Chicago-based United Airlines apologized for December flight delays in a letter to the editor in today’s Chicago Tribune.

United Airlines apologized for having more than double the delays of its competitors last month calling it “the most challenging December in United’s history.” The airline says it was affected by winter weather in Chicago and Denver where most of their flights begin or end. But members of United’s pilots’ union say a staffing shortage was the real problem and not severe weather conditions.

In a separate release today, the federal government had some bad news about United flight delays. The report says United had the worst on-time arrival rate of any major carrier during the month of November. About 24 percent of its flights arrived late. A United spokesperson did not immediately return calls.