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.