"Fed up with flying, complaints from travelers have soared, according to a new study," says Philip LeBeau at CNBC. On Monday, researchers at Wichita State University and Purdue University released their annual Airline Quality Rankings, and the news wasn't all bad for the 14 largest U.S. airlines and their customers — more flights arrived on time in 2012 than the year previously.

But overall, U.S. flyers are increasingly "irritated," says The Associated Press' Joan Lowy. And it's not hard to see why.

Carriers keep shrinking the size of seats in order to stuff more people into planes. Empty middle seats that might provide a little more room have vanished. And more people who have bought tickets are being turned away because flights are overbooked. [AP]

"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," Wichita State's Dean Headley tells the AP. "People are saying, 'Look, I don't fit here. Do something about this,' At some point airlines can't keep shrinking seats to put more people into the same tube." Headley, a business professor and 23-year veteran of the report, compiles the ratings with Purdue's Brent Bowen, weighing on-time arrivals, denied boardings, mishandled luggage, and customer complaints.

Here's a sampling of their findings for 2012, giving a numerical explanation for why today's airline customers are so grumpy:

Complaints from air customers to the U.S. Department of Transportation in 2012. "The complaints are regarded as indicators of a larger problem because many passengers may not realize they can file complaints with the Transportation Department, which regulates airlines," says the AP's Lowy.

Complaints per 100,000 passengers in 2012

Percentage increase in complaints from 2011, when 1.19 passengers per 100,000 complained

Complaints per 100,000 passengers on United — the worst score among U.S. carriers

Complaints per 100,000 passengers on Southwest — the best score among U.S. carriers

Ticketed passengers bumped (denied boarding) from oversold flights, out of every 10,000 passengers. "With carriers trying to fill every seat and maximize the profit of every flight don't be surprised if the number of denied boardings continues to climb in the future," says CNBC's Philip LeBeau.

Ticketed passengers (per 10,000) bumped from full flights in 2011

Passengers per 10,000 denied boarding on regional carrier SkyWest — the worst among U.S. airlines

Passengers (per 10,000) denied boarding on Virgin America

Passengers (per 10,000) denoted boarding on Jet Blue — the best among U.S. airlines

Average percentage of flights that arrived on time in 2012

Average percentage of flights that arrived on time in 2011. See, "at a time when U.S. airlines are a whipping post for passenger complaints about crowded flights, tight seats, costly tickets, and unsatisfactory service, there is a glimmer of hope," says CNN's Jim Barnett.

Percentage of on-time flights at Hawaiian Airlines — the best record among U.S. carriers

Percentage of on-time flights at American and ExpressJet — the worst record among U.S. carriers

Mishandled bags per 1,000 passengers in 2012

Mishandled bags per 1,000 passengers in 2011

Mishandled bags per 1,000 passengers in 2007 — the worst year on record. But even this apparent success comes at a cost to passengers, says the AP's Lowy. "The industry's shift to charging for fees for extra bags, or sometimes charging fees for any bags, has significantly reduced the rate of lost or mishandled bags. Passengers are checking fewer bags than before, and carrying more bags onto planes when permitted."

Sources: AP (2), CNBC, CNN