Authorities pinpoint cause of 787 battery fire

The aircraft was on the ground in Boston on January 7, 2013 when the auxiliary power unit battery (APU) of a JAL 787 experienced severe fire damage.

This and other incidents led the US Federal Aviation Administration (FAA) to issue an emergency airworthiness directive that required airlines to temporarily cease flights of the 787 whilst battery failures were investigated.

‘US airlines carry about two million people through the skies safely every day, which has been achieved in large part through design redundancy and layers of defence,’ said NTSB chairman Deborah A.P. Hersman. ‘Our task now is to see if enough - and appropriate - layers of defense and adequate checks were built into the design, certification and manufacturing of this battery.’

After an examination of the JAL lithium-ion battery, which was comprised of eight individual cells, investigators determined that the majority of evidence from the flight data recorder and both thermal and mechanical damage pointed to an initiating event in a single cell.

According to NTSB, that cell showed multiple signs of short circuiting, leading to a thermal runaway condition, which then cascaded to other cells. Charred battery components indicated that the temperature inside the battery case exceeded 500 degrees Fahrenheit.

As investigators work to find the cause of the initiating short circuit, they ruled out mechanical impact damage to the battery and external short circuiting. It was determined that signs of deformation and electrical arcing on the battery case occurred as a result of the battery malfunction and were not related to its cause.

Hersman said in a statement that potential causes of the initiating short circuit currently being evaluated include battery charging, the design and construction of the battery, and the possibility of defects introduced during the manufacturing process.

During the 787 certification process, Boeing studied possible failures that could occur within the battery. Those assessments included the likelihood of particular types of failures occurring, as well as the effects they could have on the battery. In tests to validate these assessments, Boeing found no evidence of cell-to-cell propagation or fire, both of which occurred in the JAL event.

In a related development the FAA has announced that it has granted Boeing permission to conduct test flights of 787 aircraft to collect data about the battery and electrical system performance while the aircraft is airborne.