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.
As of 4:15 p.m., a total of 40 departing flights were canceled at Midway, tying the airport for the sixth most cancellations in the U.S. Sunday, according to the flight-tracking website FlightAware. Fifty-three flights scheduled to arrive at the Chicago airport were also canceled Sunday, the website stated.
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 Federal Aviation Administration said there were a "few hours" of flight delays on Friday afternoon because of severe weather and staffing issues at Jacksonville Air Route Traffic Control Center, which controls airspace in five parts of Alabama, Georgia, Florida, North Carolina and South Carolina.
But the FAA stated air traffic shortages haven't been reported since then.
"Some airlines continue to experience scheduling challenges due to aircraft and crews being out of place," officials said in a statement. "Please contact the airlines for details about current flight schedules."
Dallas-based Southwest canceled 808 flights on Saturday, according to FlightAware. American Airlines, which operates a large hub in Miami, in comparison, canceled 63 mainline flights, or 2% of its operation and Fort Lauderdale, Florida-based Spirit Airlines canceled 32 flights, 4% of its schedule.
Staffing shortages fueled hundreds of cancellations at Southwest over the summer. The airline trimmed its schedule after the summer to avoid further disruptions. Other airlines faced a shortage of workers after encouraging thousands to take leave or buyouts at the height of the pandemic, only for travel demand to return faster than expected this summer.
Southwest did not immediately comment on whether staffing shortfalls contributed to the cancellations this weekend.
The disparity between Southwest's operation and other airlines fueled speculation on social media that employees were calling out sick.
The Southwest Airlines Pilots Association, Southwest pilots' labor union said that "we can say with confidence that our Pilots are not participating in any official or unofficial job actions."
"Our Pilots will continue to overcome SWA management's poor planning, as well as any external operational challenges, and remain the most productive Pilots in the world," it said.
Earlier on Saturday, the union noted that the company's recent announcement that it will comply with the Biden administration's requirement that federal contractors must mandate staff Covid vaccinations is contributing to distractions for aviators.
"Make no mistake about it – due to months of staffing issues and inefficient scheduling practices we have been operating at a higher than normal operational risk," the union's safety committee told members in a post on Saturday.
It said reports of fatigue, which require pilots not to fly, are triple historic norms.
"All of these challenges have led to an added distraction in the cockpit," it said. "This week's COVID-19 vaccine mandate announcement by the Company only exacerbates the situation."
On Friday, the labor union asked a federal court in Dallas to temporarily block the implementation of the vaccine mandate, saying it was a unilateral decision and instead, requires negotiations with the union.