U.S. National Transportation Safety Board (NTSB) members and technical specialists questioned representatives of Boeing and the Federal Aviation Administration on Tuesday about assumptions they made in determining the probability of lithium-ion batteries failing on the Boeing 787 Dreamliner. Boeing stood by those assumptions despite two incidents of malfunctioning lithium-ion batteries that led authorities to ground the worldwide 787 fleet in mid-January. “When an event like this happens early in the life of the fleet, it doesn’t automatically change our assumptions,” said Michael Sinnett, Boeing’s 787 chief project engineer.

The NTSB conducted the first day of a two-day hearing into the design and certification processes Boeing followed to install lithium-ion batteries on the Dreamliner. NTSB Chairman Deborah Hersman said the hearing will inform the board’s investigation into the January 7 APU battery fire aboard a Japan Airlines 787 parked at Boston’s Logan International Airport, which remains in the “fact-finding” stage. Representatives of battery manufacturer GS Yuasa of Japan and Thales Avionics Electrical Systems of France, which supplies the 787’s electrical power conversion subsystem, also appeared at the hearing in Washington, D.C.

NTSB questioners interrogated Sinnett and FAA representatives about the special conditions the FAA developed to certify the 787’s lithium-ion batteries. Those conditions took effect in November 2007 as a supplement to the aircraft’s Part 25 airworthiness certification.

“We identified a number of design features on the 787 aircraft, including the lithium-ion main and APU batteries, where the current safety standards were not adequate or appropriate to address the novel or unusual design features,” said Dorenda Baker, director of the FAA’s aircraft certification service. “At the time, there was no aviation industry standard for rechargeable lithium-ion batteries.” Baker said the intent of the special conditions is to produce a level of safety equivalent to that of other types of batteries, and she described the certification process as “robust.”

Sinnett said Boeing’s “internal fault tree analysis” of the probability of a lithium-ion battery short-circuiting on the 787 is once in 10 million flight hours. The estimate was based on the experience of other types of batteries and also on GS Yuasa’s experience with “14,000 cells and millions of operating hours” with lithium-ion batteries. “What we can’t do is we can’t account for every single possible method of short-circuit, particularly what we would consider the unknown unknowns,” he said.