As Ethiopian Airlines and other Boeing 787 customers prepared to return their Dreamliners to service with battery system modification kits, the U.S. National Transportation Safety Board (NTSB) conducted an exhaustive, two-day investigative hearing into the design and certification of the lithium-ion batteries implicated in the airplane’s grounding. Sixteen witnesses testified and answered questions during the hearing on April 23 and 24 at the Board’s Washington, D.C., headquarters.

NTSB chairman Deborah Hersman said the hearing will inform the Board’s investigation of the January 7 APU battery fire aboard a Japan Airlines 787 parked at Boston Logan International Airport, which remains in the “fact-finding” stage.

“While we do not know the cause of the JAL battery fire, within a month our forensic work identified the origin of the event: short circuits in [battery] cell number six that cascaded, in a thermal runaway, to other cells. The temperature inside the battery case exceeded 500 degrees Fahrenheit,” Hersman said in closing remarks. “The questions that were raised at this hearing’s outset, about how best to ensure safety and whether or not the certification process is flexible enough to incorporate new knowledge, are certainly pressing as we conclude this hearing… We must take a hard look at how best to oversee and approve emerging technology in the future.”

Representatives of Boeing, the Federal Aviation Administration, battery manufacturer GS Yuasa of Japan and Thales Avionics Electrical Systems of France, supplier of the 787’s electrical power conversion subsystem, appeared at the hearing.

NTSB technical experts interrogated Boeing 787 chief project engineer Michael Sinnett and FAA representatives about nine special conditions the FAA developed in conjunction with the manufacturer to certify the use of lithium-ion batteries on the 787, considered a “novel or unusual” design feature at the time. Those conditions took effect in November 2007 as a supplement to the aircraft’s Part 25 airworthiness certification. One line of questioning concerned the parties’ decision to forgo adopting a stricter set of requirements for permanently installed, rechargeable lithium-ion batteries known as DO-311, issued by standards organization RTCA in March 2008.

“That standard does not take into consideration aircraft systems that could mitigate some of those requirements,” said Ali Bahrami, manager of the FAA transport airplane directorate. “We actually tasked RTCA to start developing those standards for us. What’s really important to recognize is that we did not believe that there is an unsafe condition [with the 787 batteries] by not going to the later standards. Therefore, we didn’t take initiative to change the standards applicable to the 787 because we didn’t see any need for it.”

Sinnett said that Boeing’s testing regimen for lithium-ion batteries included driving a nail into a battery cell that “resulted in a short-circuit of the cell that wasn’t as energetic as we have seen in service.” While the company thought the nail penetration test would effectively induce a battery failure, “in retrospect I believe we don’t feel that it was conservative enough,” he added.