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From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

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And I'm Audie Cornish.

The Head of the National Transportation Safety Board said today that some of the assumptions used to certify Boeing's troubled 787 were wrong, and she called into question the process used to certify the new plane. It's been one month since the battery caught fire while the Dreamliner jet was parked at a gate in Boston. And as NPR's Wendy Kaufman reports, safety investigators still don't know precisely what caused the fire.

WENDY KAUFMAN, BYLINE: For the first time, the NTSB has identified what happened on that Japan Airlines plane in Boston. Speaking at a news conference today, the NTSB's Deborah Hersman said evidence pointed to a short circuit in one of the eight cells within the battery. The cell reached more than 500 degrees and created a chemical chain reaction that spread to the other cells and produced a fire. To get its airplane approved by the Federal Aviation Administration, Boeing had said that a problem in one cell would not spread to another, and the company predicted that the batteries were likely to emit smoke less than once in every 10 million flight hours. So far, the 787s have logged less than 100,000 hours in commercial flight, but as Herman pointedly said today...

DEBORAH HERSMAN: There have now been two battery events resulting in smoke, less than two weeks apart on two different aircraft. The assumptions used to certify the battery must be reconsidered.

KAUFMAN: Exactly what that will mean is not yet clear. Hersman noted investigators are still trying to figure out exactly what caused the short circuit in the first place.

HERSMAN: We are looking at the design of the battery. We are looking at the manufacturing process. And we are also looking at the cell charging. There are a lot of things that we are still looking at.

KAUFMAN: Meanwhile, the 787 remains grounded worldwide, though late today, the FAA said it would allow Boeing to conduct new test flights of the jet in order to get additional data about the battery. Boeing says those tests will begin soon.

Meanwhile, the company continues to work on fixes and improvements to the battery that would convince the FAA to allow commercial flights of the jet to resume even though the precise cause of the battery fires remains a mystery. In the past, the agency has allowed planes, including the 747 and the 737, to continue flying even though it didn't know exactly what caused fatal crashes involving those jets. But Guy Norris, a senior editor at Aviation Week says...

GUY NORRIS: In those circumstances, there was enough of an understanding of at least some of the mechanisms involved. But with a situation like this, you can see where the smoking gun is, and now you even see the smoke coming out of the gun. But you still don't know why it fired in the first place. So that's a different circumstance.

KAUFMAN: And as one former top official at the FAA said today, the agency will likely be very cautious in what it says and does this time. It's likely to be a month or more before the FAA gives a green light for commercial operations of the 787 to resume.

Wendy Kaufman, NPR News.

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