Boston airport firefighters encountered sizzling liquid and a hissing, “exploding” battery when they entered the 787 at the center of a two-month-long National Transportation Safety Board investigation, according to documents released Thursday.
The NTSB said Thursday it plans two public hearings next month, one to explore lithium-ion battery technology in general and another to discuss the design and certification of the Boeing 787 battery system.
But the NTSB still has found no root cause of the battery fire on a Japan Airlines 787 fire at the Boston airport in early January.
“The NTSB’s investigation into the probable cause … is continuing,” the safety agency states in an interim factual report and 499 pages of related documents on its investigation.
Among the findings in the documents released Thursday:
• Boeing outsourced key analysis and testing of the battery system’s safety to its subcontractor, Thales of France, and to the battery maker, GS Yuasa of Japan.
Winning Federal Aviation Administration certification for the 787 entailed a battery functional hazard assessment, a fault tree analysis, a failure mode and effects analysis, and a battery/battery charger system safety assessment.
“These analyses and tests were performed by Thales/GS-Yuasa and reviewed by Boeing,” the NTSB states.
• Boeing did its own separate safety assessment of the entire electrical power system that “also included an analysis of lithium-ion battery cell failure modes.”
Its analysis determined that “overcharging was the only known failure mode” that could result in fire. Boeing therefore built safeguards into the system to “to ensure that the likelihood of occurrence of an overcharge event” was less than 1 in 1 billion — the usual FAA standard in providing for potentially catastrophic events.
But there is no indication in the NTSB documents that the battery that caught fire was overcharged. Investigators inspected a hefty electrical contactor — a relay switch — that is part of the battery management system and was designed to open the electrical points and disconnect the cells in the event of an overcharge.
The heavily blackened contactor was found to be “in the de-energized closed orientation,” meaning that no overcharge had registered with the system and the contactor had not disconnected the cells.
• The NTSB a month ago established that the fire instead started with an internal short circuit of a single cell in the eight-cell battery.
Boeing’s pre-certification testing did try to evaluate the effect of an internal short circuit. In this test, a cell was punctured with a nail to induce a short circuit.
“This test resulted in venting with smoke but no fire,” the NTSB reported.
Boeing also consulted with other companies about their experience with the use of similar lithium-ion battery cells and “based on this information, Boeing assessed that the likelihood of occurrence of cell venting would be about 1 in 10 million flight hours.”
The 787 that caught fire in Boston had logged just 169 flight hours, the report states.
And the entire operational fleet of 787s had logged a total of 51,662 in-service hours, plus about 6,000 flight test hours.
• Testing of the battery charging unit system was done by a Thales sub-contractor, Securaplane of Tucson, Ariz.
Early developmental testing of this system resulted in a major battery fire in 2006 that burned down a Securaplane building. After this, actual batteries were used only for isolated tests, with most of the testing instead using equipment that provided an electrical load representative of what a battery would provide.
The NTSB notes that there doesn’t seem to have been any testing of the charging system and battery together as an integrated system inside the airplane.
“No records have been seen that documented the performance of the individual Li-ion battery cells in testing that involved a battery/BCU set or in a complete Model 787 airplane,” the NTSB report states.
As part of its investigation, the NTSB last month began integrated system tests at a Boeing lab in Seattle. It is still doing data review and analysis.
• On the day of the Boston fire, the battery did not behave as Boeing or subcontractor Thales predicted.
The battery’s power discharge was “not at the constant rate described by the Boeing or Thales documents and included large changes and reversals of power within short periods of time,” according to the NTSB’s preliminary report.
• Sitting on a rack above the battery that burned was a smaller lithium-ion battery, also supplied by Japanese manufacturer GS Yuasa, that is used to provide emergency power for the jet’s flight controls “for a minimum of 10 minutes when no other electrical power is available.”
Investigators found the exterior of this battery had been “lightly scorched” by the fire below and noted that its case had openings at the corners.
• No heat damage was found to any primary airplane structure. But the floor panel and carbon fiber floor support material, which are considered to be secondary structure, “were found to be heat-damaged beneath where the APU battery had been installed.”
• The firefighters who were called to put out the fire did not know they were dealing with a lithium-ion battery, and had great difficulty putting out the intense fire.
When Capt. Mark Munroe of the airport’s aircraft rescue and firefighting unit entered the plane, he “saw heavy white smoke billowing through the floor” of the passenger cabin.
After locating the fire inside the electronics bay in the belly of the airplane, firefighters entered the compartment through dense smoke and applied shots of Halotron fire extinguisher to the battery.
Lt. David Hoadley of the firefighting unit reported that “It seemed like the fire did not want to go out; it kept rekindling.”
Then the battery “exploded,” according to Capt. Monroe.
“Capt. Munroe heard the battery hissing still and pushing white smoke or steam. There was liquid sizzling over the sides of the battery and still heavy smoke conditions. … The battery continued to hiss before exploding.”
Monroe related that “he felt something hit him in the neck while he was in the airplane,” and he was sent out for medical treatment. “Something had burned his neck.”
Firefighters attempted to remove the battery from the jet, but found that the “quick disconnect” mechanism Boeing had included to allow mechanics to take out the battery for maintenance was “melted and un-recognizable” and a metal plate was preventing access.
The battery had to be cut out from the rack where it sat.
“With a hot battery and a gloved hand (Lt. Hoadley) could not access the bolts on the lower rails with tools. They attempted with pliers to remove the bolts for maybe 20 minutes. What looked like Teflon slides were burnt away and the battery would not move. There were 3 more screws that could not be removed.”
Firefighters cut away the metal plate, severed the battery wire, then “pried the battery loose with hydraulic spreaders and removed it.”
The battery was passed down to a firefighter and placed on the tarmac about 50 feet from the airplane.
The fire was declared under control an hour and 40 minutes after the initial notification.
Boeing’s entire fleet of 787s has been grounded since a second battery incident during a flight in Japan, a week after the Boeing fire.
The company’s proposed fix for its battery system is currently under review by the FAA.
The agency expected to make an initial recommendation next week that will lay out a testing and certification path toward getting the airplanes back into service with airlines.