If you're planning to fly any time soon, expect more delays ahead.

According to the Federal Aviation Administration, four U.S. Airlines are now being investigated for failing to follow agency regulations. The agency did not name the carriers in violation.

The FAA looked at nearly 2,400 planes during the past two weeks and found six that were overdue for inspection. Overall, the audit found a 99-percent compliance rate.

"The bottom lines is our system works. Flying today is safer than anytime in the past," said Bobby Sturgell with the FAA.

Ironically the report was released on the same day United was forced to cancel more than 100 flights. The airline says a routine safety check had been skipped during inspections.

Thousands of passengers were stranded as planes were grounded so that those tests could be conducted.

"I'm OK as long as I know that they're taking care of the plane," said David Daniels, a passenger in Chicago.

Last month, Southwest, American and Delta were all forced to ground planes for required inspections. The increased scrutiny comes after Southwest was fined a record $10 million for flying jets it knew had to be adequately inspected.

The incident highlights what some charge is a serious breakdown in oversight by the FAA.

"If we find a problem, we need to be able to report it and not feel like we're putting ourselves in jeopardy," said Tom Brantley with the Professional Aviation Safety Specialists.

Stating it still works for the same goal, the FAA announced its new Safety Issue Reporting System (SIRS).

"SIRS provides a totally new avenue for the employees to raise attention and get results," said Sturgell.

Congress will hold a hearing on airline safety on Thursday. There are two major questions that are sure to come up -- does the passenger need to be concerned about airplane safety, and why isn't the FAA doing its job?

The chairman of the House committee holding the hearing, James Oberstar, D-Minn., says the string of maintenance problems is due to what he calls a "culture of coziness between airlines and senior FAA management."

On the list of those scheduled to testify is a whistle blower who exposed the problems at Southwest Airlines.

Southwest now faces a record fine for continuing to fly dozens of Boeing 737s that had not been inspected for cracks in the fuselage as required.