U.S. airlines performed worse across the board in 2014, after a record best year in 2013, according to an annual quality report on the industry. The Airline Quality Rating report, to be released on Monday, found that the industry's performance worsened in all four areas reviewed -- baggage handling, consumer complaints, denied boardings and on-time arrivals -- and that most of the 12 major airlines the report studied performed worse that year.

“It’s unfortunate, and we hope it’s not the beginning of a downward trend,” Brent Bowen of Embry-Riddle Aeronautical University, a co-author of the study, told the Dallas Morning News. He added that the decline in performance came after four straight years of improvement, which may be due to consolidations as the industry struggles to keep up with a changing economy. “Airline performance after two airlines merge always goes down for several years while they’re working out the kinks,” Bowen said.

The industry’s rate of on-time arrivals fell from 78.4 percent in 2013 to 76.2 percent last year, and only two airlines -- Frontier Airlines and JetBlue Airways -- improved their on-time arrivals rate in 2014.

The rate of customer complaints rose from 1.13 per 100,000 passengers in 2013 to 1.38 in 2014. Most of the complaints registered by the U.S. Department of Transportation were for flight problems (37.8 percent), which included delays, baggage problems, reservations, ticketing and boarding and customer service issues.

The industry denied boarding to 0.92 per 10,000 passengers in 2014, an increase from 0.89 in 2013. The worst offenders were ExpressJet Airlines and SkyWest Airlines, which tied for the position with 2.71. Virgin America (0.09) performed the best in this area.

The rate of complaints for mishandled baggage rose from 3.21 per 1,000 passengers in 2013 to 3.62 in 2014.

Wichita State University's Dean Headley, who also co-authored the study, said passengers already knew the quality of their air travel was worsening. “We just got the numbers to prove it," he told the Houston Chronicle.

Headley said that airlines performed better in the years after 2001, when a dip in demand for flights meant that planes were less crowded. Airlines also saw their profits fall during the same period, however, and only returned to profitability when they began to merge together, reducing competition and limiting the number of flights to keep fares high. The report found that the average plane is over 80 percent full at most major airlines, and many flights are oversold.

"They have put the same number of people in fewer airplanes," Headley told the Houston Chronicle. "Anytime the system ramps up, it goes haywire."

He said that the new figures showed the industry was failing in its promises to reinvest and improve customer experience, after it reaped a year of record income in 2014, when net profits nearly doubled.

“We hope airlines take notice and work to improve their performance in the year ahead,” Bowen said, the Dallas Morning News reported.

The report will be publicly released in a press conference at the National Press Club in Washington, D.C. at 9:30 a.m. EDT on Monday.