Officials in the FAA office near Seattle that certifies new planes as safe for flight are reviewing a Boeing proposal to revamp the 787's lithium ion batteries to prevent them from catching fire, or to protect the plane in case of fire, Administrator Michael Huerta said.

Once he receives their evaluation, it's still up to Huerta to decide whether to accept the plan. He declined to say when he might make that decision, or how long it might be before the planes are back in the air.

"It's a very long proposal (with) a lot of technical detail in it," Huerta said. "I'm reviewing it myself, as well as relying on the teams that are reviewing it."

Boeing officials presented the plan to Huerta last week.

The planes have been grounded since Jan. 16 after a battery caught fire in a 787 parked at Boston's Logan International Airport and a smoking battery in a different 787 forced an emergency landing in Japan. There are a total of 50 of the planes in service worldwide, and Boeing had orders for 800 of the airliners at the time they were grounded.

Calling the plan "very comprehensive," Huerta said Boeing engineers worked with outside experts to narrow the potential causes of the incidents to a few possibilities, and then redesigned the batteries. The 787 has two identical 32-volt batteries, each with eight cells.

Investigators have said the incidents began with short-circuiting in a single cell, leading to a chemical reaction that causes progressively hotter temperatures. That spread the short-circuiting and fire to other cells.

But the root cause of the initial short-circuiting hasn't yet been identified. That places the FAA in the awkward position of being asked to approve a fix for a problem with its origins still unknown.

The agency has been in this situation before. After TWA Flight 800 exploded off the coast of Long Island, N.Y., in 1996 killing all 230 people aboard, investigators ultimately pinned the cause on flammable vapors in one of the Boeing 747's fuel tanks. But they were never able to conclusively identify the source of the spark that ignited vapors. Nevetheless, FAA engineers were ultimately able to come up with new requirements for fuel tanks to prevent an explosion.

Boeing's plan for the 787 includes redesigning the batteries to prevent individual cells from catching fire, Huerta said. Should that fail, the plan includes steps to prevent a fire from spreading to other cells or outside the box that contains all eight of the cells.

"What Boeing has assembled is a team to look at the universe of potential causes, and their proposal is to mitigate all of them," Huerta said.

If the plan is approved, the next step would be extensive engineering and testing before any final determination could be made on resuming flights, he said. He described the process as "effectively a certification plan."

"We have to be assured that this is a good plan and that it is going to result in a safe situation," Huerta said.

The 787 is the world's first airliner made mostly from lightweight composite materials. It also relies on electronic systems rather than hydraulic or mechanical systems to a greater degree than any other airliner. And it is the first airliner to make extensive use of lithium ion batteries, which are lighter, recharge faster and can hold more energy than other types of batteries.

Boeing has billed the plane to its customers as 20 percent more fuel efficient than other mid-sized airliners. That's a big selling point, since fuel is the biggest expense for most airlines.

Airlines have been forced to tear up their schedules while the planes are out of service. Last week, United Airlines cut its six 787s from its flying plans at least until June and postponed its new Denver-to-Tokyo flights. United is the only U.S. carrier with 787s in its fleet.

LOT Polish Airlines has said the grounding of its two 787s is costing it $50,000 per day. Most affected has been All Nippon Airways, which has 17 of the planes.

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