Southwest Airlines Co. won’t schedule any flights on Boeing Co.’s 737 Max jets until April 13 — more than a month later than previously planned — further clouding the outlook for the best-selling aircraft that’s grounded globally after two fatal crashes.

The airline, the biggest operator of the single-aisle jet, is extending the timeline from March 6 because of “continued uncertainty” around the plane’s return, it said in a statement on Tuesday. The announcement comes a day after Boeing said it would temporarily halt production of the 737 Max in January.

Southwest’s decision follows that of American Airlines Group Inc., which said last week that it is removing the 737 Max from flight schedules through early April, extending the plane’s absence from the skies for more than a year. Southwest has previously said it anticipates a month or more of preparations after the Max wins approval to fly from the U.S. Federal Aviation Administration. United Airlines Holdings Inc. currently plans to resume service with the jet in early March.

Southwest, which had 34 of the planes when they were ordered to be parked, plans to reduce last-minute flight cancellations and unexpected disruptions to travel plans by keeping the 737 Max off its schedule, according to the statement. Affected passengers on about 300 weekday flights will be re-accommodated and notified, the airline said.

Grounding of the jet has already proved to be costly for Southwest. The flying ban has cut Southwest’s operating income by $830 million this year, Chief Executive Officer Gary Kelly said in an interview last week. Boeing said in July it had taken a $5.6 billion pretax charge to cover potential costs incurred by airline customers because of the grounding.

Regulators halted Max flights worldwide after an Ethiopian Airlines jet plunged into a field on March 10, following a Lion Air flight that crashed off the coast of Indonesia five months earlier. The disasters combined killed 346 people and prompted the longest flying ban for a U.S. airliner in the jet age.