Southwest Airlines Boeing 737 MAX aircraft are parked on the tarmac after being grounded, at the Southern California Logistics Airport in Victorville, California on March 28, 2019.

Southwest Airlines on Thursday took the Boeing 737 Max out of its schedules until Feb. 8, later than any U.S. airline, as the low-cost carrier called the timeline for the plane's return to service "still uncertain."

That's a month later than it previously expected.

The lack of clarity on when regulators will allow airlines to operate the planes, grounded since mid-March after the second of two fatal crashes that killed a total of 346 people, have forced airlines to repeatedly change their schedules.

Southwest's pilots said last week they didn't expect the Max to return until February at the earliest.