By Joan Lowy

Associated Press

WASHINGTON (AP) — The government should reassess its safety approval of the Boeing 787 lithium ion batteries, the nation’s top accident investigator said Thursday.

The National Transportation Safety Board’s investigation of last month’s battery fire in a Japan Airlines 787 “Dreamliner” while it was parked in Boston shows the fire started with multiple short-circuits in one of the battery’s eight cells, said the board’s chairman, Deborah Hersman. That created an uncontrolled chemical reaction known as “thermal runaway,” which is characterized by progressively hotter temperatures. That spread the short-circuiting to the rest of the cells and caused the fire, she said.

The findings are at odds with what Boeing told the Federal Aviation Administration when that agency was working to certify the company’s newest and most technologically advanced plane for flight, Hersman said. Boeing said its testing showed that any short-circuiting was contained within a single cell, preventing thermal runaway and fire, she told reporters at a news conference.

Boeing’s testing also showed the batteries were likely to cause smoke in only 1 in 10 million flight hours, she said. But the Boston fire was followed nine days later by a smoking battery in an All Nippon Airways plane that made an emergency landing in Japan. The 787 has recorded less than 100,000 flight hours, Hersman noted.

Investigators are still trying to determine why the first battery cell short-circuited, but the board’s findings appear to raise doubts about the thoroughness of FAA’s safety certification of the 787’s batteries and whether Boeing will be able to make a quick fix that returns the planes to the skies.

The same day as the ANA emergency landing, FAA officials ordered the only U.S. carrier with 787s — United Airlines, which has six of the planes — to ground them. Aviation authorities in other countries swiftly followed suit. In all, 50 planes operated by seven airlines in six countries are grounded.

The groundings have become a nightmare for the company, which has about 800 Dreamliner orders from airlines around the world.

GE Aviation, based in Evendale, supplies engines used in some 787s.