SEATTLE -- Boeing will propose to regulators, perhaps this week, a short-term fix to bolster the 787 Dreamliner's protection against battery fires like those that grounded the jet this year.The goal is to get the planes flying again, according to multiple sources with knowledge of the matter, while Boeing works on a comprehensive redesign of the lithium-ion battery system that could take nine months or more to implement.The interim fix includes a heavy-duty titanium or steel containment box around the battery cells, and high-pressure evacuation tubes that, in the event of a battery fire, would vent any gases directly to the outside of the jet.Boeing's approach implicitly acknowledges that investigators have still not pinpointed the cause four weeks after two batteries overheated and one one caught fire while the plane was on the ground.That leaves Boeing little option but to engineer a solution that will better contain any fires and protect the airplane.However, it's unclear if the Federal Aviation Administration is ready to accept containment of an overheated battery cell rather than prevention."We're not there yet," said a government official with knowledge of the discussions, who spoke declined to be identified. "It wouldn't surprise me if we're still talking weeks before everyone is comfortable."Even if the FAA agrees, the short-term fix will take at least three months to design, test, certify and retrofit, said a source who knows details of Boeing's proposed solutions.That would mean the earliest the Dreamliners could restart commercial flights would be May. If it's much longer than that, assembly of new jets will probably have to be slowed and Boeing's plan to increase production will be severely disrupted, he said.Unlike Airbus, which last week said it will switch to nickel cadmium main batteries for its forthcoming A350 jet to avoid the possibility of delays, Boeing insists it will stick with the high-energy lithium-ion batteries.Aviation experts are increasingly worried.Adam Pilarski of consulting firm Avitas warns that though Wall Street currently accepts Boeing's optimism that the 787 grounding will be relatively short, this forgiving attitude may not last."Boeing is trying to play it down to some degree, hopeful the solution is just around the corner," Pilarski said. "But it may take much longer. And it could have a significant financial impact."