In this Thursday, Sept. 27, 2012 photo, passengers travel through an airport in Miami. Private researchers, who have analyzed federal data on airline performance, say in a report being released Monday, April 8, 2013, that consumer complaints to the Department of Transportation surged by one-fifth last year even though other measures such as on-time arrivals and mishandled baggage show airlines are doing a better job. "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,î says Dean Headley, a business professor at Wichita State University, who has co-written the annual report for 23 years.

WASHINGTON — Airline passengers are getting grumpier, and it's little wonder.

Airlines keep shrinking the size of seats to stuff more people onto planes, those empty middle seats that once provided a little more room are now occupied and more people with tickets are being turned away because flights are overbooked.

Private researchers who analyzed federal data on airline performance also said in a report being released Monday that consumer complaints to the Department of Transportation surged by one-fifth last year, even though other measures such as on-time arrivals and mishandled baggage show airlines are doing a better job.

“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 who has co-written the annual report for 23 years.

“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,” he said.

The industry is even looking at ways to make today's smaller-than-a-broom closet toilets more compact in the hope of squeezing a few more seats onto planes.

“I can't imagine the uproar that making toilets smaller might generate,” Headley said, especially given that passengers increasingly weigh more than they use to. Nevertheless, “will it keep them from flying? I doubt it would.”

In recent years, some airlines have shifted to larger planes that can carry more people, but that hasn't been enough to make up for an overall reduction in flights.

The rate at which passengers with tickets were denied seats because planes were full rose to 0.97 denials per 10,000 passengers last year, compared with 0.78 in 2011.

Consumer complaints were significantly higher in the peak summer travel months of June, July and August when planes are especially crowded.

The complaints are regarded as indicators of a larger problem because many passengers may not realize they can file complaints with Transportation Department.

At the same time complaints were increasing, airlines were doing a better job of getting passengers to their destinations on time.

The industry average for on-time arrival rate was 81.8 percent of flights, compared with 80 percent in 2011. The industry's on-time performance has improved in recent years, partly due to airlines' decision to cut back on the number of flights.

Airline passengers filed 11,445 complaints with the Transportation Department last year, an increase of more than one-fifth from 2011.The rate of complaints per 100,000 passengers also rose, from 1.19 in 2011 to 1.43 last year.A breakdown of that rate for the 14 largest U.S. carriers from a report being released today on airline quality by researchers at Purdue University and Wichita State University. The first rate is for 2012; the second for 2011.✓ Air Tran: 0.91; 0.72✓ Alaska: 0.51; 0.48✓ American: 1.80; 1.46✓ American Eagle: 1.27; 1.45✓ Delta: 0.73; 1.23✓ ExpressJet: 1.07✓ Frontier: 1.05; 0.76✓ Hawaiian: 0.89; 0.70✓ JetBlue: 0.79; 1.08✓ SkyWest: 0.88; 0.73✓ Southwest: 0.25; 0.32✓ United: 4.24; 2.21✓ US Airways: 1.74; 1.91✓ Virgin America: 1.50✓ 2011 rates were not available for ExpressJet and Virgin America.