WASHINGTON -- Experts at the Federal Aviation Administration are expected to say next week whether they recommend accepting Boeing's plan to fix its troubled 787 Dreamliners so the planes can resume flying, the agency head said Wednesday.Officials in the FAA office near Seattle that certifies new planes as safe for flight are reviewing a Boeing proposal to revamp the 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, Huerta will decide whether to accept the plan. He declined to say when he might decide or how long it might be before the planes are 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 on a 787 parked at Boston's Logan Airport and a smoking battery in a different 787 forced an emergency landing in Japan.Fifty of the planes are in service worldwide, and Boeing had orders for 800 at the time they were grounded.Calling the plan "very comprehensive," Huerta said Boeing engineers worked with outside experts to narrow the potential causes to a few possibilities, then redesigned the batteries. The 787 has two identical 32-volt batteries, each with eight cells.Investigators have said the incidents began with a short circuit in a single cell, leading to a chemical reaction that causes progressively higher temperatures. That spread the short circuits and fire to other cells.But the root cause of the initial short circuit hasn't been identified. That places the FAA in the awkward position of being asked to approve a fix for a problem whose origins are unknown.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 cells.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."