Delta Air Lines has edged ahead of United Airlines as the second-largest carrier in the US based on annual passenger traffic.
Delta posted 209.6 billion revenue passenger miles in 2015 compare to United’s 208.6 billion.
American Airlines Group Inc. which includes the newly integrated US Airways, remains the nation’s largest.
United is still grappling with challenges since its merger with Continental Airlines which has included recent IT glitches that grounded flights and ongoing labor contract disputes.
Led by new CEO Oscar Munoz, who is currently on sick leave after a heart transplant, United has vowed to improve its customer service delivery and it showed significant improvement in on-time performance last year.
In contrast Delta had a much smoother merger with Northwest Airlines in 2008.
"The reality is, Delta offers better service and a better product than United. My first thought was people probably prefer to fly Delta now," Adam Hackel, an associate analyst at Sterne Agee CRT told Bloomberg.
