NTSB chief: 'Dreamliner' probe ‘weeks away' from completion

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Hersman said the NTSB was conducting a "methodical" investigation of the 787.

"Some of it is about ruling things out," Hersman said. "There are things that are ruled in, but there's also things that we want to rule out. 'Did this happen?' If it didn't, we want to take those things off of the table."

The 787 was grounded by the Federal Aviation Administration (FAA) last month because of a pair of battery issues that led to at least one electrical fire.

The FAA ordered U.S. airlines to stop flying the 787 after a fire broke out on an airplane that was being operated by Japan Airlines at Boston's Logan International Airport. Other worldwide aviation agencies quickly followed the FAA's lead, resulting in a worldwide grounding of the Dreamliner that has lasted for nearly a month.

The NTSB has said that its examination of the battery that caught fire revealed that the lithium-ion device accelerated temperature increases known as "thermal runaway" and short-circuiting.

The agency said it has ruled out the batteries being overcharged as a possible cause of the failures experienced in the roll-out of the 787.

The NTSB's review of the 787's battery issues is separate from the FAA's probe, which will determine when the airplane is allowed to fly again.

Hersman has emerged as a leading candidate to replace outgoing Transportation Secretary Ray LaHood.

She said Wednesday, however, that the NTSB's investigations would not be affected by any possible appointments she might receive in the future.

"We have the most amazing technical staff," Hersman said at the breakfast. "They are the one who go out to the accident sites. They do the investigations. They work in the lab. They are the backbone of the NTSB."

Boeing has asked the FAA to allow it to begin flying test flights of the 787, signaling it believes it is making progress in solving the airplane's battery issues.

The FAA has said that it is "evaluating" the airplane manufacturer's request.