(Bloomberg) -- Delta Air Lines Inc.’s chief executive officer apologized to customers for an “unacceptable” level of canceled and delayed flights, an unusual step that reflects mounting frustration among waylaid summer travelers.

Delta is adding extra boarding time for flights, building in more of a buffer in crew schedules to mitigate disruptions and using white collar salaried workers to help speed customers through airports, Chief Executive Officer Ed Bastian said in a letter to its SkyMiles frequent flier program members Thursday. 

“We’ve spent years establishing Delta as the industry leader in reliability, and though the majority of our flights continue to operate on time, this level of disruption and uncertainty is unacceptable,” Bastian said.

The apology comes just before the July 4 holiday weekend in the US, when travelers are expected to pack airports and fill planes. It also follows Delta’s unusual offer to let passengers change their travel plans across its network during the “potentially challenging” holiday without paying a fare difference or change fee. 

Bastian warned that “things won’t change overnight” and said his airline is working with the US Federal Aviation Administration on ways to minimize delays and cancellations due to weather.

Delta has canceled 4% of its flights, or 3,593, since the start of the summer travel season on Memorial Day weekend. Another 20% have been delayed for an average 58 minutes, according to FlightAware.com. That compares with a 2.8% cancellation rate and 25.4% delays for the seven largest US airlines.

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