Making an ardent pitch in Japan, home to two of the largest buyers of its next-generation 787 plane, Boeing said that more than 500 engineers had worked with outside experts to put in more than 200,000 hours of analysis, engineering work and tests to understand what may have caused the batteries to overheat in two aircraft in January. Those incidents prompted American and Japanese regulators to ground the 50 Dreamliners already delivered.
Michael K. Sinnett, the Dreamliner’s chief engineer, acknowledged that Boeing had not pinned down the exact cause of the overheating, and said the company might never know what, and by extension who, is responsible. A lithium-ion battery caught fire aboard a parked plane operated by Japan Airlines; another emitted smoke during an All Nippon Airways flight, forcing the jet to make an emergency landing.
The batteries are made by the Japanese manufacturer GS Yuasa of Kyoto. The Japanese have not yet ruled out that a problem other than the battery, such as a surge in current, could have caused the malfunction. But Boeing has ruled out any possibility the batteries may have been overcharged, citing measurements from instruments aboard the plane.
“We may never get to a single root cause,” Mr. Sinnett said. But he said that engineers had examined 80 potential problems that could lead to a battery fire, grouped them into four categories and designed solutions for each category.
“We looked at everything that could impact the battery,” Mr. Sinnett said, “and we applied a broad set of solutions that encompasses everything that this large team of experts believed someday could happen. And it led to a very robust solution,”
He also said that the fix, which includes a new battery enclosure made of stainless steel, was not designed to contain a fire, but to keep the battery from ever having a fire to begin with, by quickly starving any flame of oxygen.
“We’ve been able to demonstrate that no fire is possible inside the enclosure,” Mr. Sinnett said. If battery were to heat up, its vaporized electrolytes would be vented directly out of the plane, he said, protecting the bay and other electronics.
He said the additional weight of the fix, about 150 pounds, was “a wash” compared with heavier batteries. The 787 is the first passenger jet that uses lithium-ion batteries, which are more powerful, easier to charge and lighter than older battery technologies. But they have also proven to be more volatile.
At the news conference in Tokyo, Boeing executives offered little in the way of self-criticism beyond an apology to Japanese airlines and customers from Ray Conner, executive vice president of the Boeing Company.
Mr. Sinnett said that in both incidents in January, the batteries performed as designed and stressed that there had been no injuries or extensive damage to the planes.
Earlier this week, American regulators approved Boeing’s plan to test its fixes.
Mr. Sinnett said that about 75 percent of Boeing’s test plans for the 787 had been approved and that 25 percent of the testing was complete. He said that he expected flights to resume in weeks, and if Boeing missed that time frame, it would “be by a little, not by a lot.”
“I get often asked whether the airplane is still safe,” Mr. Sinnett said. “My answer is ‘absolutely.’ I’d gladly have my family fly in this airplane.”
Japanese authorities must also approve Boeing’s fix and its test results. The Japanese transport minister, Akihiro Ota, said Friday morning it was too soon to say when Tokyo might allow the Dreamliners back in the air.
“We will work closely with the F.A.A. to examine and confirm the safety of the aircraft,” Mr. Ota said. “The tests are only beginning.”
Hisako Ueno contributed reporting from Tokyo, and Jad Mouawad from New York.
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