WASHINGTON -- Boeing attempted a major step Friday toward getting its 787 Dreamliners flying again, proposing a fix for the plane's troubled batteries that could allow the flights to resume as early as April, congressional officials said.The next question is whether the Federal Aviation Administration will agree to let the planes fly even though the root cause of a battery fire in one plane and a smoking battery in another is still unknown.A Boeing team led by CEO Ray Conner presented the plan to Federal Aviation Administration head Michael Huerta. The airliners, Boeing's newest and most advanced, have not been allowed to fly since mid-January.The plan -- a long-term solution, rather than a temporary fix -- calls for revamping the aircraft's two lithium-ion batteries to ensure that any short-circuiting that could lead to a fire won't spread from one battery cell to the others, officials said. That would be achieved by placing more robust ceramic insulation around each of the battery's eight cells. The aim is to contain not only the short-circuiting, but any thermal runaway, a chemical reaction that leads to progressively hotter temperatures.The additional spacers will enlarge the battery, requiring a bigger battery box to hold the eight cells. That new box would also be more robust, with greater insulation along its sides to prevent any fire from escaping and damaging the rest of the plane, officials said.The plan will require testing and partially recertifying the safety of the plane's batteries, said the officials, who spoke on condition of anonymity because they weren't authorized to speak publicly.The testing and recertification will take time, with engineers currently estimating completion sometime in April at the soonest, they said. Even after the batteries are recertified, it could take some more time to get the planes back into the air. Boeing will have to send teams to eight airlines in seven countries to retrofit their planes.It's up to Huerta to decide whether to approve the plan. But Boeing's plan is not a surprise, since the company has kept regulators closely informed, the officials said."The FAA is reviewing a Boeing proposal and will analyze it closely," the agency said in a statement Friday. "The safety of the flying public is our top priority and we won't allow the 787 to return to commercial service until we're confident that any proposed solution has addressed the battery failure risks."Boeing also acknowledged the meeting, but spokesman Marc Birtel would not discuss what was said. "We are encouraged by the progress being made toward resolving the issue," the company's written statement said.Boeing, the FAA and the National Transportation Safety Board still have not identified the root cause of a Jan. 7 fire that erupted in an auxiliary power unit battery of a Japan Airlines 787 about a half-hour after the plane landed at Boston's Logan International Airport. The safety board is investigating that incident, but NTSB officials didn't attend Friday's meeting and declined to comment on the proposal.Engineers and battery experts gathered by Boeing developed a list of possible causes for the fire and a plan to modify the batteries to address the spread of a fire created by any of those causes, officials said.Nine days after the Boston fire, an All Nippon Airways 787 with a smoking battery made an emergency landing by in Japan. The FAA and aviation authorities overseas ordered the planes grounded soon afterward. There are a total of 50 of the planes in service, and Boeing had orders for 800 of the airliners at the time they were grounded.