Flight delays and cancellations continued to mar thousands of Fourth of July travelers on Friday, with United Airlines passengers bearing the brunt of the problems.

The Transportation Security Administration expects to screen 17.7 million people from June 29 through July 5, peaking on Friday at more than 2.8 million people. That would be an single-day record for daily screenings and one of the clearest signs yet of air travel’s strong recovery from the Covid pandemic.

More than 4,800 U.S. flights were delayed on Friday, though United had more delays than competitors.

By 5 p.m. on Friday, the carrier had canceled more than 230 mainline flights, 8% of its operation, while more than 790 flights, or more than quarter of its schedule, were delayed, according to flight-tracker FlightAware.

That was still far fewer than its disruptions on Thursday and a notable improvement from last weekend when a slew of thunderstorms along the East Coast at some of the country’s most congested airports kicked off the chaos. Some airline executives blamed the Federal Aviation Administration’s shortfall of air traffic controllers for exacerbating the problems for their customers.

Customers throughout the week sprawled out on airport floors, waiting for hours for flight information or new schedules, with seats on other flights, or other airlines scarce. They also faced long lines for customer service and lost bags.

Even United Airlines’ CEO couldn’t get a seat out of the New York area. On Wednesday, Scott Kirby took a private jet from New Jersey’s Teterboro Airport to Denver, Colorado.

A spokeswoman for the airline told CNBC that United did not pay for his flight. Kirby apologized to staff and travelers on Friday for taking the private jet when so many others were stranded.

“Taking a private jet was the wrong decision because it was insensitive to our customers who were waiting to get home,” Kirby said in a statement to CNBC. “I sincerely apologize to our customers and our team members who have been working around-the-clock for several days — often through severe weather — to take care of our customers.

“Watching our team firsthand with our customers at four different airports and during countless meetings this week, it’s clear to me they represent the best of United, and I regret that I have distracted from their professionalism,” he continued. “I promise to better demonstrate my respect for the dedication of our team members and the loyalty of our customers.”

United said on Friday afternoon that its performance was improving into the holiday weekend. The airline has been offering waivers to travelers affected so they can rebook their trips without paying fare differences.

But it also cautioned that: “Storms in Denver, Chicago and the East Coast will continue to be a challenge, but most of today’s cancellations were made in advance to give customers time to adjust.”

Transportation Secretary Pete Buttigieg on Friday called United out for the challenges over the past week, saying the airline’s disruptions were “elevated but moving in the right direction.”

Airlines are under political and public pressure to run reliably after their overambitious schedules and staffing shortages worsened routine challenges like bad weather. The struggles come as travel demand bounces back from pandemic lows.

More storms and challenges like wildfire smoke from Canada are likely to plague airlines in the days ahead, though the worst of the disruptions this week have mostly subsided. (Of course, if your flight is canceled or delayed, here’s what airlines owe you.)

More than 42,000 flights operated by U.S. airlines were delayed from Saturday through Thursday, and more than 7,900 were scrubbed altogether, according to flight-tracker site FlightAware. More than 5% of U.S. schedules were canceled, about four times higher than the cancellation rate so far this year.

Over that six-period period, half of United’s mainline flights arrived late, amounting to average delays of 106 minutes, according to FlightAware data. Another 19% of its schedule was canceled.

Union leaders blamed United for some of the problems, which stranded crews along with passengers during the disruptions. Flight disruptions often snowball because crews and aircraft are out of position and long delays can have them run into federally-mandated work limits.

United has been offering flight attendants triple pay to pick up shifts over the peak holiday period.

“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,” Ken Diaz, president of the United chapter of the Association of Flight Attendants, said in a statement Thursday. “The airline actually ‘lost’ crews in the system for days on end because there was such a significant breakdown in running the operation.”

Garth Thompson, a United captain and chairman of the United chapter of the Air Line Pilots Association union, accused the company of not investing in the operation.

“Summer flying can be challenging, but this summer will be unnecessarily memorable,” he said. “To those caught up in management’s unforced errors, I’m truly sorry.”

Both unions are engaged in contract negotiations with the company and are seeking compensation and scheduling improvements.

United CEO Kirby on Monday wrote to staff that some of the issues last weekend stemmed from air traffic controller understaffing, and said that “the FAA frankly failed us” when it slashed arrival and departure rates at Newark Liberty International Airport in New Jersey, a major United hub.

The FAA had warned about staffing shortages in the New York City area earlier this year, and some airlines agreed to reduce capacity to avoid overloading the system.

“It led to massive delays, cancellations, diversions, as well as crews and aircraft out of position,” Kirby wrote in a staff note, which was seen by CNBC. “And that put everyone behind the eight ball when weather actually did hit on Sunday and was further compounded by FAA staffing shortages Sunday evening.”

JetBlue also blamed the FAA for similar issues.

“We are working with the FAA to better understand what led to the significant and unexpected ATC restrictions this week that affected thousands of flights across carriers,” JetBlue’s COO, Joanna Geraghty, said in a memo to employees Wednesday. “The severity and lengthy duration of the latest programs were worse than we have seen in the past with similar weather and this has left tens of thousands of our Customers inconvenienced and, in many cases, blaming JetBlue for a situation outside of our control.”