United Airlines‘ CEO Scott Kirby said that without more gates the airline will have to reduce or change schedules to handle frequent gridlock at its Newark, New Jersey, hub, a message that came after mass flight delays marred July Fourth holiday weekend travel. The carrier gave 30,000 frequent flyer miles to customers who were most affected by the chaos.

“This has been one of the most operationally challenging weeks I’ve experienced in my entire career,” Kirby said in a note to staff Saturday.

He said that the airline needs more gates at Newark Liberty International Airport because of frequent aircraft backups there. “We are going to have to further change/reduce our schedule to give ourselves even more spare gates and buffer — especially during thunderstorm season,” he added. United didn’t provide more detail on the schedule reductions.

A day earlier, Kirby apologized for taking a private jet out of New Jersey’s Teterboro Airport while thousands of passengers were stranded, CNBC first reported Friday.

Problems began with a series of thunderstorms in some of the country’s most congested airspace along the East Coast last weekend, cutting off routes for aircraft. While most airlines recovered, United’s problems continued during the week, angering both customers and crews. United and JetBlue Airways executives said air traffic control problems worsened the disruptions.

The difficult week was also among the busiest. The Transportation Security Administration said it screened a record of nearly 2.89 million people on July 1 alone, topping a previous high on the Sunday after Thanksgiving in 2019.

Kirby laid out the weeklong troubles and said long-term changes were needed. He said that extensively delayed departures, which piled up at its hub at Newark since last weekend, hurt its operation. Takeoffs were reduced by as much as 75% for longer than eight hours in some cases from Sunday through Tuesday.

“Airlines, including United, simply aren’t designed to have their largest hub have its capacity severely limited for four straight days and still operate successfully,” he wrote.

Aircraft and crews were then left out of position, something that happens often during severe weather and can spark a cascade of disruptions for customers.

Unions complained about hours-long waits for crew members to get assignments and get hotels, forcing them to stay at airports longer.

Ken Diaz, president of the United chapter of the Association of Flight Attendants, which represents the company’s cabin crews, said in a note to members on Friday that the airline is short on crew schedulers. He said that problems became so severe over the past week that schedulers weren’t clear on which city some crews were in.

Kirby said that United must improve the platforms so crews can get assignments and accommodation more easily on its app, saying what happened over the past week isn’t acceptable.

Kirby called for more investment in the FAA and air traffic control to avoid delays and staffing shortages, some of which occurred after hiring and training paused early in the Covid pandemic.

United sent the 30,000 miles to customers who were delayed overnight or didn’t get to their destination at all, a spokeswoman said. She declined to say how many customers received the email.

The sum is enough to redeem for a domestic roundtrip ticket to many destinations, though the miles required vary based on demand for that flight or route.

More than 63,000 U.S. flights arrived late from June 24 through Sunday, and more than 9,000 were canceled — or more than 4% of airlines’ schedules — a rate nearly three times the average so far this year, according to flight-tracker FlightAware. United fared worse than competitors with 47% of its mainline schedule arriving late and 15% canceled over that period, FlightAware data show.

On Sunday, 7,650 U.S. flights were delayed and more than 630 canceled — driven in large part by thunderstorms on the East Coast. Nearly 900 United flights were delayed, or a third of its operation, while close to 1,000 American Airlines flights were delayed and more than 300 JetBlue Airways flights were late. New York City-area airports, led by Newark, were the hardest hit.

Thunderstorms are one of airlines’ biggest challenges because they can pop up suddenly and are harder to predict compared with hurricanes or winter storms, when airlines can cancel flights ahead of time to avoid stranding passengers and crews.