Delta passed United in 2015 based on traffic figures for paying passengers that the airlines released for the year, reports The Wall Street Journal: American came in first with reported traffic of 223 billion revenue-passenger miles, a 2.4% increase over the previous year; Delta ended 2015 year with 209.6 billion miles flown by passengers, a 3.3% increase from 2014; and United’s final numbers amounted to 208.6 billion miles, a 1.5% bump from the year prior. Those figures take into account regional-carrier partners the airlines work with.
It seems United is having a bit of trouble playing catch-up while it’s in the process of tying up all the loose ends of its merger with Continental back in 2010. While Delta stayed mum on its success, United said it’s “continuing to focus on running a reliable operation and investing in the tools and services that allow us to deliver the flier-friendly experience our customers deserve and expect.”
These numbers aren’t the be-all, end-all measurement for airline success, however, though it’s one of a few useful tool for comparing airlines. You can rank the airlines by the number of passengers carried (American, 201.2 million; Delta 179.4 million; Southwest 144.6 million; and United 140.4 million in 2015), or you can stack them up based on annual revenue, where Delta remains in second behind American before 2015.
Delta Surpasses United for No. 2 Airline Spot by Traffic [The Wall Street Journal]