Lithium-metallbatterier - UK`s AAIB enig med NTSB

UK urges FAA to review lithium-metal

British air accident investigators are urging their US counterparts at the Federal Aviation Administration (FAA) to develop tougher certification requirements for lithium-metal batteries in aviation equipment.

The UK's Air Accident Investigation Branch (AAIB) found that a ground fire on an unoccupied Boeing 787-8 operated by Ethiopian Airlines at London Heathrow Airport on 12 July, 2013, was caused by all five cells in the emergency locator transmitter (ELT) battery experiencing thermal runaway.

"Strong physical evidence from the ELT, supported by the manufacturers' root cause testing, identified the most likely cause of the ELT fire as an external short-circuit of the battery, due to the improperly routed battery wires, most probably in combination with the early depletion of a single cell," stated the AAIB in its report.

The investigators found that neither the cell-level nor battery-level safety features were able to prevent the single-cell failure, which then propagated to adjacent cells, resulting in a cascading thermal runaway, rupture of the cells which generated smoke, fire and flammable electrolyte.

The pinched wires further prevented the battery cover-plate from forming an effective seal with the ELT case and provided a gas path for flames and ejected battery decomposition products to escape from the ELT to the surrounding aircraft structure.

The UK in its recommendations said the US regulator should take account of current industry knowledge on the design, operational characteristics and failure modes of lithium-metal batteries to develop tougher certification standards.

It made several further recommendations:

The FAA should require that electrical performance and design-abuse certification tests for lithium-metal batteries are conducted with the battery installed in the parent equipment, to take account of battery thermal performance.

The FAA should work with industry to determine the best methods to force a lithium-metal cell into thermal runaway and develop design-abuse testing that subjects a single cell within a lithium - metal battery to thermal runaway in order to demonstrate the worst possible effects during certification testing.

The FAA should require equipment manufacturers wishing to use lithium-metal batteries to demonstrate (using the design-abuse testing described in Safety Recommendation 2014-022) that the battery and equipment design mitigates all hazardous effects of propagation of a single-cell thermal runaway to other cells and the release of electrolyte, fire or explosive debris.

The UK investigators also urge the FAA to review whether the Technical Standard Order (TSO) process is the most effective means for the certification of lithium-metal batteries installed in aircraft equipment, the actual performance of which can only be verified when demonstrated in the parent equipment and the aircraft installation.