Chicago-based United Airlines said a key measure of passenger revenue was flat in September as it faced new competition in China and made an accounting adjustment.
However, even with those factors, United's performance in September was worse than that of competitors. Delta Air Lines Inc. posted a 5.5 percent gain in September revenue per available seat mile compared to September 2012. US Airways Group Inc. saw the same measure rise 6 percent.
United said its own revenue was hurt by more than 1 percentage point because of an adjustment related to tickets sold on one airline and carried on another, according to the report filed on Monday evening.
Barclays analyst David E. Fintzen wrote in a note on Friday that, like United, Delta saw its revenue gains suffer for several quarters after it bought Northwest in 2008. However, United's performance on costs "has been significantly worse" than Delta, he wrote, putting pressure on profit margins. United and Continental merged in 2010, and are still working to integrate parts of the airline.
September traffic at United rose 1 percent compared to a year earlier. A 5.9 percent gain in traffic to Europe offset a 4.5 percent drop in Pacific traffic. Domestic traffic was about flat.
Flying capacity rose 0.2 percent, with a 1.5 percent cut in domestic flying and a 1.9 percent cut in seats across the Pacific. Capacity across the Atlantic rose 4.2 percent.
Because traffic rose faster than flying capacity, United's planes were a little fuller. Occupancy rose 0.6 percentage points to 82.7 percent.
For the first three quarters of the year, United traffic has fallen 1 percent. Capacity is down 2.6 percent. Occupancy has risen 1.3 percentage points to 84 percent.
Shares of United Continental Holdings Inc. fell 77 cents, or 2.4 percent, to close at $30.97 on Monday.