Boeing said this week it can move "really fast" to get its 787 Dreamliner back into the skies once regulators approve a fix for burning batteries on board the plane.

Regulators may not move so quickly.

The U.S. Federal Aviation Administration, which grounded Boeing's high-tech jet nearly seven weeks ago, faces unusually tough obstacles in approving it for flight - one of them brought on by the agency's own boss.

Transportation Secretary Ray LaHood set an impossible standard early in the crisis by promising that the Dreamliner won't return to the skies until regulators are "1,000 percent sure" of its safety.

Because no aircraft is 100 percent safe, "it is going to be a challenge for the FAA to dial back from some of the overheated rhetoric," said Richard Aboulafia, an aerospace analyst at the Teal Group in Virginia.

Boeing Co's flagship jetliner has been grounded for nearly seven weeks, costing an estimated $350 million, after lithium ion batteries overheated on two 787s in January.

The National Transportation Safety Board this week is due to issue an update on its investigation into what caused the battery to overheat and smoke but has indicated it will take longer to get to the bottom of what went wrong.

Boeing proposed a fix two weeks ago, but safety experts said approval from the FAA will be difficult as long as what caused the batteries to melt down remains a mystery.

Even if the battery failures are fully explained, safety experts said, that does not make the Dreamliner "1,000 percent safe." Plane makers and the FAA always aim to reduce risk to levels that may approach zero but never reach it.

"Nothing is 100 percent safe," said John Goglia, a former NTSB board member with 40 years of experience in the industry.

In addition, the National Transportation Safety Board has questioned the process the FAA and Boeing used to approve the plane as safe just 18 months ago.

"The FAA will need to find a way to communicate that they believe the level of risk has been reduced to a minute level that's acceptable," Aboulafia said.

The FAA declined to answer detailed questions for this story and reiterated that it is analyzing Boeing's proposal closely. "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," agency representative Laura Brown said.

Boeing also declined to comment. But at an investor conference on Monday, Boeing Commercial Airplanes CEO Ray Conner said the company is confident of its proposal. "We feel very good about the fix," he said. "We've covered the waterfront."

COSTLY GROUNDING

The FAA faces strong commercial pressure to return the plane to flight. Boeing is the nation's largest exporter, employing nearly 85,000 people in its airplane division. The company is still producing planes, but it cannot deliver them or receive payment until a worldwide grounding by regulators is lifted.

Airlines are losing money by parking the planes and must lease jets or make other arrangements to fly passengers on flights scheduled months in advance.

Goglia, the former NTSB board member, said regulators are "going to have to find a way to balance off their caution with the country's and Boeing's need to get this airplane back in the air."

Having certified the high-tech plane as safe, the FAA is facing very close scrutiny of its approval procedures. The NTSB has questioned the process Boeing and the FAA used to certify the plane.

Without these pressures the FAA might have approved Boeing's fix in a couple of weeks and had the jet flying in another month or two, said Hans Weber, a consultant who has served as an adviser to the FAA for more than 20 years.

"Now, with the other considerations, I don't even want to venture a guess," said Weber, president of TECOP International, in San Diego.

Aboulafia estimated that it would take at least four months for the 787 to get cleared to fly if the FAA approves flight tests soon, as Boeing requested. If flight testing takes longer, it could take six to nine months before the 787 is back in the sky.

BOEING's SUPERBOX

Some safety experts see the FAA eager to stand by Boeing and move quickly - even if politics slows it down some. The agency has a well-established process for vetting plane changes and has called in numerous experts to advise it on lithium-ion technology.

Former Department of Transportation Inspector General Mary Schiavo said the FAA is likely to decide that the fix is "an acceptable risk before the NTSB will."

The aviation industry, for its part, has strong incentives to avoid cutting corners, since human life, company reputations and careers are at stake.

"We put our own children on those planes," said Tom McCarty, president of SPEEA, the engineering union at Boeing.

Boeing, meanwhile, has swiftly made design changes, which include adding ceramic insulation between the cells of the battery and a stronger stainless steel box with a venting tube to contain a fire and expel fumes from the aircraft.

At the investor conference on Monday, Boeing's Conner said the company will double 787 production this year and deliver the same number of jets it planned before the battery crisis struck.

Industry experts, however, say the inability to pinpoint the cause of the battery failures could make the FAA's job of certifying a solution more difficult, since it requires a fix that can contain the worst problem that can occur without damaging the aircraft.

"People will attack this approach," said Weber. "They'll say, ���How do you know that's the worst case?'"

Weber speculated that the safety concerns may result in regulators restricting the 787's ability to make long flights over water, a standard known as ETOPS. Such a change would be a severe blow to Boeing and airlines that use the Dreamliner for long-haul direct flights with about 250 passengers, a highly lucrative market that the 787 can serve at 20 percent lower fuel cost than other planes.

"If politically they feel they cannot move aggressively toward restoring three-hour ETOPS they may restrict it to one hour, so the aircraft can land if there is a fire," said Weber. "That destroys the business case for the 787."

Boeing shares were up $1.49, or nearly 2 percent, at $78.58 in late afternoon trading on Tuesday.

(Additional reporting by Mari Saito, James Topham and Tim Kelly in Tokyo, Andrea Shalal-Esa in Washington, Deepa Seetharaman in Detroit and Karen Jacobs in Atlanta; editing by Edward Tobin and Steve oRLOFSKY)