The cause of the battery problems that have grounded Boeing’s 787 Dreamliners may never be known, a company engineer said Thursday.

FAA order

Federal regulators are telling airlines they can fly Boeing’s 787 Dreamliners again as soon as they replace its problematic lithium ion batteries with a revamped battery system.

The Federal Aviation Administration safety order lifting the grounding became official after it was posted online Thursday.

It applies to all U.S. airlines, but only one of them, United, has 787s. It has six. The FAA estimated the repair costs for those planes at $2.8 million.

There are 50 Dreamliners in service worldwide, and Boeing has purchase orders for 840 more. Newly delivered versions will come with the revamped battery system.

Separately, Poland’s national airline, LOT, said Thursday it will resume flights on its two 787s on June 5 and is selling tickets for the flights. LOT’s 787s will at first fly from Warsaw to Chicago, and later to New York, Toronto and Beijing.

Associated Press

The engineering leader for the 787, Richard Horigan, said the root cause of smoldering batteries experienced by the two 787s may never be known, because the evidence was destroyed by heat.

“When we say a root-cause we want to know exactly what happened, what size particles caused the cell to vent ... and we may never get there because the evidence is destroyed. When these cells vent there’s a lot of heat damage on those cells,” Horigan said.

He also said that all potential causes of the battery fire have been eliminated with the new, redesigned battery system. He said it was not the first incident where a problem that caused a malfunction in an aircraft has not been identified definitively, but a solution covering the problem had been found.

Horigan cited the 1996 crash of TWA flight 800. The plane crashed after a fuel-tank explosion, but the cause was never identified.

After the plane exploded off the coast of Long Island, killing all 230 people aboard, investigators ultimately pinned the cause on flammable vapors in one of the Boeing 747’s fuel tanks.

Investigators were never able to conclusively identify the source of the spark that ignited the vapors.

Horigan said that in that case, plane experts came up with a solution that “fully encapsulated and eliminated all possible causes of fuel tank fire.”

“So it’s not uncommon where you have events like this where you don’t get a definitive root cause but you have sufficient confidence in your design solution to know that whatever the root cause is, you’ve identified it and addressed it,” he said.

Boeing assembles the 787 at factories in North Charleston and Everett, Wash.

Air safety authorities grounded the jets after incidents with smoldering batteries occurred aboard two planes in January. The U.S. Federal Aviation Administration has approved Boeing’s redesigned battery system, which the company said sharply reduces the risk of fire.

Boeing said Wednesday that deliveries of the 787 should resume in early May. Most of the 50 planes that have been delivered to airlines will be fixed by the middle of the month.

Boeing South Carolina delivered four planes, all to Air India, before the grounding order.

The Post and Courier contributed to this report.