(CBS News) WASHINGTON, D.C. - Late Thursday, the FAA gave Boeing the go-ahead to conduct limited test flights of the grounded 787 Dreamliner. On that same day, a Boeing flew an empty 787 from Texas back to its factory in Washington state. The planes were grounded three weeks ago after batteries overheated twice; one of the batteries caught fire.
Federal investigators revealed the fire aboard the Dreamliner in Boston last month began with a short circuit in one of the lithium ion battery's eight cells. That caused a dangerous condition known as "thermal runaway," where a short spreads to other cells in an uncontrolled chemical chain reaction. The fire topped 500 degrees Fahrenheit.
It's something Boeing's development studies didn't predict.
Deborah Hersman, chairman of the National Transportation Safety Board, said Boeing was also incorrect in estimating a Dreamliner battery would heat up and smoke less than once in 10 million flight hours.
"We have seen two events on two aircraft less than two weeks apart," she said.
Hersman said that's good reason to question the entire process the FAA used to certify the Dreamliner's battery system as safe and airworthy.
When the Dreamliner was developed, federal guidelines for its planned use of lithium ion batteries didn't exist. So the FAA issued what's called "special conditions," and relied largely on Boeing's own studies to approve the plane.
Eventually, guidelines for the batteries were created in 2008. They require specific technical tests to prove the battery's safety casing would contain any fire and prevent a possible explosion.
But the Dreamliner was grandfathered in and didn't have to meet them.
Former National Transportation Safety Board member John Goglia said that may change. On whether Boeing have to go back in light of the questions and meet the standard that's in effect, he said: "Now that they're in a certification review, the FAA may very well hold them to the highest standard," he said
Boeing late Thursday said it "will not hesitate to make changes that lead to improved testing products and processes." As far as the investigation, investigators said they are now focused on the battery charger, battery design and construction, and also possible manufacturing defects.