Chicago-based United Airlines came in third in a ranking of airlines with the most complaints last year, according to a research report.
The report from the U.S. Public Interest Research Group Education Fund, released Thursday, ranked 13 major airlines in the United States by the number of complaints per 100,000 passengers.
The rankings come from 2013 data, but the research group has data from the past five years. United Airlines has consistently stayed at the No. 2 and No. 3 ranking during those years.
The airline with the least amount of complaints is Southwest Airlines, which has held that ranking for the past five years. Spirit Airlines has consistently had the most complaints all five years.
The report found that most complaints were about delayed or canceled flights. Over the past five years, the number of these complaints has increased overall. The report also noted the increase in complaints about crowding of seats, gates, runways and airways.
"When airlines cut corners, it causes all sorts of headaches for passengers," Laura Murray, a consumer associate for the research group, said in a press statement. "These complaints show that the airlines and policymakers should act to improve service."
Complaints about baggage have dropped since 2009, and fewer reports of mishandled bags have been sent to the Department of Transportation.
The report unsurprisingly found that the number of complaints about flight problems correlated with the airline's statistics for on-time performance. In the past few years, United Airlines has dealt with major delays due to computer glitches, prompting passengers nationwide to file complaints.