Spirit Airlines canceled more than half its schedule today, and American Airlines struggled to recover from weekend storms at its Texas home, stranding thousands of passengers at the height of the summer travel season.

By early evening, Spirit had canceled more than 400 flights, or nearly 60% of its schedule, according to the FlightAware tracking service. Nearly 100 other flights were late. The blame appeared to lie at least partly with a technology outage affecting crew scheduling.

American Airlines had already canceled nearly 350 flights. It is much larger than Spirit, so those flights amounted to 11% of its schedule — still an unusually high rate.

About three-fourths of the American cancellations appeared to be due at least partly to a lack of pilots, according to a company log.

The disruptions at Spirit and American are just the latest examples of airlines scrambling to deal with an increase in travel this summer. Airlines have thousands fewer employees than they did before the pandemic, but U.S. air travel has recovered to about 80% of 2019 levels.

A Spirit spokesman said the low-cost carrier was proactively canceling some flights — dropping them before most passengers drive to the airport — to “reset” the operation.

“We’re working around the clock to mitigate the travel disruptions caused by overlapping operational challenges including weather, system outages and staffing shortages in some areas of the operation,” spokesman Erik Hofmeyer said. “We’re working to provide refunds for cancellations and, when possible, to re-accommodate our guests” on other flights.

A person familiar with the situation said Spirit experienced an outage Tuesday morning affecting crew scheduling, preventing airline officials from rescheduling crews to cover gaps. The person, who was not authorized to speak publicly, said crews were stranded in many places around the country and unable to get to assigned flights.

On Monday, Florida-based Spirit scrapped more than 330 flights, or 42% of its schedule, more than double the rate of American Airlines, the next poorest performer among major U.S. carriers. Spirit canceled about 20% of its flights on Sunday.

American’s difficulties Tuesday came after the airline canceled about 560 flights, or 18% of its schedule, on Monday and nearly 300 on Sunday, according to FlightAware. Most were at Dallas/Fort Worth International Airport, where thunderstorms disrupted flights for several hours Sunday.

The union representing American’s pilots accused the airline’s management of poor planning and not having enough employees.

“It’s pretty simple. They don’t have enough pilots, and they don’t have modern scheduling practices to do more with what they have,” said union spokesman Dennis Tajer. He said that bad weather “hits every airline, but American is the last to recover. This has to change.”

American denied that it has a pilot shortage. Spokeswoman Whitney Zastrow said in a statement that Tuesday’s cancellations were largely related to Sunday’s storm in the Dallas-Fort Worth area. She said employees were working around the clock to take care of customers.

Southwest canceled about 2% of its flights Tuesday, and other large U.S. airlines had even lower cancellation rates, according to FlightAware. The numbers don’t count flights on smaller planes that are branded as American Eagle, United Express or Delta Connection.

Customers who called Spirit and American also complained about being put on hold for hours. Airlines received $54 billion in taxpayer money to keep people employed through the pandemic but reduced staff anyway. Now they are adding call center workers and filling other jobs.

David Donovan, a retired lawyer in Washington whose future flight to Des Moines, Iowa, was changed, said American promised in a recorded message to call him back within four hours on Monday, then called at midnight, when he was trying to sleep. He elected to try again Tuesday but couldn’t get through.

“With all the bailout money we gave them, they ought to have enough people to answer the dang phone,” Donovan said.