Airlines for America (A4A) said short staffing at FAA air traffic control (ATC) centers is leading to repeated flight delays and cancellations.

In a letter to US Transportation Department (DOT) Secretary Pete Buttigieg, A4A CEO Nick Calio said US airlines have collectively cut 15% of planned June-August capacity, yet are still experiencing increasingly frequent service disruptions.

Staffing issues at airlines, including a growing pilot shortage at regional carriers, have been blamed for many of the service disruptions. But ahead of what is expected to be an extremely busy Independence Day holiday weekend at US airports, Calio noted airlines are not in full control.

“The industry is actively and nimbly doing everything possible to create a positive customer experience since it is in an airline’s inherent interest to keep customers happy, so they return for future business,” Calio wrote. “However, not every air traffic variable is within an airline’s control.”

He said an A4A member airline–the specific carrier was not disclosed–reported ATC issues played a role in one-third of its recent cancellations.

Calio acknowledged that weather causes delays and cancellations, but he said ATC staffing has also been a major factor, explaining: “We have … observed that FAA ATC staffing challenges have led to traffic restrictions under blue sky conditions.”

Calio said FAA’s Jacksonville, Florida enroute ATC center, one of 20 enroute centers in the US, was understaffed for 27 of 30 days between May 25 and June 24. Short-staffing at the Jacksonville center is “crippling to the entire East Coast traffic flows,” Calio wrote.

In the letter, the A4A CEO requested a meeting with Buttigieg “to discuss how we can work together to better understand FAA’s controller staffing plan for the upcoming July 4th weekend and summer travel season.”

Noting that the Transportation Security Administration (TSA) “has shared its staffing plan for the July 4th travel period with” airlines, Calio added: “As a government partner, it would be helpful for the FAA to share its staffing plan with airlines so we can plan accordingly.”