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The Federal Aviation Administration (FAA) said Friday it received a formal proposal from Boeing to fix the lithium-ion battery problems in the 787 Dreamliners and “will analyze it closely.”

But the agency indicated it won’t rush to get the Dreamliners back in the air despite the crisis the grounding of the planes has brought to Boeing and its customers.

“The safety of the flying public is our top priority, and we won’t allow the 787 to return to commercial service until we’re confident that any proposed solution has addressed the battery failure risks,” said an FAA statement.

The Dreamliners have been grounded since mid-January after two battery failures caused a battery fire in a jet on the ground in Boston and then a smoldering battery on a flight in Japan.

Though no one was seriously hurt in either incident, the 787 grounding has already lasted a day longer than the 1979 grounding of the DC-10 fleet, which was prompted by an air crash that killed 273 people.

A Boeing team led by Commercial Airplanes chief Ray Conner presented the company’s proposed fix in Washington, D.C., on Friday to FAA head Michael Huerta, deputy Transportation Secretary John Porcari and other FAA officials.

Boeing issued only a short statement saying the two sides had “a productive meeting,” providing no further detail.

Boeing remains tight-lipped about its proposed fix, but according to multiple sources, it includes:

• A stronger, sealed containment box enclosing the eight battery cells

• A system of venting tubes that in case of an incident would channel any flammable vapors or liquids directly out of the airplane

• Continuous monitoring of temperature and voltage of individual cells within the battery

• Better thermal separation of the cells, with some barrier such as high-temperature glass inserted between them

Battery experts suggest these could all be part of a good engineering solution to try to prevent what the National Transportation Safety Board (NTSB) says happened in Boston: an internal short circuit in one cell caused overheating that spread to adjoining cells and led to uncontrolled thermal runaway and fire.

Brian Barnett, a battery specialist with technology-development company TIAX in Lexington, Mass., said battery-cell overheating can be contained.

“You can calculate the energy released and understand the kinetics for the release of that energy, then you should be able to build a containment,” Barnett said.

What’s unclear is how much testing the FAA will require to validate the safety of the revamped battery.

Barnett said the tests conducted on the batteries by Boeing during the original certification process — baking the battery in an oven, puncturing it with a nail, crushing it, and overcharging it — are standard industry tests but don’t reflect what typically happens when a battery fails due to an internal short circuit.

He said TIAX researchers have perfected a way to implant a tiny metal particle inside a cell and artificially trigger such a short circuit — “it’s very difficult to do” — allowing them to study how the heat can cascade from cell to cell and cause thermal runaway.

To prove the new battery setup performs as promised, Boeing may have to do similar testing. Such tests would have to be done in a controlled lab environment not on a flight test, Barnett said.

The FAA will likely demand flight tests will be needed, however, to prove the new venting system works, putting it through multiple pressurization cycles as the jet takes off and lands.

How fast could all the testing be completed?

In January, Transportation Secretary Ray LaHood, who oversees the FAA, said the 787 wouldn’t be allowed to fly again until authorities are “1,000 percent sure” it is safe.

Boeing indicated in private meetings with key members of Congress on Wednesday that it hopes to get permission for the 787s to fly passengers again by April.

Issaquah-based aviation analyst Scott Hamilton, of, said that schedule is “aggressive.”

He points out that the NTSB’s preliminary report of its investigation into the 787 fire in Boston isn’t expected until early March.

And the FAA will want thorough testing of Boeing’s fix, he said.

“The last thing the FAA can stand is to fast-track this and then have something else go wrong,” Hamilton said.

After the fire in Boston, and before the second battery incident, the FAA promised a sweeping review of the 787’s certification process to find out why the original certification missed this battery vulnerability, despite the extensive battery testing by Boeing to prove it met the agency’s stringent requirements.

“They cannot afford to get it wrong this time,” Hamilton said.

A further uncertainty, he said, is that 24 of the 50 Dreamliners that are grounded worldwide are operated by just two Japanese airlines.

Whatever the FAA decides, the Japan Transport Safety Board is expected to be conservative in allowing those jets to fly.

Dominic Gates: 206-464-2963 or