Failures by Boeing and its battery suppliers, and insufficient oversight by the US Federal Aviation Administration, resulted in a lithium-ion battery fire aboard a Boeing 787 in Boston early last year, according to a report released today by the National Transportation Safety Board (NTSB).
The board’s report called out Boeing and the FAA for not properly anticipating the chance of a so-called thermal runaway event in the aircraft’s lithium-ion batteries, and highlighted shortcomings by the battery’s manufacturers.
“The probable cause of this incident was an internal short circuit within a cell of the APU [auxiliary power unit] lithium-ion battery, which led to thermal runaway that cascaded to adjacent cells, resulting in the release of smoke and fire,” says the NTSB in its 1 December report.
“The incident resulted from Boeing’s failure to incorporate design requirements to mitigate the most severe effects of an internal short circuit within an APU battery cell, and the FAA’s failure to identify this design deficiency during the type design certification process,” adds the board.
The incident occurred on 7 January 2013, when the APU lithium ion battery on a Japan Airlines (JAL) 787-8 caught fire while the aircraft was on the ground at Boston Logan International airport.
The incident preceded a similar event on 16 January of that year, when a battery caught fire on an All Nippon Airways 787, forcing the crew to make an emergency landing.
The NTSB’s reports notes insufficient oversight down the regulatory-manufacturing chain – from the FAA to Boeing, from Boeing to battery designer Thales Group, and from Thales to the battery manufacturer, Japanese company GS Yuasa.
The FAA failed to recognise the potential for thermal runaways and therefore did not require its engineers to perform thermal runaway tests as part of the certification process, says the NTSB.
Meanwhile, Boeing provided insufficient guidance to the battery manufacturers, adds the NTSB. Boeing’s battery safety assessment assumed a short circuit would not spread to other cells, but the company did not justify that assessment.
Boeing also did not consider the consequence of its assessment possibly being incorrect, which it was, the NTSB writes.
In addition, Thales insufficiently oversaw GS Yuasa.
“GS Yuasa did not test the battery under the most severe conditions possible in service, and the test battery was different than the final battery design certified for installation on the airplane,” says the NTSB.
The board says that during a visit to GS Yuasa it found that the company’s post-assembly battery inspections “could not reliably detect manufacturing defects” caused by foreign object debris generated during manufacturing.
Those defects can lead to short circuits, says the NTSB.
The NTSB also notes that the flight data recorders on the JAL 787 continued to record seemingly valid parameters after the source stopped providing data, delaying the NTSB’s understanding of the data.
Following the two battery fires, the FAA grounded the 787 fleet for three months while Boeing devised a fix.
Boeing redesigned the installation of the battery, adding a stainless steel battery case, and improved the battery monitory software.
But it retained the lithium-cobalt-dioxide chemistry of the battery’s eight cells, which store 72Ah of power.
The changes will not prevent a cell from “venting” – a thermal event in which burning electrolytes spew heat and vapour.
Rather, Boeing has said the new design should prevent actual flames and keep the venting cell from affecting other cells – the thermal runaway.
The improvements are also intended to channel heat and vapour from a venting cell directly off the aircraft.