The National Transportation Safety Board's preliminary findings, issued Thursday, marked an important step forward for the closely-watched probe after little public indication of progress since it started in mid-January. The safety board said multiple short-circuits in one of the battery's eight cells touched off "an uncontrolled chemical reaction at high temperatures" that spread to other cells and ended up with the 63-pound battery in flames.

Investigators stopped short of identifying the root cause of the Jan. 7 blaze on the Japan Airlines Co. plane, which they indicated could range from contamination to hard-to-detect problems with the plane's electrical grid.

With the probe ongoing and investigators highlighting a series of past miscalculations by Boeing and regulators, prospects for speedy resolution of 787 safety matters appeared to dim. Industry officials have said that historically, redesigning, testing and deploying a new generation of batteries can take up to a year. This time, there is intense pressure on all participants to move faster, but experts predict it is still likely to take months.

NTSB Chairman Deborah Hersman said at a news briefing that investigators "have a long road ahead" to flesh out all of the possible hazards—including potential manufacturing defects—raised by malfunctions of the 787's novel lithium-ion batteries. She said the NTSB plans to release an interim report of its findings in 30 days.

The latest findings directly challenge the criteria and testing standards Boeing and federal regulators relied on six years ago to certify the safety of the power packs. The continuing investigation—loosely coordinated with a Japanese examination of a second 787 battery that burned in midair on a plane in Japan—demonstrates that "some of [those] assumptions were not met" and "a short-circuit in a single cell can propagate to adjacent cells and result in smoke and fire," Ms. Hersman said.

For the Chicago jet maker, which initially used testing and other data to persuade the FAA that such a dangerous scenario was practically impossible, Thursday's developments amounted to a rare public rebuke of its design and engineering processes.

Late Thursday, the Federal Aviation Administration approved Boeing's request to begin closely-monitored test flights to evaluate the 787's batteries and electrical system. The flights, which will be limited to Boeing aircrew and essential test personnel, will give Boeing the opportunity to collect data and try to replicate the conditions that caused the batteries to fail.
Boeing has told airlines that it hopes to have Dreamliners flying passengers again by March. But answering the NTSB's pointed criticism of the plane's initial safety assessments—combined with anticipated delays in verifying fixes and locking in a new battery design—could take months, according to people briefed on the process.

Boeing spokesman Marc Birtel said it "welcomes the progress" in the NTSB investigation. "We are working collaboratively to address questions about our testing and compliance with certification standards, and we will not hesitate to make changes that lead to improved testing processes and products," he added.

Boeing says each individual component inside the battery was tested before the company undertook 5,000 hours of battery testing at laboratories and more than 25,000 hours on the overall electrical system. The company described the harsh treatment it used to cause the batteries to fail during testing including "baking the battery to induce overheating, crush testing and puncturing a cell with a nail to induce a short circuit."

Since the world-wide fleet of 50 Dreamliners was grounded in mid-January, the Federal Aviation Administration has said the jets won't be cleared to fly until the agency is convinced all hazards have been mitigated.

Following the briefing, FAA chief Michael Huerta and his boss, Transportation Secretary Ray LaHood, released a joint statement emphasizing that investigators must finish their work "before reaching conclusions about what changes or improvements the FAA should make going forward." As part of a previously announced review of the 787's design and assembly, according to the statement, "the FAA is looking at both the certification process and specifically, at the required tests and design" of the suspect batteries.

Ending weeks of mystery about whether the short circuit caused or stemmed from the heat buildup inside the battery, the NTSB chief said "we believe that the evidence points" to a sequence of events in which "the short-circuit came first."

According to the safety board, Boeing's initial battery testing indicated that the likelihood of smoke coming from a lithium-ion battery was roughly one such event in 10 million flight hours. So far, the Dreamliners fleet has flown about 50,000 total hours in revenue service and there have been a pair of events causing batteries not only to emit smoke but to rupture and burn. "There have now been two battery events resulting in smoke, less than two weeks apart, on two different aircraft," Ms. Hersman said.

The safety board, an independent agency that issues non-binding recommendations, has a tradition of friction with the FAA, which is responsible for regulating safety. Often, that has produced pubic spats and resulted in industry receiving conflicting signals. But in the case of the Dreamliner, the two organizations have seemed to work unusually well together.
Jim Hall, a former chairman of the NTSB, says the criticism leveled at the FAA is ultimately designed to strengthen, not hurt, the relationship between the regulator and the air-accident watchdog. Mr. Hall said the relationship is "successful because it is built on tension."

In addition to highlighting internal battery issues, investigators continue to look at the possibility that an external problem stemming from battery charging or some other type electrical glitch touched off the cascade of events. "There are a lot of things we are still looking at," Ms. Hersman said.

The investigators' interim findings, though, don't seem to offer much guidance for Boeing's plans to develop a more-robust outside container for the battery. Based on investigative efforts so far, Ms. Hersman said it isn't clear whether the existing protective casing did its job.

The safety board said it has ruled out an external short-circuit and impact damage.
Some of the safety board's preliminary findings coincide with Boeing's current efforts to devise a potential new internal battery structure, including ways to make individual cells more rigid and increase the separation between them. Along these lines, Ms. Hersman said investigators are "looking at the physical separation of the cells, their electrical inter-connections and their thermal isolation from each other."

Sketching out broader policy and testing issues the safety board is looking at, Ms. Hersman said "we need to understand what tests were done" before the planes went into service and "how they were verified, not just by Boeing but by the regulator."