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 in the
safety assessment. As a result, the potential for
cell-to-cell propagation of thermal runaway was not
thoroughly scrutinized 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. "The aviation industry is continually
benefitting from technological advances, and we are
hopeful that the lessons learned in this investigation
will further enhance the industry's ability to safely
bring those innovative technologies to market," said
Hart. (Final Report)
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