United Airlines and American Airlines, the biggest carriers at Chicago O’Hare airport, are among the most complained about carriers in America over the past five years, with ultra discounter Spirit Airlines garnering by far the most complaints relative to its size, according to a report released Thursday by the U.S. PIRG Education Fund [embedded below].

The report, “The Unfriendly Skies: Five Years of Airline Passenger Complaints to the DOT,” draws on U.S. Department of Transportation data over a five-year period, from 2009 to 2013. Not surprisingly, most complaints are about delayed or canceled flights, the top complaint category each year.

“When airlines cut corners, it causes all sorts of headaches for passengers,” Laura Murray, U.S. PIRG Education Fund Consumer Associate, said in a statement. “These complaints show that the airlines and policymakers should act to improve service.”

Spirit Airlines, known for low fares but many add-on fees, generated the most complaints per 100,000 passengers, a rate about three times higher than any other airline. Frontier Airlines, Chicago-based United and American were also at the top of the rankings, with complaint rates steadily increasing over the past few years.

Airlines with the lowest complaint rates were Southwest Airlines, Alaska Airlines, AirTran Airways and JetBlue Airways. Delta and JetBlue generated fewer complaints per 100,000 passengers steadily since 2010.

PIRG also released a consumer tip sheet that explains a consumer’s rights for flights that are delayed or cancelled, or if bags are lost or damaged by airlines.

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Photo Credit: Spirit Airlines CEO Ben Baldanza really doesn't care. Cristobal Herrera / Sun Sentinel/MCT