Updated March 7, 2013, 10:03 p.m. ET
By ANDY PASZTOR
The Wall Street Journal
The battery that burst into flames two months ago on a Boeing Co. jet in Boston experienced dramatic power fluctuations and other failures its designers had considered practically impossible, according to the latest report by U.S. air-safety regulators.
Their update, however, didn't clear up the mystery over the fire's initial cause.
The National Transportation Safety Board interim report released Thursday did provide some new details about how a short-circuit that began in a single battery cell spread to seven other cells and ended up in an uncontrollable thermal reaction that quickly reached more than 1,250 degrees Fahrenheit, burning through the battery's metal container. The fire aboard the Japan Airlines Co. plane parked in Boston created smoke so intense that at one point, a firefighter couldn't see the battery when he entered the electronics bay containing it, the report said.
A fire captain on the scene told investigators that a bit later, the battery was "hissing loudly and that liquid was flowing down the sides of the battery case" before it "exploded."
The report detailed that years earlier, Boeing and federal regulators grossly underestimated hazards from the 787's lithium-ion batteries, considering the likelihood of such a blaze as "extremely improbable," or less than one in one billion flight hours.
Yet before the world-wide fleet of Dreamliners was grounded in January, the dozens of planes in service had logged about 50,000 flight hours—and batteries burned on two of them. The second plane to experience a battery incident, though the battery didn't burst into flames, was a 787 operated by All Nippon Airways Co. that made an emergency landing on a flight in Japan on Jan. 16.
The 48-page report also indicated that investigators essentially determined that various electronic components connected to the battery—including a charging unit and a sophisticated surge protector—were intact and didn't exhibit any failures or defects. Tests after the fire showed that those components generally worked as expected, the report said.
Such findings are likely to focus more public attention on whether some type of defect or problem within the battery itself could have prompted the sequence of events that led to the fire.
The report, and hundreds of pages of supporting documents, offer at least one tantalizing hint that certain wires may have been damaged by excessive current flowing outside the battery. But as it has in the past, the safety board stopped short of indicating whether an external or internal cause was responsible.
As expected, much of the report was devoted to an examination of the Federal Aviation Administration's procedures and its oversight of Boeing tests used to certify the safety of the 787's batteries. The safety board previously challenged the validity of the engineering assumptions and risk analyses on which the FAA and the Chicago-based plane maker relied to demonstrate the safety of the batteries manufactured by Japan's GS Yuasa Corp.
Before certification of the 787, Boeing's hazard assessment deemed that a battery fire would be a "catastrophic" event that could cause the loss of an aircraft, according to the report. Boeing determined that a battery fire could be caused only by overcharging. In conjunction with regulators, the plane maker concluded that redundant safety systems, many of which would be installed specifically to prevent battery overcharging, essentially would be foolproof.
Investigators previously said they were convinced that the battery on the Japan Airlines plane wasn't overcharged.
The report also said that all the tests initially to determine the safety of the 787's battery system were conducted by Boeing or its subcontractors and were reviewed by Boeing engineers, along with company employees designated to serve as the FAA's representatives.
The FAA previously said it approved the type of testing that was done, helped determine standards for passing those tests and was kept informed about Boeing's risk assessments.
Investigators continue to review manufacturing and quality-control issues related to the batteries, Thursday's report said, which highlights Boeing's outsourcing of some safety analyses to subcontractors. The safety board, among other priorities, is paying "particular attention to the coordination of responsibility and authority of the contractors and subcontractors" that worked on the plane's battery system, according to the report.
In another section, the report reiterates that Boeing initially determined that the likelihood of a 787 battery emitting smoke, without catching on fire, was one in 10 million flight hours.
According to the board's investigators, Boeing's initial round of safety tests did try to assess the hazards of an internal battery short circuit. But the test used a nail to puncture a cell to induce the fault, and no flames were observed. The report also notes that from the start, the battery manufacturing process was intended to "prevent, detect and eliminate contamination as a source" of internal cell short circuits.
With the 787's grounding, resulting in international investigations and recertification efforts under way simultaneously, NTSB Chairman Deborah Hersman said "it is essential to provide the aviation community, policy makers and the public with the factual information we are developing"
In addition to Thursday's release of documents, Ms. Hersman said the board will convene a pair of public sessions next month to delve deeper into lithium-ion issues. A forum in mid-April is intended to concentrate on lithium-ion battery technology and transportation safety, while a later hearing will focus on the design and FAA approval of the 787's battery system.