-of-the-art aircraft made of lightweight composite materials that greatly improve the jet's fuel efficiency.