With no end in sight to the grounding of Boeing's 737 Max, one airline is warning travelers that their Thanksgiving and Christmas travel plans might be disrupted.

Southwest Airlines on Thursday became the first U.S. airline to pull the planes, grounded by regulators worldwide in mid-March after two fatal crashes within five months of one another, from its schedule until next year. American and United expect the planes to return in early November.

Boeing this week said that it's testing a software fix for an automated flight-control system it added to the Max jetliners that was implicated in two crashes — one in Indonesia in October and another in Ethiopia in March — that killed 346 people. CEO Dennis Muilenburg said he expects the company will submit those fixes and other information needed for the plane to fly again to the FAA in September. He said the company anticipates the planes will be flying again early in the fourth quarter, but it warned that more delays are possible.

Airlines have already canceled thousands of flights during the peak summer travel season as the Boeing 737 Max grounding wears on for months longer than expected. The groundings and paused deliveries of the planes to airlines cost Boeing more tan $7 billion in the second quarter: money it set aside to compensate to airline customers and increased costs.

"It's really all about the Max," said Southwest CEO Gary Kelly on an earnings call. The airline posted higher-than-expected profits despite the prolonged Max grounding thanks to strong travel demand. "That's the only issue that we are dealing with."

The airline's operating income took a $175 million-hit due to the grounding, the airline said.

Kelly said everything else at Southwest "is rock solid."

Hamstrung by the lack of its 737 Max planes, currently parked in the California desert, Southwest can't grow as planned, or ramp up flying as much meet demand during busy travel periods like the end-of-year holidays after it pulled the planes from its schedules until Jan. 5.

Southwest, which doesn't charge travelers fees to change or cancel their flights, will waive fare differences for customers who need to change their trips because of the extended schedule disruption.

"If demand remains strong as it appears to be, looking into the fall, I think there's going to be plenty of competition by consumers for the cheapest seats," said Henry Harteveldt, a former airline executive and founder of travel-industry consulting firm Atmosphere Research Group.

"I think the airlines are worried," he added. "Neither Boeing nor the FAA can provide 100% accurate insight right now because they're still working on the various fixes on the plane that have to be made."

Once the regulators clear the planes to return to service, airlines will have to train their pilots on the changes Boeing made aboard, a process that airline executives say can take more than a month.