United Airlines Struggles to Resolve Flight Disruptions

United Airlines was struggling on Friday to recover from a week of flight delays and cancellations, testing the resilience of its operation as people head to airports ahead of the busy July 4 holiday.

The airline’s troubles began last weekend in the New York area. At the time, United blamed the disruption on thunderstorms and staffing shortages at federal air traffic control facilities. Other airlines suffered flight delays and cancellations at the time, too, but by Wednesday United’s problems stood out as they spread to its operations across the country.

The situation appeared to improve somewhat on Thursday. After canceling about a quarter of its flights on Tuesday and Wednesday, United scrubbed about 18 percent of its schedule, according to FlightAware, a flight tracking firm. Still, the number of flights canceled by United on Thursday, more than 520, eclipsed cancellations by other carriers. SkyWest Airlines, which operates flights for United and several other major airlines, ranked second, canceling just over 100 flights.

The airline said that it was closely monitoring the weather in Denver and Chicago, two of its hubs, and that it hoped to have fewer last-minute cancellations. As of midmorning on Friday, United had canceled more than 200 flights, or 7 percent of its schedule for the day, according to FlightAware. Another 280 flights were delayed.

“We’re seeing continued meaningful improvement today after an overnight effort to further repair schedules and match separated crews with aircraft,” United said in a statement on Thursday afternoon. “As the recovery progresses, delays and cancellations will continue to decline as we head into what we expect to be a very busy holiday weekend.”

Pete Buttigieg, the transportation secretary, singled out the airline on Twitter on Friday morning, noting that other carriers had recovered from bad weather earlier in the week.

The disruption comes during one of the busiest periods for air travel in years. The Transportation Security Administration reported screening more than 2.7 million people at airport checkpoints on Thursday, one of its busiest days since 2019. Only four other days have been busier since the pandemic began, all in recent weeks. AAA, the travel club, said it expected nearly 4.2 million people to fly this weekend, up 6.6 percent from 2019.

Throughout the week, United passengers have reported having to sleep in airports and waiting in line for hours to rebook flights. Some travelers said they have had to wait days to retrieve checked bags.

The disruptions have left pilots and flight attendants frustrated, too. Many have had to wait on hold for hours to get reassignments after flights were canceled. Some flight attendants slept at airports, too, according to social media posts. The complaints from the airline’s employees echo those of flight attendants and pilots at Southwest Airlines during a much bigger operational meltdown at that company around Christmas.

“Weather last weekend affected everyone, but United is the only airline still struggling to recover and we know why,” Ken Diaz, president of the United chapter of the Association of Flight Attendants-CWA, which represents more than 25,000 United flight attendants, said in a statement on Thursday. “United management’s failure to properly staff crew schedulers, the flight attendant support team and more has exacerbated these operational issues and left passengers and flight attendants waiting for answers for hours at a time.”

Mr. Diaz said United had “lost” crews in its system for days because of the breakdown. He also said that the union had warned management last year of problems that could contribute to more disruptions, but that the airline “charged ahead” with an ambitious flight schedule this summer. United used some of the union’s recommendations to get through the current disruption, including making changes to its schedule and agreeing to pay flight attendants three time their normal pay to pick up trips through July 6, Mr. Diaz said.

Pilots have expressed similar frustrations.

“It is United Airlines management that is failing our loyal customers by ignoring the warning signs and failing to properly plan,” Capt. Garth Thompson, president of the United chapter of the Air Line Pilots Association union, which represents more than 15,000 of the airline’s pilots, said in a statement.