Consumer advocacy groups want federal regulators and Congress to investigate whether U.S. airlines are canceling or rerouting flights at the last minute when they have not sold enough seats to make them profitable.

They say airlines are canceling flights to save money, but telling passengers it’s because of mechanical problems or weather delays. And they are ready to take legal action against carriers.

Commercial airline officials deny the claim, saying flights are not canceled for bottom-line reasons.

Tim Wagner, a spokesman for American Airlines, said grounding flights because of the number of seats sold “has been a common misperception for some passengers for decades, so this isn’t some new speculation. It simply has never been true.”

“I’m not an attorney, but yes, we’ll file lawsuits,” said Kate Hanni, founder of the California-based Coalition for an Airline Passengers’ Bill of Rights.

Hanni wants the U.S. Department of Transportation to chronicle and investigate airlines’ cancellation claims, specifically last-minute cancellations citing mechanical problems. She also wants Congress and USDOT to investigate flights that routinely are canceled.

Harvey Rosenfield, founder of Consumer Watchdog, another California group, said canceled flights are an increasing problem, and his organization wants the federal government to press airlines more for cancellation information. That could include filing legal actions and regulatory complaints against airlines, he said.

Canceling flights because not enough seats are sold is against federal regulations. Airlines are allowed to cancel flights for mechanical, weather or air traffic control reasons.

USDOT spokesman Bill Moseley said the agency fields passenger complaints about flight cancellations, but has found no evidence airlines are doing it to cut costs.

Representatives of Tempe, Ariz.-based US Airways Group Inc. and Dallas-based Southwest Airlines Co. — the dominant carrier at Baltimore/Washington International Thurgood Marshall Airport — said their carriers are operating by the rules.

“We do not cancel flights because of the number of seats sold,” said US Airways spokeswoman Valerie Wunder in an e-mail response to the Phoenix Business Journal.

Southwest spokeswoman Whitney Eichinger said the carrier is honest with customers about delays and cancellations, and does not cancel flights for lack of demand.

USDOT tracks cancellations, flight delays and consumer complaints.

Airlines canceled 9,900 U.S. flights in September 2008, compared with 6,500 in September 2007, according to USDOT. Some of the cancellations this year were attributed to severe tropical storms and hurricanes hitting Gulf Coast states.

Rosenfield said a recent Southwest Airlines flight he booked from Oakland, Calif., to Los Angeles was canceled for mechanical reasons, and he was put on a later flight. He said a Southwest employee told him several flights were canceled that day because they weren’t full.

Some passengers say airline employees told them flights sometimes are changed or canceled because of how many seats are sold. Others suspect the cancellations may be for economic reasons.

Robyn Itule, a public relations consultant with Armstrong Troyky Public Relations & Advertising in Phoenix, Ariz., thinks a recent US Airways flight she was booked on from Phoenix to San Diego was canceled because it was not full. She said another flight on the same route, which was full, had left a few minutes earlier.

“While no one ever came right out and said our flight was canceled because of it not being filled to capacity, it doesn’t take a huge leap in logic to come to that conclusion,” she said.

An official with Alaska Airlines, who asked not to be identified, said airlines will cancel flights if they are not full enough for them to be profitable.

“I do have knowledge that this has been used with other carriers when circumstances demand shoring up resources, such as aircraft, crew, fuel costs,” the official said.

Scott Theuer, communications chairman of the US Airline Pilots Association, the union that represents US Airways, said economic cancellations are rare. He said flight schedules, routes and staffing needs throughout the system make it difficult for airlines to cancel flights on a frequent basis. He said mechanical delays and cancellations are often just that.

“None of this is to say pre-emptive cancellations can’t occur, but based on the above constraints they are probably fairly rare,” Theuer said.

US Airways’ Wunder said from an operations standpoint, the airline needs to keep planes flying on the published schedule.

“Maintenance schedules, crew plans and passengers all depend on every flight operating. Enormous effort is required to protect passengers booked on subsequent flights (and) perform routine maintenance on schedule,” she said.

JetBlue Airways spokesman Sebastian White said nixing flights at the last minute would cause more than customer service issues for carriers.

“It would be a tricky proposition,” he said. “You can’t cancel a single flight without having an impact downline at other cities.”

Robert E. Mittelstaedt, dean of Arizona State University’s W.P. Carey School of Business, said speculation of bottom-line cancellations has been around for a while.

“I think these rumors are exaggerated, which does not mean it has not happened on occasion,” he said. “However, I have also been on some weekend flights that were not full where it was clear they were losing money flying the leg.”