Story highlights The Japanese decision follows a directive from U.S. aviation authorities

Japanese airlines, the biggest Dreamliner operators, are installing new batteries

It remains unclear exactly when they will resume flights of the Boeing planes

Japan has authorized passenger airlines to resume flights of Boeing 787 Dreamliners in the country starting Friday, authorities said.

The move follows the U.S. Federal Aviation Administration's online posting Thursday of a directive outlining the modifications necessary for the Dreamliner to fly again after faulty battery systems grounded the aircraft earlier this year.

The directive goes into effect upon publication Friday in the U.S. Federal Register.

The Japanese transport ministry said All Nippon Airways and Japan Airlines, the world's two largest Dreamliner operators, have already begun installing modified lithium ion batteries on their 787 jets.

But it remains unclear precisely when the two airlines will resume scheduled passenger flights of the Boeing planes.

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Nearly 50 Dreamliners around the world have been grounded for more than three months, after two incidents on jets operated by the Japanese airlines called the battery systems into question.

Last week, the FAA cleared Boeing to make fixes to the problematic battery system. Boeing said it was deploying teams to help the airlines install the redesigned system.

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.