WASHINGTON - Spirit Airlines canceled 60% of its flights Wednesday and apologized to customers for severe disruptions in recent days, the latest in ongoing issues for air travelers seeking normalcy as airlines struggle to bounce back from pandemic lows.

The union representing Spirit's flight attendants said the Florida-based company was addressing the meltdown by using procedures developed to recover from hurricanes.

Aviation industry experts said the busy summer travel season, combined with lingering impacts from the pandemic, has complicated airline scheduling and added to logistical problems. Spirit's woes this week are the latest for an industry strained by rising demand on the heels of a pandemic-induced slump in travel.

Experts say some carriers are facing difficulties in finding pilots, flight attendants and ground crew to service flights as they recover from historic declines in air travel, leaving them vulnerable when more routine problems occur.

Jon Jager, an analyst at aviation data firm Cirium and a former schedule planner for a major U.S. carrier, said airlines are used to planning for events such as cyclical weather patterns. But staffing constraints - as travel demand outpaces airline employees returning after the worst of the pandemic - are creating difficulties in planning as the end of summer approaches.

"This is the last best chance for airlines to make revenue - and the demand is there," Jager said. "People are wanting to travel. This causes pressure on the airline to maximize their schedule with fewer employees available to work due to pandemic staffing levels."

Southwest Airlines, the nation's fourth-largest air carrier, faced disruptions in late June and around the July Fourth holiday, spurring a surge in cancellations and delays.

It came after the airline notched steep growth in recent months, with passenger volumes rising 45% between March and June, Southwest Chief Operating Officer Michael Van de Ven told investors last month. He cited technology issues in June that led to "extreme delays" as the company also works through staffing challenges.

Van de Ven said the carrier has searched for flight instructors to support training for pilots returning after an extended time away from the cockpit.

"It feels really good to finally be in a position where we can add flights and pick up our operating momentum," he said. "It was a bit messy as we throttled down our activity and it doesn't surprise me that it's a bit messy as we're accelerating it."

Most recently, Spirit is facing its own far-reaching problems.

The airline canceled more than 800 flights over two days this week, including 61% of its Tuesday flights, according to aviation firm FlightAware. The airline said many of those cancellations were part of a "thorough reboot of the network, allowing us to reassign our crews more efficiently and restore the network faster."

The company blamed "overlapping operational challenges" for the problems. It said a combination of bad weather, computer problems and staffing shortages caused "widespread irregularities" - issues exacerbated by high levels of summer travel and fuller flights industry-wide.

The Association of Flight Attendants-CWA, which represents nearly 50,000 flight attendants at 17 airlines, including those at Spirit, said the airline had an IT outage Tuesday that prevented employees from using the crew scheduling system for more than an hour, compounding other challenges and stymieing efforts to rearrange flight plans for flight attendants.

"In an operational breakdown like this, the airline needs to 'reset' in order to recover quicker and prevent further disruptions that could last weeks," the union said in a statement. "The reset will see significant cancellations in the short term, but avoid long term disruptions."

That means canceling more trips and nearly rebuilding the flight attendants' schedules from scratch, said union spokeswoman Taylor Garland.

American Airlines faced significant cancellations earlier this week from storms at its Dallas-Fort Worth International Airport hub.

"It was sustained wind, hail and rain. It was not a great situation for the operation," said American spokesman Curtis Blessing. "The result of that has been weather recovery for the last couple days."

Scores of diverted planes Sunday left crews out of position, he said, leaving lingering effects. "We're coming out of it," Blessing said.

FlightAware data indicates American canceled 377 flights, or 12% of its total, on Tuesday. It canceled more than 100 flights Wednesday.

Southwest also faced weather-related delays Tuesday, with 1,215 of its flights - about one-third of them - coming in late.

"Mother Nature provided challenges yesterday with convective thunderstorms around the Denver area and across a large stretch of the Gulf Coast, southeast, and Florida," the airline said in a statement. But it added, "we're happy to report that we're not, currently, experiencing the same operational disruptions" the airline faced in June and July.

Spirit said it is working through "proactive cancellations" that will enable a quicker reassignment of crew members.

"As a result, cancellation numbers will progressively drop in the days to come," the airline said in a statement. "The last three days were extremely difficult for our Guests and Team Members, and for that we sincerely apologize. We continue to work around-the-clock to get our Guests where they need to be."

The delays are coming as the Transportation Security Administration said it screened 1,797,120 people Tuesday, compared with 543,601 on the same day last year and 2,387,115 in 2019. Tuesday's total was the lowest checkpoint volume at the nation's airports since June 22.

Tuesday's dip in screenings came two days after a pandemic-era high on Sunday, when more than 2.2 million passengers moved through airport checkpoints.

The TSA has battled its own staffing struggles in recent months, alongside those of airlines.

The acting head of the agency issued a memo earlier this summer warning that the nation's largest airports would face shortages and asked office workers to volunteer to assist with checkpoints, such as managing queues and aiding with administrative tasks.

The agency has promised recurring monthly bonuses, allowed part-time workers to become full-time, adjusted shifts and increased the use of overtime.