Deborah Hersman, the chairwoman of the National Transportation Safety Board, told reporters that the problems seemed to have originated in the battery, when one of the eight cells had a short circuit and the fire spread to the rest of the cells. But she said that Boeing’s tests showed no indication that the new lithium-ion batteries on its 787 planes could erupt in flame and concluded that they were likely to emit smoke less than once in every 10 million flight hours.

Once the planes were placed in service, though, the batteries overheated and emitted smoke twice last month, and caused one fire, after about 50,000 hours of commercial flights.

“The assumptions used to certify the batteries must be reconsidered,” Ms. Hersman said.

Late Thursday, the F.A.A. said it would allow Boeing to conduct test flights with its 787 to collect data on the batteries and the plane’s electrical system. The agency said the flights “will be an important part of our efforts to ensure the safety of passengers and return these aircraft to service.” It did not immediately specify the number of test flights or when they would begin.

Ms. Hersman, at her news conference, said that before the F.A.A. certified the batteries, Boeing’s tests found no evidence that a short circuit in one of the eight cells could spread to other cells.

But Ms. Hersman said the fire on a 787 parked at an airport in Boston on Jan. 7 started with a short circuit in one cell and then spread to the others.

She said investigators had still not been able to tell what caused the short circuit in that cell and led to a “thermal runaway,” overheating up to 500 degrees, that then cascaded to the rest of the cells.

Still, she said, “This investigation has demonstrated that a short circuit in a single cell can propagate to adjacent cells and result in a fire.”

Battery experts said that the finding pinpointed one step Boeing could take to make the batteries safer: it could expand the size of the battery to create more physical separation between the cells. Ralph J. Brodd, a battery industry consultant in Henderson, Nev., said Boeing and its Japanese battery subcontractor, GS Yuasa, could make the design and manufacturing changes needed to do that fairly quickly.

But unless investigators can determine what caused the first cell to short-circuit, federal officials said, Boeing will also be required to make other changes to prevent any of the possible causes and to better contain or vent any overheated materials. And given the safety board’s findings about how poorly Boeing gauged the original safety risks, the F.A.A. is likely to take its time in assessing the validity of any new tests.

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, when a 787 made an emergency landing in Japan after the pilots smelled smoke in the cockpit. That incident occurred nine days after the Boston fire.

In searching for the cause of the fire on the plane in Boston, Ms. Hersman said the safety board was still looking at the battery’s charging mechanism and potential manufacturing defects or contamination, and whether the cells were not as isolated as they should have been.

Investigators have so far ruled out two possible reasons for the short circuit — a mechanical or electrical shock from outside the battery.

“We have not yet identified what the cause of the short circuit is,” she said. “We are looking at the design of the battery, at the manufacturing, and we are also looking at the cell charging. There are a lot of things we are still looking at.”

Boeing said in a statement Thursday that it viewed the safety board’s findings as narrowing the likely source of the problem to within the battery itself, as opposed to other components of the plane’s extensive new electric system. But company officials said they also recognized that it would take a combination of changes to restore confidence in the battery system.

Article source: http://www.nytimes.com/2013/02/08/business/us-officials-fault-faa-for-missing-787-battery-risk.html?partner=rss&emc=rss