Southwest Airlines This weekend, more than 1,800 flights were canceled, thousands of customers’ travel plans mixed up and flight crews stranded.

“I know this is incredibly difficult for all of you, and our customers are not happy,” said Alan Kasher, executive vice president of Daily Flight Operations, in a statement on Sunday viewed by CNBC.

The airline blamed bad weather and an “FAA-imposed air traffic management program” for the initial problems.

“Although we were staffed for the weekend, we could not foresee the significant disruption caused by unexpected ATC problems and bad weather at our Florida stations,” said Kasher.

Other airlines have canceled relatively few flights. Southwest did not comment on the inequality.

The Federal Aviation Administration said there were “several hours” of flight delays on Friday afternoon due to severe weather and staffing problems at the Jacksonville Air Route Traffic Control Center, which controls the airspace in five parts of Alabama, Georgia, Florida, North Carolina and South Carolina.

“No staff shortages have been reported to the FAA in air traffic since Friday,” said the FAA. “Some airlines continue to have scheduling problems due to out of seat aircraft and crews. Please contact the airlines for details on current flight schedules.”

Southwest’s top destinations, Denver, Baltimore, Dallas Love Field and Chicago Midway, were hardest hit by Sunday’s cancellations.

Southwest apologized to travelers for long customer service waits. The airline said in a statement that it is expected to get close to normal operations by Sunday, but disruptions worsened.

Southwest’s Kasher admitted to staff in his note on Sunday that some crew members were out of hotel rooms last night and said disruptions can arise quickly when flight crews reach contractual and state working limits.

“Right now is ours [network operations center] The teams are working to protect our crew network and prevent bad connections – both for our crews and for our customers – that would have an even bigger impact, ”he said. “And the teams are working to determine the best course of action to be the quickest way to reset our network. “

The Dallas-based airline canceled 1,019 flights on Sunday, 28% of its schedule, after canceling 808 flights on Saturday, according to flight tracking website FlightAware. American Airlines, which operates a major hub in Miami, has canceled 66 mainline flights, or 2% of its operations on Saturday and Fort Lauderdale, Florida, by comparison Spirit Airlines 32 flights canceled, 4% of the flight plan.

Staff shortages resulted in hundreds of cancellations at Southwest this summer. the The airline has cut its flight schedule after the summer to avoid further disturbances. Other airlines have also faced labor shortages after encouraging thousands to take or buy vacations at the height of the pandemic, only to have travel demand return faster than expected this summer.

“Although we made changes to the timetable by autumn, our route system has not fully recovered – that will take time,” said Kasher. He said the airline had fewer frequencies between major airports to reroute or rebook travelers. Southwest doesn’t have what is called the interline arrangement that major airlines like delta and American Travelers have to book with other airlines.

Southwest did not immediately comment on whether staffing shortages contributed to the cancellations this weekend.

The airline is struggling to hire new employees. New CEO Bob Jordan told CNBC last month that the airline was ready to cut flights during the spring break if it doesn’t have enough staff to support the operation.

The inequality between the operations of Southwest and other airlines fueled speculation on social media that employees called in sick, with some suspecting it was in protest of the vaccine mandate.

The Southwest Airlines Pilots Association, the Southwest pilots union, said, “We can say with confidence that our pilots are not participating in any formal or unofficial work-related activities.”

“Our pilots will continue to overcome poor planning by SWA management 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 would comply with the Biden government’s requirement that federal contractors commission Covid vaccinations for employees is adding to the distraction for airmen.

“Make no mistake – due to months of personnel problems and inefficient planning practices, we are working with a higher than normal operational risk,” the union’s safety committee told members in a post on Saturday.

It is said that reports of fatigue requiring pilots not to fly are triple historical norms.

“All of these challenges created an additional distraction in the cockpit,” it said. “The company’s announcement this week of the COVID-19 vaccine mandate only exacerbates the situation.”

On Friday, the union asked a federal court in Dallas to temporarily block implementation of the vaccine mandate as it was a unilateral decision and required negotiations with the union instead.