A congressional investigation has found more potential safety problems at Southwest Airlines and possible lapses at other US carriers are being reviewed, a senior lawmaker said on Friday.

Rep. James Oberstar, a Minnesota Democrat and chairman of the House of Representatives Transportation Committee told reporters his panel has received information from government airline safety inspectors who claim their reports about violations are routinely not acted upon by superiors at the Federal Aviation Administration (FAA).

"So many FAA inspectors have given up reporting failures by carriers because there is a cozy relationship between the FAA management and airline management," Oberstar said. "That has to stop."

Oberstar's comments came one day after the FAA proposed a record USD$10.2 million fine against Southwest for failing over a period of several months in 2006 and 2007 to complete required aircraft safety inspections on 46 older 737s.

The airline voluntarily notified the FAA about the lapse in checks for fuselage cracks, but continued to fly the planes until the work was done -- about eight days. Safety and congressional officials said the aircraft should have been grounded.

Southwest, the largest low-cost airline, said it consulted with the FAA and thought it was in compliance with all rules. The airline said the fine was unfair.

There were no safety incidents that resulted from flying the uninspected planes, officials said.

But Oberstar said Southwest had "systematic flaws" in its program for monitoring inspections required by the government, and asserted a certain complacency at the "highest levels" of FAA management.

And there are two other investigations of possible lapses at Southwest.

Senior FAA officials are investigating a potential safety issue that they will not disclose and, separately, Oberstar said the airline may have failed to complete mandatory rudder inspections on 70 planes.

Moreover, Oberstar said his investigators have begun looking at other airlines, based on reports from information provided by inspectors.

"There is a strong likelihood that there were similar lapses at other carriers," he said.

Oberstar did not identify any other airlines.

FAA officials said there was a "twofold" breakdown in inspection monitoring at Southwest and called it an isolated case that began as an inadvertent misstep by the airline.

"This is a clear failure by the air carrier and one of our employees," Peggy Gilligan, a senior FAA safety official, said about communications between the carrier and an agency official in Dallas.

Southwest said it consulted 737 manufacturer Boeing on its response to the missed inspections and was told it could safely fly the aircraft for a brief period while the work was completed. Boeing confirmed this account.

Southwest also said it received guidance from the FAA that allowed it to operate uninspected planes. Southwest said six planes were found to have small cracks, which have been repaired.