Boeing engineers and federal regulators appear to have vastly underestimated the possibility of a lithium-ion battery fire before the 787 Dreamliner was certified.
An interim report released last week by the National Transportation Safety Board said Boeing engineers believed an incident like the Jan. 7 fire on a Japan Airlines plane at Boston's Logan International Airport had a probability of happening just once in a billion flight hours. So far, the 787 fleet has logged approximately 52,000 hours and has already had two incidents in which batteries were burned.
A fire captain who responded to the Boston fire "reported that the battery was hissing loudly and that liquid was flowing down the sides of the battery case," the NTSB said in its 48-page report. "He heard a 'pop' sound," and smoke began pouring out of the electronics equipment bay. The captain "received a burn on his neck when the battery, in his words, 'exploded.'"
Click on the image below to see different views of the charred battery.
The auxiliary power unit (APU) battery area showed damage consistent with smoke, hot gases, and discharged electrolyte.
The incident occurred while the aircraft was being cleaned after a flight. A mechanic told the NTSB that he found heavy smoke in the electronics equipment compartment and then discovered two small flames at the connectors on the front of the battery case. After trying unsuccessfully to douse the flames with a fire extinguisher, he called the airport's fire department, which used a handheld thermal imaging camera to discover what a firefighter called "a white glow about the size of a softball" amid the smoke. The temperature of the battery reached 1,250F, the NTSB told Design News.
That fire was the first of two battery-related incidents for the 787. Nine days later, a 787 operated by All Nippon Airways Co. made an emergency landing at Takamatsu Airport in Japan. Though the batteries sustained heat damage, it's not yet clear whether a fire was involved in that incident.
The two incidents appear to contradict Boeing's original functional safety hazard assessment of the 787 electrical power system. That assessment, made before certification, identified two potential hazards pertaining to the aircraft's main and auxiliary power unit (APU) batteries. The first hazard, "battery vents smoke/fire," was assessed with an average probability of 1 x 10-9 per flight hour, according to the NTSB report. The probability for the second, "battery vent and/or smoke without fire," was classified as 1 x 10-7 per flight hour. The assessment now appears to make the events about 20,000 times less likely than what has occurred at this point.
Boeing evaluated the probabilities by puncturing a battery cell with a nail to induce short circuiting. "This test resulted in cell venting with smoke but no fire," the report said. The company's engineers then combined those results with information from other manufacturers to determine that the likelihood of cell venting (without fire) would be about once per 10 million flight hours.
The NTSB still does not know what caused the JAL fire. It examined the battery and its eight cells. "All of the cells were found to be electrically short circuited except for cell 8," and four of the cells "exhibited a darkened, charred appearance."
Still, no one as yet knows exactly why the short circuit occurred. Before certification, Boeing's analysis determined that overcharging was the only failure mode that could cause a cell to vent fire. However, NTSB investigators have said the JAL battery was not overcharged.
The agency is "continuing to review the design, certification, and manufacturing processes for the 787 lithium-ion battery system."