US safety investigators have widened their probe of 787 battery failures to include the certification process used by the Federal Aviation Administration and Boeing to declare the lithium-ion batteries safe before the aircraft entered service.

The move by the National Transportation Safety Board could complicate reported efforts by Boeing to gain approval from the FAA to start testing an interim solution to the battery problem.

"The assumptions used to certify the battery must be reconsidered," says Deborah Hersman, chairman of the NTSB.

Boeing's pre-certification analysis determined that a battery failure could lead to the emission of smoke in less than one of 10 million flight hours by the 787.

"The 787 fleet has accumulated less than 100,000 flight hours and there have now been two battery events resulting in smoke less than [two] weeks apart on two different aircraft," Hersman says.

"We know that some of the assumptions they made to ensure there was not a smoke event were not met, much less a fire event."

The NTSB also has narrowed its search for a root cause of battery fire on a Japan Airlines 787 last month to the source of a short-circuit in one of the battery's eight cells, but the team still remains "probably weeks away" from reaching a conclusion, Hersman says.

Microscopic and flight data recorder examinations have pointed to the sixth cell as the source of the short circuit, which triggered a thermal runaway that "cascaded" to other cells and generated temperatures above 260e_SDgrC (500e_SDgrF), Hersman says.

The NTSB is evaluating three possible reasons why the sixth cell in the battery failed. There could be a problem in the 787's overall electrical system that charges the battery, Hersman says.

The NTSB also is still considering possible contamination of the cell during manufacturing, she says. Finally, the review is evaluating the design of the battery as the potential root cause.

Source: Air Transport Intelligence news