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The F.A.A. could still demand changes in Boeing's proposed new battery design if problems develop in the laboratory and in flight tests, which will take several weeks. But the decision to start the tests will mark a major step in Boeing's efforts to get the innovative jets, which have been grounded since mid-January, back in the air.

The federal approvals are expected to come late this week or early next week, even though some battery experts remain concerned that investigators have not found the precise cause of two incidents in which the jetliner's new lithium-ion batteries emitted smoke or fire.

The National Transportation Safety Board has found that a short-circuit in one cell caused a battery in a jet parked at a Boston airport to overheat and burst into flame on Jan. 7. The board plans to release a preliminary report on that incident on Thursday.

The F.A.A.'s Seattle office on Wednesday was wrapping up its recommendation to approve Boeing's plan to carry out the tests needed to certify that the battery fixes would work, federal officials said. The plan is still subject to approval by Michael P. Huerta, the head of the F.A.A., and Ray LaHood, the transportation secretary, who will be briefed on it over the next several days.

Mr. LaHood said in January that the planes "won't fly until we're 1,000 percent sure they are safe to fly." Department officials said Mr. LaHood and Mr. Huerta had been kept informed of the details of the proposal as it was created, and they are expected to sign off on it.

Boeing officials say they have identified the most likely ways in which batteries could fail. They contend that the changes would virtually eliminate the odds of future incidents and protect the plane and its passengers if a problem does arise.

Boeing has delivered 50 787s so far to eight airlines. The company has much at stake with the plane, which is the first commercial jet to be built mostly out of lightweight composite materials that reduce fuel costs. Boeing has orders for 800 more of these planes.