The unofficial start of summer over the Memorial Day weekend offers a troubling glimpse of what lies ahead for travelers during the peak vacation season.

U.S. airlines canceled more than 2,800 flights from Thursday through Monday, or about 2% of their schedules, according to tracking service FlightAware.

Delta Air Lines, usually among the top performers, had the worst record among major carriers with more than 800 canceled flights over the five-day span.

“This was a chance for airlines to show that last summer’s delays would not be repeated this summer, and yet, it was not to be,” said Helane Becker, an analyst for banking firm Cowen. She blamed the disruptions on bad weather, air traffic control delays, airline crew members calling in sick, and long security lines at some airports.

“We expect a busy summer, and are concerned about the industry’s ability to handle the demand,” Becker said.

When asked to comment Tuesday about its weekend troubles, Delta pointed to a statement it issued last week, when it said it faced challenges including rising COVID-19 cases among workers.

Delta ran 13% more flights in May than it did a year ago, but it announced last week that it would trim its schedules for July and August by up to 3% to make the remaining flights more reliable. The pilots’ union said it has warned the airline for months about crew shortages.

“We understand our customers’ frustration, especially over the weekend,” said Evan Baach, a Boeing 767 captain at Delta and an official with the Air Line Pilots Association. “Delta has just not properly staffed the airline with pilots for the number of flights they want to fly.”

The good news was that flight cancellations were down sharply on Tuesday. FlightAware reported only about 80 by late afternoon on the East Coast.

Various forecasts of high numbers of travelers over the weekend proved to be accurate. The Transportation Security Administration reported screening more than 11 million people at airport checkpoints from Thursday through Monday.

That was down 9% from the same days in 2019, but an increase of almost 25% over last year. Crowds of just under 2.4 million on both Thursday and Friday nearly matched the pandemic high set on the Sunday after Thanksgiving last year.

That meant many flights were packed, too, because airline schedules still have not returned to pre-pandemic levels, according to figures from travel-research firm Cirium.

The U.S. airline industry hopes to push passenger numbers higher, in part by removing one of the last U.S. pandemic-related travel restrictions. Industry representatives said they met Tuesday with White House officials to repeat their request to end the requirement that travelers test negative for COVID-19 within a day of flying into the United States.

Trade group Airlines for America said its member airlines estimate that lifting the requirement would lead to 4.3 million more international passengers over one year. Airlines believe many Americans are unwilling to travel overseas because they could be stranded if they contract the virus on their trip.

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