WASHINGTON (AP) — Boeing's beleaguered 787 could be flying again within a week after federal officials approved a fix for its batteries, even though the root cause of a fire on one plane and smoke on another still isn't known.

The Federal Aviation Administration said Friday it would send airlines instructions and publish a notice next week lifting the 3-month-old grounding order that day. Airlines will be able to begin flying the planes again as soon as the new systems are installed and they have approval from safety regulators in their own countries. Dreamliner flights could resume within a week, the agency told members of Congress.

Boeing is eager to get the planes flying. It has stationed 300 workers on 10 teams around the world to do the work, some of it beginning on Friday, 787 chief engineer Mike Sinnett said on a call with reporters. It will take about five days to install the revamped lithium-ion battery system on each plane, he said.

The FAA gave Boeing permission last month to test the revamped system, which includes additional insulation around each of the battery's eight cells to prevent a short circuit or fire in one of the cells from spreading to the others. The new system also includes enhanced venting of smoke and gas from inside the battery to outside the plane. A strengthened box to hold the battery is an effort to ensure that if a fire were to occur, it wouldn't escape to the rest of the plane.

Boeing has completed 20 separate tests of the new system, FAA Administrator Michael Huerta told Congress earlier this week.

The system involved in the emergencies in January had been extensively tested, too.

"We always learn more as we dig deeper into things," Sinnett said. "We have learned a lot about how to test batteries, and to be conservative" in testing.

Boeing had delivered 50 planes to eight airlines in seven countries when a fire erupted in a battery aboard a Japan Airlines 787 parked at Boston's Logan International Airport on Jan. 7. Nine days later another incident forced an emergency landing in Japan by an All Nippon Airways 787. That prompted the FAA and other authorities to ground the entire fleet.

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Boeing said new batteries and kits with the parts for the new battery systems have been shipped to Boeing supply centers around the world and are ready to be installed. The 787s will get the fix in approximately the order they were delivered, Boeing said.

The FAA's action directly affects the six 787s flown by United Airlines, the only U.S. airline with the plane. But aviation authorities in other countries are expected to follow suit swiftly. Boeing deferred questions about approval in other countries to those aviation authorities.

United Airlines already has domestic 787 flights scheduled for May 31. Spokeswoman Christen David said no other schedule changes have been made yet. Its launch of Denver-to-Tokyo Narita flights is still planned for June 10, but that will depend on installing the battery fix by then, she said.

"We are mapping out a return-to-service plan, and we look forward to getting our 787s back in the air," she said by e-mail.

LOT Polish Airlines spokesman Marek Klucinski noted that they need permission from the European Aviation Safety Agency to resume flights. He said they hope that a decision on Friday would mean they can resume flights in the middle of next week. LOT has two planes, one in Warsaw and one that was stranded in Chicago by the grounding.

Boeing has orders for 840 of the planes from airlines around the globe. The grounding halted deliveries, which were expected to resume "in the weeks ahead," after it installs the changes on planes at the two factories where they're assembled, Boeing said. It still expects to hit its target of delivering at least 60 787s this year, and that the battery issue "will have no significant impact" on its financial guidance for the year, the company said.

Boeing shares rose $1.84, or 2 percent, to close at $87.96 on Friday.

The plane's grounding on Jan. 16, an enormous black eye for Boeing, marked the first time since 1979 that FAA had ordered every plane of a particular type to stay out of the air for safety reasons.

The 787 is Boeing's newest and most technologically advanced plane. It 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.

The 787 has two identical lithium-ion batteries: One near the front of the plane, which powers cockpit electrical systems, the other toward the rear and used to start an auxiliary power unit while the plane is on the ground, among other functions. The rear battery was involved in the fire and gushed smoke on the plane in Boston, which had recently landed after an overseas flight. It was the front battery that failed on the plane in Japan.

Every item that is part of an airplane, down to its nuts and bolts, must be certified as safe before FAA approves that type of plane as safe for flight. The two events have raised questions about why the FAA and Boeing didn't uncover problems with the batteries before the FAA certified the plane as safe for flight in 2011. In recent years, the FAA has relied to a greater extent on designated employees of aircraft makers to conduct the safety testing necessary of certification. Some aviation safety experts have questioned whether FAA has the in-house expertise to oversee the safety of cutting-edge technologies that haven't been in planes before.

Lithium batteries are much more likely to experience uncontrolled high temperatures that can lead to fires if they are damaged, exposed to excessive heat, overcharged or have manufacturing flaws. Despite their safety risks, they are increasingly attractive to aircraft makers as a way to cut weight and thus improve fuel efficiency.

The National Transportation Safety Board is investigating the Boston battery fire and the process by which the FAA certified the 787's batteries were certified as safe. The board has scheduled a two-day hearing beginning Tuesday at which FAA and Boeing officials are slated to testify.

NTSB officials have said the Boston battery fire began with a short circuit in one of the battery's eight cells, leading to uncontrolled temperatures and short-circuits in the rest of the battery's cells. Firefighters who responded to the incident reported dense clouds of white smoke and two small flames on the outside of the box that contained the battery cells.

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Freed reported from Minneapolis. Associated Press writers Monika Scislowska in Warsaw, Poland, contributed to this report.

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