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Fine detail: NTSB materials engineer examines the damaged batter casing
Shortcomings in design and certification ultimately led to the fire in a lithium-ion battery installed on a Boeing 787 jetliner that had just completed an intercontinental flight to Boston last year, an official investigation has revealed.
On 7 January 2013, ground workers discovered smoke and flames coming from an auxiliary power unit lithium-ion battery in a Japan Airlines 787. The battery was manufactured by GS Yuasa.
Early in the investigation, officials from the US National Transportation Safety Board (NTSB) said that the fire began after one of the battery’s eight cells experienced an internal short-circuit leading to thermal runaway of the cell, which propagated to the remaining cells causing full battery thermal runaway. This condition caused smoke and flammable materials to be ejected outside the battery’s case and resulted in excessive heat and a fire.
The battery failure did not result from overcharging, overdischarging, external short-circuiting, external heating, installation factors, or environmental conditions of the aeroplane, said NTSB.
Christopher Hart, NTSB acting chairman, said: “The investigation identified deficiencies in the design and certification processes that should have prevented an outcome like this. Fortunately, this incident occurred while the airplane was on the ground and with firefighters immediately available.”
Because the auxiliary power unit and main lithium-ion batteries installed on the 787 represented new technology not adequately addressed by existing regulations, the Federal Aviation Administration (FAA) required that Boeing demonstrate compliance with special conditions to ensure that the battery was safe for use on an aircraft, said NTSB.
Investigators said that Boeing’s safety assessment of the battery, which was part of the data used to demonstrate compliance with these special conditions, was insufficient because Boeing had considered, but ruled out, cell-to-cell propagation of thermal runaway (which occurred in this incident) but did not provide the corresponding analysis and justification. As a result, the potential for cell-to-cell propagation of thermal runaway was not thoroughly scrutinised by Boeing and FAA engineers, ultimately allowing this safety hazard to go undetected by the certification process.
As a result of its findings, the NTSB is recommending that the FAA improve the guidance and training provided to industry and FAA certification engineers on safety assessments and methods of compliance for designs involving new technology.
“Through comprehensive incident investigations like this one, safety deficiencies can be uncovered and addressed before they lead to more serious consequences in less benign circumstances,” said Hart.
NTSB investigators also identified a number of design and manufacturing concerns that could have led to internal short-circuiting within a cell.
As a result of the investigation, the NTSB made 15 safety recommendations to the FAA, two to Boeing, and one to GS Yuasa.