The troubled Boeing 787 Dreamliner is back in the sky, three months after it was grounded worldwide because of problems of over-heating batteries.

An Ethiopian Airlines flight took off from Addis Ababa on Saturday morning on a two-hour flight to the Kenyan capital Nairobi. Regulators cleared the aircraft after modifications were made to the lithium ion battery packs.

Boeing's chief engineer for the Dreamliner on Saturday said changes made to the battery were fully sufficient to ensure the aircraft's safety despite being unable to find the root causes of the battery failures earlier this year.

Mike Sinnett gave a briefing on the battery changes to reporters in Tokyo as Japanese carriers prepared to resume flying the 787.

Batteries aboard two 787s failed less than two weeks apart in January, causing a fire aboard one plane and smoke in another. The root cause of those incidents is still unknown.

Boeing has since developed and tested a revamped version of the battery system, with changes designed to prevent a fire, and to contain one should it occur using an "enclosure," a casing around the battery to ensure heat would not be released within the aircraft.

"Even if we never know root cause the enclosure keeps the airplane safe, it eliminates the possibility of fire, it keeps heat out of the airplane, it keeps smoke out of the airplane, and it ensures that no matter what happens to the battery, regardless of root cause, the airplane is safe," Sinnett said, adding that "in some ways it almost doesn't matter what the root cause was."

Boeing's modifications were approved by the U.S. Federal Aviation Administration last week, and by Japanese authorities on Friday, clearing the way for Boeing to install the new system on the 50 Dreamliners in service worldwide.

Japanese carrier All Nippon Airways, which currently has the largest fleet of the 787s in the world, is planning to conduct a test flight using a modified Dreamliner in Japan on Sunday.

The 787, Boeing's newest and most technologically advanced plane, is the first airliner to make extensive use of lithium-ion batteries.

There were two battery failures when the entire 787 fleet had clocked less than 52,000 flight hours.

The first came Jan. 7 aboard a Japan Airlines 787 parked at Boston's Logan International Airport shortly after landing from an overseas flight.

Nine days later, a smoking battery aboard an All Nippon Airways 787 led to an emergency landing in Japan.

The FAA ordered all US-registered 787s grounded the same day, and aviation authorities in other countries swiftly followed suit.

Boeing has orders for 840 of the planes from airlines around the globe.