More U.S. flights are arriving on time and airlines are losing fewer bags, yet more consumers are complaining about air travel.
Traveler complaints jumped 34 percent last year, to the highest level since 2000, according to an annual ranking of airline quality from Wichita State University and Embry-Riddle Aeronautical University. The top frustration: "problem flights," which include delays and cancellations and has been the No. 1 gripe for 16 years.
The latest report, released Monday, reviewed public information from the U.S. Department of Transportation to rate the airlines for on-time performance, baggage handling, bumping passengers because of oversold flights and complaints filed with the government.
How did the individual airlines score? Click ahead for a look at the researchers' rankings of top U.S. airlines.
1. Virgin America
This boutique airline has ranked No. 1 four years in a row. Researchers say its score for mishandled baggage improved since last year. While Virgin America's scores for on-time performance, bumping passengers and customer complaints slipped, that wasn't enough for the airline to lose its top spot.
Virgin America (VA), the brainchild of British billionaire Sir Richrd Branson, is based at San Francisco International Airport. Separately on Monday, Alaska Air Group announced it will buy Virgin America for more than $2 billion.
JetBlue (JBLU), based out of New York's John F. Kennedy Airport, came in second. That's an improvement for the airline, which was ranked No. 4 last year.
Researchers noted that JetBlue gained across all four criteria -- on-time performance, baggage handling, bumping passengers because of oversold flights and complaints filed.
JetBlue, which transports 35 million passengers annually, started out as an all-coach airline. But in 2014 it rolled out a premium product called "Mint," with flatbed seats, toiletries and upscale dining. Mint seats can be booked on select transcontinental flights.
Atlanta-based Delta Air Lines (DAL) held on to its No. 3 spot and is the highest-rated of the "Big 3" legacy airlines (American, Delta and United).
The researchers said Delta's overall score improved since last year and that its baggage handling, denied boarding and on-time performance all rose. The airline did see a slight uptick in the number of customer complaints.
Delta, which started as a crop-dusting service, now carries about 160 million passengers annually.
Honolulu-based Hawaiian Airlines (HA) ranked fourth, down from second place just a year ago. While on-time performance slipped, the airline did perform better in that category than all other carriers included in the report.
Hawaiian Airlines' scores for customer complaints and mishandled baggage also declined.
But it bumped fewer passengers.
Hawaiian took to the skies as Inter-Island Airways in 1929, when it flew from Honolulu to Hilo, on the Big Island (Hawaii). The flight, which included a stop in Maui, took 3 hours and 15 minutes. Today, a nonstop flight from Honolulu to Hilo is scheduled for a mere 52 minutes.
Alaska Airlines (ALK), founded in Anchorage but nowadays headquartered in Seattle, ranked No. 5 on this year's list. Alaska Airlines saw improvement in its on-time performance, and it bumped fewer customers. But it mishandled more luggage and had more customer complaints.
The airline, which announced Monday it's buying Virgin America for more than $2 billion, started with a three-seat airplane under the name "McGee Airways." It now serves more than 100 destinations.
Southwest (LUV), which saw improvement in on-time performance, lost luggage and number of passengers bumped, came in sixth in this year's rating. The customer complaint rate was virtually unchanged.
Southwest, based in Dallas, got its start in 1971 with three Boeing 737 jets. As of the end of 2015, the airline said it has more than 700 jets and operates more than 3,900 flights a day.
St. George, Utah-based SkyWest (SKYW) landed in seventh place, an improvement over its No. 10 ranking a year ago. SkyWest improved in all four categories -- on-time performance, luggage delivery, number of passengers bumped and complaints.
The airline isn't a household name, but chances are you've flown it. SkyWest partners with American, Delta, United and Alaska, and serves some of their regional destinations. The airline has a fleet of 351 aircraft and transported 30.1 million passengers last year.
United Airlines (UAL), based in Chicago, saw its ranking go up one notch to No. 8.
The airline improved in on-time performance, luggage delivery and number of passengers bumped. But it was the target of more customer complaints. United merged in 2010 with Continental Airlines.
United's hubs include Newark, Chicago, Denver, Houston and San Francisco.
ExpressJet, which operates commuter flights on behalf of American, Delta and United, saw its ranking improve to No. 9, from 11th place last year. The airline's on-time performance, luggage delivery, number of bumped passengers and customer complaint ratings all improved.
The Atlanta-based airline has 8,500 employees and carried 26 million passengers in 2015.
American Airlines (AAL) landed in spot No. 10, down three notches from last year. The airline's score includes performance results for US Airways, which merged with American in 2013 (the last US Airways flight took off in October 2015).
While American saw improvement in on-time performance, it slipped in baggage handling and number of passengers bumped. It also faced more customer complaints.
Denver-based Frontier Airlines dropped three spots to No. 11. The good news is the airline bumped fewer passengers. But it slipped in on-time performance, lost more bags and racked up more customer complaints. Frontier collected gripes at a rate of 7.86 per 100,000 passengers -- that's double the 3.91 complaint rate it had in 2014.
The discount airline serves more than 40 cities and operates 275 flights a day.
12. Envoy Air
Envoy Air, a subsidiary of the American Airlines Group (AAL), managed to hold on to its 12th-place ranking. The airline did improve its on-time performance, mishandled fewer bags and received fewer customer complaints. But it did bump more passengers in 2015 than it did a year prior.
The company was founded in 1998 as American Eagle but changed its name in 2014. It operates nearly 180 regional jets.
Spirit Airlines (SAVE) made its debut on the airline quality rating report at No. 13 -- dead last. Its on-time performance of 69 percent is below the industry average of 79.9 percent. And its customer complaint rate of 11.73 per 100,000 far exceeds the industry average of 1.9 per 100,000 passengers.
Bright spots include a low number of bumped passengers as well as its baggage handling performance.
Spirit is based in Miramar, Florida, and operates 35 jets to 49 destinations.