United plans to resume 787 service in May April 9, 2013 -- Updated 1507 GMT (2307 HKT) A 787 Dreamliner passenger jet is tested above the Boeing factory at Paine Field in Everett, Washington state on March 20, 2011. United Airlines is only U.S. carrier that flies the 787; 50 in service worldwide The 787 was grounded in January after two battery fires in Boston and Japan The Federal Aviation Administration must still sign off on Boeing's battery redesign Washington (CNN) -- United Airlines plans to resume flying the 787 Dreamliner in May, the carrier told CNN in anticipation of regulatory approval of Boeing's remedy for battery problems that forced the jetliner's worldwide grounding. While United is the only U.S. carrier that flies the wide body, a signal from the world's biggest carrier indicates that others are also likely making plans to resume service in coming weeks. For the first time in months, it also indicates a clear time line for Boeing to return the aggressively promoted but troubled jetliner to the skies after it was grounded by regulatory authorities globally in January. United said it plans to start flying 787 again on May 31. Final battery test on Dreamliner 'straightforward' "We are in the process of formulating our domestic flying plans and will be making additional schedule changes as we gain visibility to the time line for certification and modification work," Christen David, an airline spokeswoman said. United said it will fly the plane on both domestic and international routes. Boeing modified the 787's lithium-ion battery system after fires in Boston and in Japan prompted aviation authorities globally to ground it. It completed new test flights last week. The Federal Aviation Administration (FAA) will review the test data and must sign off on the redesign. Transportation Secretary Ray LaHood has said the Dreamliner will not be allowed back in service until Boeing proves the new design is safe. Boeing has placed a huge bet on the lightweight, mostly carbon-composite jetliner. Only 50 Dreamliners were flying worldwide at the time of its grounding, but the world's largest aircraft manufacturer has orders for several hundred in the pipeline.