United Airlines said Tuesday it expected to keep its Boeing 787s out of service through March 30, a fresh sign of how long the aircraft could remain grounded worldwide to fix a battery problem. United, the only US airline that operates Boeing\'s cutting-edge aircraft, said it was scheduling alternate planes through March 30 while its seven 787 Dreamliners remained grounded. \"We are further adjusting our published flight schedules to show alternate aircraft in place of our six Boeing 787s through March 30,\" Christen David, a spokeswoman for United, said in an email. \"We will make further adjustments in the future as we gain more visibility on the question of when service will be restored,\" she said. All 50 787s in service around the world have been banned from flight since mid-January after battery smoke forced an emergency landing of one and a battery fire occurred on another. US and foreign investigators have reported progress in the probe of the lithium-ion batteries but have yet to pinpoint the cause of the problems. Boeing reportedly is looking for a temporary fix for the batteries, according to the Seattle Times, a daily that covers a region including a major Boeing plant. The aerospace giant may encase the battery cells in a titanium or steel box fitted with a high-pressure vent to contain any fire that erupts in flight, the newspaper reported Monday, citing people familiar with the plan. Even if the FAA agrees to the interim solution, it would take at least three months to design, test, certify and retrofit the planes, a source told the newspaper. The US Federal Aviation Authority refused to comment on the report, while Boeing said the report was speculative and contained unspecified errors. The Dreamliner, a long-haul fuel-efficient airliner built largely with composite materials, is key to Boeing\'s business strategy as it battles against European rival Airbus. On January 16, the 50 Dreamliners in service around the world were grounded after a battery fire on a Japan Airlines plane parked in Boston, and battery smoke on an All Nippon Airways flight forced an emergency landing. On Tuesday, a Japanese safety board official said that investigators found a battery on the ANA flight that initially was believed to be intact had also been damaged. Detailed examination of the auxiliary power unit battery revealed that two of its eight cells were misshapen. Boeing has suspended deliveries of the aircraft but continues to build them at a rate of five per month. Airbus said last week it will switch from lithium-ion batteries to cadmium batteries for its new A350 long-range aircraft, due to enter service in the second half of 2014.