The Boeing 787 "Dreamliner" batteries that caused the airplane to be grounded exhibited signs of short-circuiting and an accelerated temperature increase, the National Transportation Safety Board (NTSB) said Thursday.
NTSB Chairwoman Deborah Hersman said the findings were an early result of the agency's investigation into a fire on a 787 at Boston's Logan International Airport earlier this month that was caused by a lithium battery on one of the planes.
Hersman said the agency has not yet determined if the symptoms that have been identified in its examination of the damaged battery were the cause of the fire that led to the 787 being grounded by the Federal Aviation Administration (FAA).
"We know the lithium ion battery experienced a thermal runaway, we know there were short circuits, and we know there was a fire," Hersman said during a news conference at the NTSB's Washington headquarters.
"We are not determining the cause of the event," she continued. "Just sharing some characteristics [of the damaged batteries]."
Hersman, the NTSB chief who has been mentioned as a possible replacement for Transportation Secretary Ray LaHood, took pains to make clear that the NTSB's investigation of the 787 was separate from the FAA review that has resulted in the planes being taken out of the skies.
The FAA, which is a part of the Department of Transportation, certified the 787 originally for flight and took it out of service after a pair of battery incidents last week. The NTSB's members, who are appointed by the president but independent of the DOT, have investigated the circumstances of the incident that occurred in the U.S.
Citing that distinction between the agencies Thursday, Hersman would not offer an option on when she thought the 787 should be allowed to return to flight.
"It's important to understand that the NTSB and the FAA have different roles," she said. "[The FAA] made the decision to ground the plane, they will make the decision to allow them to fly again."
The order from the FAA for U.S. airlines to stop flying 787s until the mechanics were reviewed came after a battery fire at the Boston airport and a similar incident that occurred in flight in Japan.
Other worldwide aviation associations quickly followed suit, creating a worldwide ground stop — and a public relations headache — for the beleaguered Boeing aircraft.
Aviation industry observers have said that problems are typical with new airplane models, but Hersman said Thursday that the 787's battery issues were an "unprecedented event."
"This is a very serious air safety concern," she said. "This aircraft has been in the air for less than 100,000 hours, and to see two battery events in its early lifespan is not what we would expect.
"We have to understand why this battery resulted in a fire when there were so many protections designed to prevent it."
Hersman added that the 787 battery that has been examined did not show signs of being overcharged.
"The data traces that we have … do not show that the battery charged beyond the 32.2 [volt] limit," she said.
-This post was updated with a scientific definition of thermal runaway on Friday at 2:15 p.m.