WASHINGTON (AFP) ― The U.S. Federal Aviation Administration issued formal approval of Boeing’s 787 battery fix that will clear the way for the troubled aircraft to fly again after a three-month grounding.

The move was quickly followed by regulators in Japan who said they would also allow the resumption of flights by the grounded aircraft following the FAA order, which they said would come into effect Friday.

Japan Airlines and domestic rival All Nippon Airways account for half the Dreamliners in service, and have said it could take several months to complete the battery fix at center of the crisis and get the planes in the air.

The FAA and other regulators grounded the 50-strong worldwide Dreamliner fleet in mid-January after two failures of the innovative lithium-ion batteries on the jetliner.

The FAA’s new airworthiness directive for the next-generation plane requires the installation of modified battery packs 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 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 grounding.

The FAA action technically affects just the six 787s of United Airlines, the sole U.S. airline owning the aircraft.

“But we expect foreign civil aviation authorities will order the same action,” the FAA spokesman said.

Shortly after, an official at Japan’s transport ministry said: “As the FAA is to approve the resumption as of Friday morning U.S. time, we will take the same step.”

ANA declined to comment on the development, while JAL could not be immediately reached.

Other airlines flying the Dreamliner include Ethiopian Airlines and Polish carrier LOT.

In the FAA’s directive, the air-safety regulator said the battery modifications would minimize the safety risk posed by the overheated batteries in the January incidents.

Investigators are still unable to pinpoint the cause of the battery failures, the FAA noted.

The National Transportation Safety Board, which is probing the January 7 battery fire aboard a Japan Airlines 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 ANA plane 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 U.S.-registered airplanes at $2.8 million.

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.