But John Goglia, a former NTSB board member and aviation safety expert, said NTSB's findings mean the government will likely require Boeing to re-certify the batteries.
“Certifications aren't exactly painless and quick,” he said. “It could be a big, drawn-out thing.”
The significance of the NTSB's findings “is if this can happen — and the safety analysis assumed that it would not happen — then the safety analysis is no longer valid,” said Jon Hansman, a Massachusetts Institute of Technology aeronautics professor and a member of the FAA's Research and Development Advisory Committee.
Battery experts said Boeing could try to build more safeguards into the battery by using a greater number of smaller cells and putting more insulation between them. Or, they said, the aircraft maker could switch to a different type of lithium ion battery already approved for aviation. Some business jets use lithium ion batteries as their main batteries.
Switching to another type of battery, such as lead-acid or nickel-cadmium battery, is another possibility, but that would involve changing the charging system as well, they said. The new batteries — and, presumably, a revised charging system — would need to be designed and tested by Boeing and approved by the FAA before they could be installed.
Boeing issued a statement saying it is working to address questions about its testing and compliance with certifications requirements, “and we will not hesitate to make changes that lead to improved testing processes and products.”
Defending its original testing, Boeing said there were more than 5,000 hours of laboratory testing on the battery itself, including normal operations and simulated failures. Examples of these tests include baking the battery to induce overheating, crush testing and puncturing a cell with a nail to induce a short circuit.
The battery was tested on a plane, both in flight and on ground, for more than 10,000 hours in total, the company said.
The same day as the emergency landing in Japan, 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 787 is the first airliner to make extensive use of lithium batteries. Besides being lighter, the batteries recharge faster and can store more energy than other types of batteries of an equivalent size, and can be molded to fit into odd spaces on planes. The Airbus A350, expected to be ready next year, will also make extensive use of lithium ion batteries. Manufacturers are also looking to retrofit existing planes, replacing other types of batteries with lithium ion.
But lithium batteries in general are more likely to short-circuit and start a fire than other batteries if they are damaged, if there is a manufacturing flaw or if they are exposed to excessive heat.
In 2007, the FAA issued special conditions that Boeing had to meet in order to use lithium ion batteries in the 787, because at that time the agency's safety regulations didn't include standards for such battery systems.
The 787 relies to a greater extent than any previous airliner on electrical systems, as opposed to hydraulic or mechanical ones. The batteries help run those electrical systems and also are used to start a power-generating engine in the rear of the aircraft.
The batteries are made by GS Yuasa of Japan. Japanese aviation investigators probing the cause of the ANA battery failure have also found there was thermal runaway.
Investigators have ruled out mechanical damage or external short-circuiting as possible causes of the initial, internal battery short-circuiting, Hersman said. Investigators and technical experts are now looking for evidence of flaws inside the batteries like pinches, wrinkles or folds, she said.
“We are looking at a number of scenarios,” Hersman said, including the state of charge of the battery, its manufacturing processes and the design of the batteries.
“We haven't reached any conclusions at this point,” she said. “We really have a lot of work to do.”