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Flight delays are growing longer and more frequent on the nation’s airlines, according to a yearlong government study released Monday, prompting new calls in Congress for sweeping legislation to better protect consumers.

The report by the U.S. Department of Transportation’s inspector general found rising dissatisfaction among passengers one year after the airline industry staved off a “passenger bill of rights” that would have required certain measures to improve customer service. The airlines instead introduced a voluntary program.

Overbooked flights are on the rise, the information the carriers provide about delays and cancellations is often unreliable and the congestion at airports is expected to get worse over the next two years, Inspector General Kenneth Mead reported.

While some congressional leaders said new laws were necessary to address the airlines’ dismal record, the prospects remained slim that a mandatory program would win support from the Bush administration and a Republican-controlled Congress reluctant to impose more regulations on airlines.

Mead’s report, which stops short of calling for congressional mandates, did find some areas of improvement since the voluntary initiatives were introduced. The report said the airlines were better at quoting the lowest available fares, providing more legroom between seats and increasing compensation for lost luggage.

But the positive efforts have only nibbled at the edges of the deeper problems that threaten to wrap the commercial aviation system in gridlock again this summer, Mead said.

The Air Transport Association, which represents the major airlines, characterized Mead’s report as “fair and reasonable” and said it reflected the solid progress the industry has made.

But the numbers tell a different story. The number of flight delays increased by 19 percent in just a year, and passenger complaints rose 14 percent. According to the report, more than a quarter of flights were delayed, canceled or diverted to other airports last year, inconveniencing 163 million travelers.

David Perrine of Charlotte, who flies two to three times a week, is among those who said he hasn’t noticed any improvement in service despite the airlines’ pledges.

“The myth about the airlines calling you at home if your flight is canceled, well, they do that maybe once every 10 times,” Perrine said Monday at O’Hare International Airport, referring to one of the voluntary programs the airlines adopted.

While modernization of the air-traffic system and the construction of new runways are beyond the airlines’ powers, the carriers are making a difficult situation worse by regularly scheduling more flights than airports can handle at peak periods, Mead said.

“The road ahead is long, and aggressive progress will be required by the airlines, airports and the FAA if consumer confidence is to be restored,” Mead said.

U.S. Sens. John McCain (R-Ariz.) and Ron Wyden (D-Ore.) said they would introduce legislation requiring the airlines to stand behind their customer service plans, which include assurances that passengers bumped from flights will be quickly rebooked and will be given meals and hotel rooms if they experience excessive delays.

“The airline industry argues that the answer to problems with customer service is simply more runways and better air-traffic control,” Wyden said. But “passengers should not have to wait until more runways are built for the airlines to stop making excuses and start telling passengers what they know and when they know it.”

U.S. Rep. William Lipinski of Illinois, the ranking Democrat on the House aviation subcommittee, said he was disappointed that “the airlines have not done a better job when they convinced all the parties not to go ahead with legislation.”

But in light of the reorganization and consolidation in the airline industry, some aviation experts and airline officials argued that now is not the time to issue layers of new rules.

“We are in the service business, and if we don’t find ways to provide that level of service, passengers will obviously find other airlines that can,” said James Goodwin, chairman and chief executive officer of United Airlines.

“We have made progress in all areas, but obviously the report indicates that all of us still have room for improvement,” Goodwin said.

Wyden said the legislation that he and McCain will propose gives the airlines six months to incorporate their voluntary customer service plans into the legally binding passenger contract that is printed on all airline tickets.

McCain said during a trip Monday to Northwestern University that “the airlines have not played a constructive role here in Chicago.” He said his flight Monday from Phoenix to O’Hare was delayed for three hours because of the high volume of aircraft competing for runways and terminal gates, and he urged that talks proceed at once concerning construction of a third major airport in the region.

While Mead warned that flight delays and cancellations are expected to worsen this year as the Federal Aviation Administration struggles to modernize its air-traffic control technology and push capacity-boosting improvements at airports, he said the airlines have failed to fix problems that they directly control. The shortcomings include habitual overscheduling, flights that are chronically late or canceled, and the failure of the airlines to provide passengers with timely and reliable information.

The report cautioned that the Department of Transportation’s resources to enforce consumer protection laws and to monitor anti-competitive practices by the carriers are “seriously inadequate.”

The National Airline Passengers Coalition, a watchdog group formed last week to push for a passenger bill of rights, said legislation can indeed be passed by the Republican-controlled Congress.

“The airlines failed the American people,” said Scott Reed, a spokesman for the coalition. “This is a new day in Washington, and we will take full advantage of that.”