NTSB report shows Boeing’s battery analysis fell short
The safety agency’s interim report shows Boeing mistakenly ruled out any potential causes of a battery fire other than an overcharge and failed to predict the battery’s erratic behavior on the day of the fire.
Seattle Times aerospace reporter
The most detailed report yet on the
lithium-ion battery fire on a 787 Dreamliner in January provides no answers on the root cause, but sheds new light on the safety analysis done by Boeing and its subcontractors to win Federal Aviation Administration certification and how that analysis fell short.
The National Transportation Safety Board (NTSB) released a voluminous set of documents Thursday from its investigation into the intense and persistent fire on the Japan Airlines jet parked at Boston’s Logan Airport.
Among the findings are Boeing mistakenly ruled out any potential causes of a battery fire other than an overcharge and failed to predict the battery’s erratic behavior on the day of the fire.
Still, the interim report lacks clear answers, and that increases the pressure on the FAA as the aviation regulator weighs whether to approve Boeing’s proposed battery fix, with an initial ruling expected next week.
That pressure will continue. 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.
Boeing’s safety analysis
After the Jan. 7 fire, the FAA announced it would review the plane’s electrical system and its own certification process. A week later, it grounded the fleet after another 787 battery overheated and smoldered in flight in Japan.
For the 787 to win FAA certification, the safety of the battery system had to be analyzed and tested in advance, with assessments of what failures could possibly happen and the potential impact of each.
The assessment was done at two levels. The subcontractors — Thales of France, battery maker GS Yuasa of Japan, and Securaplane of Tucson, Ariz., which designed and built the battery charging unit (BCU) system — focused on potential failures of their pieces of the system.
Boeing reviewed the supplier assessments, but it also took a more integrated look from the airplane level, including a safety assessment of the entire electrical- power system and with a specific look at “lithium-ion battery cell failure modes.”
The NTSB report seems to question the thoroughness of the testing done by Thales and Securaplane.
The report notes there doesn’t seem to have been any testing of the charging system and battery together as an integrated system inside the airplane.
“None of the Thales documents described a complete life-cycle of tests,” the report states. “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.”
Given that finding, the NTSB said that last month it began integrated-system tests at a Boeing lab in Seattle. It is still doing data review and analysis.
Boeing spokesman Marc Birtel said Thursday that “suppliers are in many cases responsible for design, build and testing of the parts they deliver to Boeing. Boeing is often involved in key tests and typically provides in-person support during those test activities.
“Regardless of who performs analysis or testing, Boeing ensures certification compliance,” Birtel said.
According to the NTSB, Boeing’s own analysis determined “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 one in a billion — the usual FAA standard in providing for potentially catastrophic events.
However, 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-battery cells and “based on this information, Boeing assessed that the likelihood of occurrence of cell venting would be about one in ten million flight hours.”
Yet all of this analysis badly missed the mark. The probabilities proved to be off by a factor of 200.
The 787 that caught fire in Boston had logged just 169 flight hours.
And the entire operational fleet of 787s had logged a total 51,662 in-service hours, plus about 6,000 flight-test hours.
On the day of the Boston fire, the battery did not behave as 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,” the report states.
The fire that day was small but intense.
Boston airport firefighters encountered heavy smoke in the passenger cabin and had to forcibly extract a smoking, hissing, popping, chemical-spewing battery from the belly of the plane.
Interviews by the NTSB revealed the firefighters did not know they were dealing with a lithium-ion battery, and they had great difficulty putting out the 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, in munroe’s words, “exploded.”
“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.”
Munroe related that “he felt something hit him in the neck while he was in the airplane,” and he was sent for medical treatment. “Something had burned his neck.”
Firefighters attempted to remove the battery from the jet but found the “quick disconnect” mechanism Boeing had included to allow the battery to be removed for maintenance was “melted and unrecognizable” and a metal plate was preventing access.
The firefighters cut away the metal plate, severed the battery wires, 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.
Still, for all the intensity, no one was badly hurt and details in the report suggest it could have been worse.
NTSB investigators found no heat damage to any primary airplane structure — that is, any part of the airframe critical to flight.
Only 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.”
And 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 10 minutes or more “when no other electrical power is available.”
Investigators found the exterior of this battery had been “lightly scorched” by the fire below and noted its case had openings at the corners.
The firefighters suppressed the fire before it could spread to that second battery.
Dominic Gates: firstname.lastname@example.org,