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Federal regulators said on Wednesday that they had approved one flight of a Boeing 787, with a flight crew but no passengers, as the company's engineers study possible changes to the plane's electrical systems that could reduce the risk of another battery fire.

The flight would be the first for a 787 since aviation authorities grounded the innovative aircraft last month after two incidents with its lithium-ion batteries. The Federal Aviation Administration said it would let Boeing return one 787 from a painting plant in Fort Worth to its plant near Seattle. It has not yet approved flights to conduct tests on the batteries.

The flight, scheduled for Thursday, will come as the National Transportation Safety Board is expected to raise questions about how the F.A.A. certified the 787's battery before it began flying passengers in 2011. The safety board, which has been performing tests of its own as part of its investigation into the battery problems, is seeking to find out why weaknesses with the batteries were not picked up in Boeing's original testing program.

The safety board is looking at whether the F.A.A. fully understood any potential issues with the volatile new batteries before it approved their use under special conditions.

Deborah Hersman, the safety board's chairman, told reporters on Wednesday that it would probably take investigators several more weeks to determine what happened with the Boeing batteries.

Boeing engineers are working on a range of possible technical overhauls. These include making the battery cells more resistant to shocks to keep excess heat from spreading from one cell to another, causing the kind of thermal runaway that occurred in the two recent events. Boeing officials have said they are also working on building more solid containment cases and better venting mechanisms in the event of overheating.

None of this has been tested or approved yet, a process that could take months. And until more is known about the cause of the recent incidents, the grounding order is unlikely to be lifted soon.

The 787 is the first commercial airplane to use large lithium-ion batteries for major flight functions. All 50 of Boeing's 787s that were delivered to airlines have been grounded since mid-January.

"I would not want to categorically say that these batteries are not safe," Ms. Hersman said during a briefing with reporters on Wednesday. "Any new technology, any new design, there are going to be some inherent risks. The important thing is to mitigate them."

Boeing officials said that they were exploring numerous ways to strengthen the batteries and that it was premature to think any of those would be approved by regulators without more information.

Boeing officials said they remained confident that they could keep using the lithium-ion batteries, and they hoped that finding a way to strengthen the batteries might allow them to do so. But officials said the company also had a team of engineers working on alternatives involving more conventional batteries in case regulators banned them.

Boeing picked the new lithium-ion technology because it provided more power than traditional batteries of the same size. But battery experts have questioned their use because, under certain conditions, they can overheat and ignite.

The F.A.A.'s decision to certify the batteries has come under scrutiny in recent weeks. While the federal regulator is stretched thin with too few inspectors, and typically relies on testing data from Boeing, lithium batteries are an area where the agency has some expertise. It has had to deal for years with fires involving lithium-ion batteries shipped as cargo or carried by passengers in their computers or cellphones.

Ms. Hersman will provide an update on Thursday on the investigation's process. But while she said the safety board was in a position to rule out some problems, it was unlikely to be able to say what happened for some time.

She said that she would not rule out the use of lithium batteries "categorically," but insisted that the safeguards Boeing had put in place failed when a Japan Airlines plane experienced a fire while parked at Logan Airport in Boston.

"Obviously what we saw in the 787 battery fire in Boston shows us there were some risks that were not mitigated, that were not addressed," she said. She added that the fire was "not what we would have expected to see in a brand-new battery in a brand-new airplane."

The safety board, she said, was also examining the special conditions the F.A.A. ordered Boeing to follow in using the batteries and whether they should have been updated later.

The F.A.A.'s conditions were fairly general, and required Boeing to create the means to contain any fire or vent any smoke to keep it from spreading into the cabin and putting the plane at risk if a battery failed.

"What happens is that when an aircraft is certified it basically gets locked into the standards that were in existence at the time," Ms. Hersman said.

The fleet's grounding is not affected by the one-time F.A.A. permit and no one except crew members will be allowed on board the plane from Fort Worth. The plane, which still belongs to Boeing, was scheduled to be delivered to China Southern Airlines.

The F.A.A. said that before takeoff, the Boeing crew should perform a number of inspections to verify that the batteries and cables showed no signs of damage. While airborne, the crew must also "continuously monitor the flight computer for battery-related messages, and land immediately if one occurs," the F.A.A. said.