The US Federal Aviation Administration on Thursday issued formal approval of Boeing's 787 battery fix that will clear the way for the troubled aircraft to fly again.
The FAA's new airworthiness directive for the 787 requires the installation of new and auxiliary power unit batteries and their respective chargers, as well as battery enclosures and ducts.
"Once the aircraft are in compliance with the AD, they can return to service," an FAA spokesman said in an email.
The FAA and other regulators grounded all 50 787s in service worldwide in mid-January after two failures of the innovative lithium-ion batteries on the jetliner.
The latest FAA airworthiness directive caps a difficult three months for Boeing and its 787 customers, which have had to cancel thousands of flights and rearrange schedules after the 787 was grounded worldwide.
The FAA action technically affects just the six 787s of United Airlines, the sole US airline owning the aircraft.
"But we expect foreign civil aviation authorities will order the same action," the FAA spokesman said.
In its directive, the air-safety regulator said the battery modifications would minimize the safety risk posed by the overheated batteries in the January incidents.
Safety investigators still are unable to pinpoint the cause of the battery failures, the FAA noted.
The National Transportation Safety Board, which is investigating the January 7 battery fire aboard a Japan Air Lines 787 parked at a Boston airport, has not yet determined the cause, it said.
The same was true for the Japan Transport Safety Board, which is investigating the battery failure on an All Nippon Airways that forced an emergency landing in Japan, the agency said.
The FAA said it was issuing the directive without waiting for public comment, citing the grounding's cost to operators of the 787.
"While necessary in the short term to address the unsafe condition, this (grounding) caused a significant economic burden on domestic and international operators of Boeing Model 787-8 airplanes," it said.
"The purpose of this AD is to allow the aircraft to return to service as soon as possible by mandating a modification that will address the unsafe condition."
The FAA estimated the cost of the required modifications on the six US-registered airplanes at $2.8 million.
According to Boeing, the FAA said, some of the costs may be covered under warranty.
United Airlines, reporting first-quarter earnings Thursday, said it took an $11 million charge related to the 787 grounding.
United chief executive Jeff Smisek said that domestic 787 flights would resume in May and the airline's first international 787 service -- a new nonstop link between Denver and Tokyo -- would begin on June 10.
"The grounding of the 787s had an impact on our bottom line and we are eager to get this remarkable aircraft back up and flying," he said.
Shares in parent United Continental Holdings tumbled 2.0 percent and Boeing shares were up 1.3 percent in late-afternoon trade on the New York Stock Exchange.
On Wednesday, Boeing, reporting a 20 percent year-on-year jump in first-quarter profit, said the 787 problems had only a "minor" financial impact on the company.
The aerospace giant said it had begun battery modifications on 10 aircraft owned by airlines and nine planes being readied for delivery, and the installations were expected to be completed by mid-May.
Boeing confirmed its target of delivering more than 60 787s during 2013, despite the holdup from the grounding, by doubling production to 10 787s per month by year-end.