Boeing Co. conducted its first test flight of its Dreamliner aircraft with modified lithium-ion batteries, two months after the all of its newest jets were grounded.

The Dreamliner spent two hours in the air on Monday after taking off from Everett, Washington, and all went according to plan, the US aircraft maker said.

Overheating and short-circuiting problems with the lithium-ion batteries, which are used in part to boost the Dreamliner’s fuel efficiency, have kept the 49 passenger jets in use around the world grounded since January 16. The groundings were ordered after a battery caught fire and another melted on the planes.

About 800 of the fuel-efficient, long-haul aircraft were due to be delivered in the coming years to airlines around the world, making the grounding an expensive and embarassing problem for Boeing.

The US airline regulator, the Federal Aviation Administration, must approve the changes Boeing is making to the batteries before it may return to commercial flights.

Data from Monday’s flight must be evaluated and then the approval process could begin, a Boeing spokesman said.

Tests on the ground and another demonstration flight are also planned to win the approval.

If all goes to plan, the Dreamliners could begin flying commercially again as soon as they get the modified batteries and Boeing could continue Dreamliner deliveries.

No specific cause of the overheating of the batteries has been determined, but Boeing’s modifications include better separation of the individual cells of the batteries to minimize short circuits, a stainless steel housing and a better vent line.