Although overall airline quality remained high last year for the second year in a row, passengers were less happy with their flight experiences.

Airline quality in 2012 declined only slightly from the previous year—in which quality ratings reached an all-time high—but customer complaints rose by roughly 20 percent, according to the “2013 Airline Quality Rating” [PDF].

Conducted annually by researchers at Purdue University and Wichita State University, the study analyzed 14 airlines in four performance areas:

• On-time arrivals. Almost 82 percent of flights arrived on time in 2012, up from 80 percent in 2011.
• Lost or mishandled luggage. The number of bags lost or mishandled dropped to 3.07 bags per 1,000 passengers from 3.35 in 2011.
• Customer complaints. The rate of complaints in 2012 was 1.43 per 100,000 customers, compared to 1.19 in 2011.
• Overbooked flights. The number of travelers unable to board planes because of overbooked flights increased from 0.78 passengers per 100,000 in 2011 to 0.97 in 2012.

“U.S. airlines have been working to improve the customer experience and have reinvested more than $9 billion last year, which has resulted in the best-ever bag handling and third-best on-time performance,” Victoria Day, a spokeswoman for the trade group Airlines for America, told the Los Angeles Times.

Complaints were higher largely due to smaller seat sizes and overbooking.

“The way airlines have taken 130-seat airplanes and expanded them to 150 seats to squeeze out more revenue I think is finally catching up with them,” said Dean Headley, a business professor at Wichita State University and coauthor of the report for the past 23 years, in an interview with the Associated Press.

Headley told the Los Angeles Times that while passenger complaints were high, they ultimately would not affect the industry’s bottom line. “A sizable number of passengers are still unhappy,” he said. “Are they going to stop flying? That’s doubtful.”

The study ranked the 14 airlines in the following order, from best to poorest performance: Virgin America, JetBlue, AirTran, Delta, Hawaiian, Alaska, Frontier, Southwest, US Airways, American, American Eagle, SkyWest, ExpressJet, and United.