Southwest Airlines flight cancellations continued at Chicago airports Monday following a weekend of major service disruptions.
As of Monday afternoon, Midway Airport reported 37 flight cancellations, all of which were Southwest Airlines flights, according to Flightaware.
Another 10 cancellations were reported at O'Hare International Airport.
By midmorning Monday, the carrier had canceled about 360 total flights and more than 600 others were delayed.
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Citing air traffic control issues and bad weather, Southwest Airlines canceled more than 1,000 flights Sunday, 27% of its schedule, including dozens set to depart from Chicago's Midway Airport.
Midway, one of the airline's major hubs, along with its other top destinations, Denver, Baltimore and Dallas Love Field, were among the hardest hit by the cancellations.
In total, 93 cancellations, both arrivals and departures, were reported at Midway of the airport's 96 overall flight cancellations, according to the Chicago Department of Aviation.
The Dallas-based airline had blamed air traffic control issues and weather for its weekend “operational challenges” that resulted in 1,900 canceled flights on Saturday and Sunday.
"We experienced weather challenges in our Florida airports at the beginning of the weekend, challenges that were compounded by unexpected air traffic control issues in the same region, triggering delays and prompting significant cancellations for us beginning Friday evening," a Southwest Airlines spokesperson said in a statement. "We’ve continued diligent work throughout the weekend to reset our operation with a focus on getting aircraft and crews repositioned to take care of our customers."
The Federal Aviation Administration, which provides air traffic control services, took the unusual step of pushing back against Southwest's explanation. Southwest Airlines was the only airline to report such a large percentage of canceled and delayed flights over the weekend.
Southwest has struggled all summer with high numbers of delayed and canceled flights. In August it announced it was trimming its September schedule by 27 flights a day, or less than 1%, and 162 flights a day, or 4.5% of the schedule, from early October through Nov. 5.
Last week, Southwest became the latest airline to announce it was imposing a vaccine mandate on its staff. The Dallas-based company said its workers must be fully vaccinated by Dec. 8 in order to remain at the airline. Employees can seek approval to skip the shots due to medical or religious reasons.
One analyst said a possible reason for the weekend outages may be a work slowdown by pilots who oppose the mandate.
"It's inaccurate," a Southwest spokeswoman said by email. "There's a lot of unfounded rumor and speculation circulating."
The pilots union denied reports that pilots were conducting a sickout or slowdown to protest the vaccine mandate. The Southwest Airlines Pilots Association said it “has not authorized, and will not condone, any job action.”
The union offered another explanation: It said Southwest's operation “has become brittle and subject to massive failures under the slightest pressure” because of a lack of support from the company. The union complained about the “already strained relationship” between it and the company.
Savanthi Syth, an airlines analyst for Raymond James, said the weekend problems will increase Southwest' costs and worsen the company's strained relations with unions.
Shares of Southwest Airlines Co. briefly fell more than 4% on Monday before a partial recovery; they were down 2% by midmorning.