Mad about delays? Blame air traffic control, United says

United Airlines is the latest carrier to blame the nation's air traffic control system for recent operational difficulties.

The Chicago-based carrier said in a message to employees on Wednesday that in the past four months, it has estimated that 50% of its delay minutes and 75% of its cancellations are due to Federal Aviation Administration traffic management initiatives.

Traffic management initiatives are delays that are implemented to slow the flow of air traffic due to weather, staffing, volume or other issues. In the message, United COO Jon Roitman acknowledges that weather is a primary cause of ATC delays, but that other issues are a factor, too.

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"Of course, we know that many delay programs are weather-related, but delays related to air traffic volume and staffing are also contributing," Roitman said. "Rightfully so, customers aren’t focused on the cause of a delay or cancellation, they expect us to get them to their destinations safely and on-time."

United's message comes a week after Delta Air Lines CEO Ed Bastian sent a similar message to employees.

“I think [our partner] that’s most stressed right now is air traffic control,” Bastian told employees in a webinar, according to Skift Airline Weekly.

Roitman went further, indicating that United views ATC staffing as a potentially longer-term issue.

"The reality is that there are just more flights scheduled industrywide than the ATC staffing system can handle (particularly in NY and FL)," Roitman wrote. "Until that is resolved, we expect the U.S. aviation system will remain challenged this summer and beyond."

The comments by United and Delta seem to put those big airlines at odds with the FAA.

While the FAA has acknowledged some staffing issues at certain facilities, the agency — along with Transportation Secretary Pete Buttigieg — have made clear that in their view, recent operational difficulties are due to airlines overscheduling their flights relative to the staffing they have available.

Related: United will cut flights from Newark as delays and congestion get worse

Indeed, United last month — with the FAA's approval — dropped up to 50 flights per day at its bustling Newark Liberty International Airport (EWR) hub, which has been a hotbed of delays and cancellations.

"It’s going to take partnership from government and industry, but as always, you can expect United to take a leadership role," Roitman wrote.

In a statement, the FAA disputed Roitman's comments about staffing.

"It is unfortunate to see United Airlines conflate weather-related Air Traffic Control measures with ATC staffing issues, which could deceptively imply that a majority of those situations are the result of FAA staffing," the agency said. "The reality is that multiple overlapping factors have affected the system, including airline staffing levels, weather, high volume, and ATC capacity, but the majority of delays and cancellations are not because of staffing at FAA."