Boeing 787 battery fire probe points to one cell: NTSB

The US Federal Aviation Authority (FAA) has allowed Boeing to carry out flight tests of its 787 Dreamliner in order to help it check its troubled electrical system and batteries which is suspected to be the root cause of battery fire in one of its planes last month.

The entire Dreamliner fleet had been grounded ever since the January 7 incident.

In a statement late yesterday, the FAA said these flight tests will allow Boeing to gather additional data.

“These test flights will be an important part of our efforts to ensure the safety of passengers and return these aircraft to service,” Transportation Secretary Ray Lahood and FAA Administrator Michael Huerta said in a joint statement.

They said the primary purpose of the test flights will be to collect data about the battery and electrical system performance while the plane is air-borne.

These test flights, however, will be subject to a number of restrictions including extensive pre-flight testing and inspections and in-flight monitoring in order to ensure the highest levels of safety.

The flights will be conducted in defined airspace over unpopulated areas, they said.

NTSB zeroed down on root cause

Earlier in the day the National Transportation Safety Board (NTSB), which is carrying out probe into incident, said it has zeroed down on the root cause.

NTSB Chairman Deborah Hersman said the flight data recorder data showed the battery voltage had unexpectedly dropped from a full charge of approximately 32 volts to approximately 28 volts.

This drop is consistent with the charge voltage of a single cell, she said.

“Based upon findings from the examinations and identifying thermal and mechanical damage, we believe that the evidence points to a single cell as the initiating event,” she said.

Hersman said short-circuit was caused in cell number six of the battery which was the root cause of the fire.

“At this time, we’ve been able to rule out two possible causes. We have ruled out the mechanical impact damage to the battery. All mechanical damage to the cells and the battery case occurred after initiating the short-circuiting in cell number six,” she said.

The board, in its probe, has also ruled out external short-circuiting of the cells or battery, she said, adding several potential causes for the short-circuiting in cell number six are being examined.

“We are looking for evidence of contamination, electro folds, wrinkles and pinches in the assembly of cells and the battery. And because the 787 battery is really a collection of eight individual cells packaged together in a box, we’re looking at the total design of the battery”, she said.

NTSB blames FAA’s certification process

The NTSB also blamed the FAA certification process for having given approval for the battery.

“Because the 787 incorporated a number of what the FAA calls ‘novel or unusual design features’, the applicable airworthiness regulations at the time did not address those new features.”

“So as a result, in 2007, the FAA issued nine special conditions for the 787 with regard to the use of lithium ion batteries. These special conditions contained additional safety requirements that the FAA deemed necessary to ensure an equivalent level of safety,” Hersman said.

“Our investigators found that during the certification process, Boeing studied possible failures that could occur within the battery.

“They, number one, assessed the likelihood of the failure occurring, and two, they assessed the effects that failure would have on the battery,” she said.

“To do this, various tests were performed on battery cells. One test was characterised as abusive, and it was intended to short-circuit the induce short-circuiting and cell venting,” she said.

“Boeing has indicated that these tests that were conducted prior to certification showed no evidence of cell-to-cell propagation or fire in the battery. However, our investigative findings with respect to the event battery show that when a short circuit did occur, it resulted in cell-to-cell propagation in a cascading manner and a fire,” Hersman said.

Another condition considered by Boeing during certification was whether a failure that resulted in a single cell venting through its ruptured disk would create smoke emission from the battery, she said.

Boeing assessed that the likelihood of a smoke emission event from a 787 battery will occur less than once in every 10 million flight hours.

The 787 fleet has accumulated less than 1,00,000 flight hours, yet there have now been two battery events resulting in smoke less than two weeks apart on two different aircraft, Hersman observed.

She said the investigation has demonstrated that a short circuit in a single cell can propagate to adjacent cells and result in smoke and fire.

“The assumptions used to certify the battery must be reconsidered. As we move forward, we will begin testing of some of the batteries that have been removed from the 787 fleet from the field,” she said.

Meanwhile in a separate joint statement, Lahood and Huerta said the FAA is looking at the certification process and specifically at the required tests and design of the aircraft’s lithium ion battery.

The FAA invited the NTSB to observe this FAA-led process,” they said.

Welcoming the progress reported by the NTSB, Boeing said the findings will provide the public with a better understanding of the nature of the investigation.

(This article was published on February 8, 2013)
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