U.S. airlines struggled to improve the quality of their service in 2015, with baggage handling and poor on-time performance still irritating flyers, according to the latest annual report on airline quality from Wichita State University.

The 26th annual Airline Quality Rating (AQR) report [PDF] compared U.S. airlines by a metric, based on U.S. Department of Transportation data, that includes on-time flights, denied boarding, mishandled luggage and other customer complaints.

It found that the quality of the airlines was about the same as one year ago, with Delta Air Lines increasing the most year-over-year and Frontier declining the most.

Virgin America, interestingly, remained ranked highest among U.S. airlines, followed by JetBlue and Delta. The lowest ranked airlines were, unsurprisingly, Spirit Airlines and Frontier.

“The Airline Quality Rating industry score for 2015 shows an industry that improved slightly in overall performance quality over the previous year,” reads the report, which was authored by Dr. Brent D. Bowen of Embry-Riddle Aeronautical University and Dr. Dean E. Henley of Wichita State University. “Six airlines (Delta, ExpressJet, JetBlue, SkyWest, Southwest, and United) showed improvement in AQR scores in 2015. ExpressJet and SkyWest had the largest improvements in their AQR scores for 2015. Six airlines (Alaska, American, Envoy Air, Frontier, Hawaiian and Virgin America) all had a decline in their 2015 AQR score. Frontier had the largest decline in AQR score for 2015.”

Here are three takeaways from the report.

Airline quality is stagnant

Overall, the report shows that while the U.S. airline industry has improved in the last two years, it’s been hovering around the same level of dissatisfaction for the last year, with cyclical decreases in satisfaction during heavy travel months.

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Flyers love Virgin America and hate Spirit Airlines

Probable Alaska Airlines acquisition Virgin America ranked as the airline with the highest quality rating over the last two years, while Spirit Airlines ranked dead last.

Check out the contrast between the two in terms of quality rating since 2014 (Spirit Airlines only began submitting usable data in 2015).

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Passenger complaints are way up

More flyers are lodging complaints with the U.S. Department of Transportation than before. Spirit, Frontier and American received the most complaints in 2015. More than a third of total complaints were about flight problems like delays or cancellations.

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Photo Credit: A Virgin America aircraft lands in Boston. David Montiverdi / Flickr