US airlines continued to grapple with flight disruptions heading into the busy weekend ahead of Independence Day, keeping the industry on edge even as the recent volatility driven by bad weather and air-traffic control limits showed signs of subsiding.

United Airlines Holdings Inc. canceled 182 flights as of early Friday, or 6% of its total, according to data from Flightaware.com. The carrier, which has been at the center of the recent turmoil due in part to heavy disruptions in the New York City area, accounted for the bulk of Friday’s cancellations in the US. JetBlue Airways Corp. hadn’t canceled any but about 11% of its flights were delayed.

Read more: JetBlue Joins United in Shifting Blame to FAA for Delays

The heavy-travel weekend will test the changes airlines have made to improve reliability after extended disruptions last year tied to manpower shortages forced them to reduce flight schedules. More than 24 million passengers are expected to fly from US airports over the holiday period from June 29 through July 5, according to booking app Hopper.com. On Friday alone, about 3.7 million passengers are expected to pack planes for the second straight day.

“We’re still not running as optimally as it needs to run, and we have improvements that we can make, substantial improvements over where we were compared to last summer,” Delta Air Lines Inc. Chief Executive Officer Ed Bastian told investors at a meeting earlier this week.

Crowded planes, a result of the extended surge in travel coming out of the depths of the pandemic, means passengers on canceled flights may have to wait days to be rebooked. Disruptions left airports including Newark Liberty International and Denver International packed with travelers this week.

Possible storms forecast for the midsection of the country could further disrupt flights.

United has been the most affected carrier this week, with some passengers posting complaints on Twitter about the issues they’ve faced.