( CNN ) -- How will the Dreamliner get back in the air ? The answers lie in a dense federal document posted online Thursday . The Federal Aviation Administration 's Airworthiness Directive for Boeing 's 787-8 was posted online Thursday and goes into effect upon publication Friday in the Federal Register . The directive outlines the modifications necessary for the much-anticipated Dreamliner to fly again after faulty battery systems grounded the aircraft earlier this year . The FAA stated that any 787 's return to service would only take place after the agency monitors modifications of the aircraft in the U.S. fleet and inspects the work . United plans to resume 787 service in May . `` An FAA inspector will have to accept the work before the airplane technically is in compliance with the ( directive ) , '' said FAA spokesman Les Dorr . Nearly 50 already-delivered Dreamliners have been grounded for more than three months , after two incidents on jets operated by Japanese airlines called the battery systems into question . The Dreamliner 's use of lightweight composite materials to greatly improve fuel economy has made it a big seller in Asia and the Middle East , where long-haul flights account for much of an airline 's business . United Airlines , which has six Boeing 787 aircraft , is the only U.S. airline to take delivery of the Dreamliner so far . It will cost the airline about $ 2.8 million to implement the fix , according to the FAA 's Federal Register filing . Calling the FAA plan `` a good step forward , '' United spokeswoman Christen David said the company is developing `` a return-to-service plan , and we look forward to getting our 787s back in the air . '' The company plans to begin domestic flights using the 787 in May and possibly launch the Denver-Narita ( Japan ) route on June 10 . Airline quality report sorts out duds from the dynamos . Boeing is sending special teams to assist in the repairs , according to Boeing spokesman Marc Birtel . `` The FAA 's publication of the Airworthiness Directive is an important step forward in returning the 787 fleet to flight status , '' Birtel said in an e-mail . The directive requires that 787 operators install `` main battery and auxiliary power unit battery enclosures ( APU ) and environmental control system ( ECS ) ducts ; and replacing the main battery , the APU battery and their respective battery chargers . '' `` Once modifications are completed on individual airplanes , our customers will determine , with their regulatory authorities , when to return their 787s to service , '' said Birtel . Although the FAA 's fix only applies to U.S. carriers , it 's likely other foreign civil aviation authorities will consider the U.S. agency 's plan , said the FAA 's Dorr . Boeing has already started installing the modified battery system on 10 aircraft in service and nine aircraft in production , said Boeing CEO Jim McNerney during Boeing 's earnings call Wednesday . `` We expect to complete the bulk of fleet retrofits by mid-May , '' said McNerney , according to a transcript of the call . McNerney said Boeing expects to resume Dreamliner deliveries in early May . CNN 's Mike Ahlers , Jennifer Liberto and Marnie Hunter contributed to this story .
FAA posts Airworthiness Directive for Boeing Dreamliner . Score: 6 It outlines required modifications that must be inspected before flight . Nearly 50 Boeing 787s have been grounded more than three months .