(CNN) - Aviation authorities from around the world may not immediately follow the FAA's lead whenever the US agency decides to allow the Boeing 737 Max to fly again, Boeing CEO Dennis Muilenburg said Wednesday.
Speaking to an investors conference, Muilenburg said what others in the industry have been suggesting for some time, that the past practice of other nations' aviation regulators following the FAA's lead on a decision to certify an aircraft might not be the case this time.
"I think a phased ungrounding of the airplane amongst regulators from around the world is a possibility," he said.
Muilenburg said he believes that there is a "broad convergence," among the different regulators, and that "the FAA is working very hard to build that collaboration amongst the regulators and bring everyone along together."
The aircraft was grounded after two fatal crashes that killed all 346 people on board. The second crash, of an Ethiopian Airlines jet minutes after it took off on March 10, brought swift action around the world to ground the 737s. By the time the FAA also decided to ground the plane on March 13, the US planes were the only ones still flying.
In past groundings, such as the case of Boeing's 787 Dreamliner--which was grounded for several months in 2013 due to concerns about battery fires--all the different aviation authorities worldwide acted virtually simultaneously to allow it to fly again.
A phased in recertification will cause problems, since most of the nearly 400 planes in service at the time of the grounding fly for non-US carriers. Southwest Airlines, American Airlines and United have fewer than 70 of the planes between them.
Muilenburg said Boeing still hopes to get approval for the plane to fly early in the fourth quarter, which beings Oct. 1. But he said the question of "regulator alignment around the world" is the greatest risk to meeting that timeline.