Federal Aviation Administration officials say an audit of airline maintenance found three airlines had missed inspection deadlines. At a briefing at Washington's Reagan National Airport, the officials said that wiring in the wheel well was one area where carriers failed to comply with "air worthiness directives."
The agency would not name the carriers under investigation. FAA officials said penalties could be levied, though it would be several months before the probe was complete.
Earlier on Wednesday, United Airlines 11 percent of its fleet while it tested dozens of Boeing 777s to make sure components of a cargo fire suppression system were operating effectively, the carrier said.
Industry experts warned that passengers can expect more headaches as the FAA and airlines work to guarantee safety amid the rise in air travel - though federal officials are quick to note that this has been one of the safest periods in aviation history.
"The bottom line is ... flying is safer today than at anytime in the past," acting FAA Administrator Robert Sturgell said at a briefing Wednesday. "It's no accident or miracle."
The last U.S. crash of a jumbo jet was Nov. 12, 2001, when American Airlines Flight 587 lost part of its tail and plummeted into a New York City neighborhood, killing 265 people.
Independent airline consultant Robert Mann said that while the issue with the 777s should have been detected beforehand, passengers shouldn't be worried about the increased maintenance glitches.
"It's clearly inconvenient for passengers but it's a matter of the system working as designed," he said. "Carriers are now being prompted to check their own records and check the facts versus the records, and the FAA is doing the same thing from its end."
Hewlett Packard Co. employee Brock Tharp of Albertville, Minn., who was flying Northwest Airlines to Los Angeles for training on Wednesday, said he had heard some reports about safety concerns in the airline industry, but he wasn't worried.
"I know the airlines are heavily regulated, and with the press putting it out there I'm sure they are working double-time on any inspections," he said. "I hope so, anyway."
Others take a dimmer view. Rep. James Oberstar, chairman of the House Transportation and Infrastructure Committee, said Tuesday that the string of maintenance problems springs from a "culture of coziness between the airlines and senior FAA management."
The Minnesota Democrat will lead a hearing Thursday at which the committee will report the findings of its investigation into the agency's safety oversight.
Among those scheduled to testify are a whistle-blower who first detailed problems at Southwest, which faces a record $10.2 million fine for continuing to fly dozens of Boeing 737s that hadn't been inspected for cracks in their fuselages, as well as safety inspectors for other airlines and the Transportation Department's inspector general.
Southwest is not the only carrier that has benefited from a "cozy" relationship with regulators, said Tom Brantley, president of the Professional Aviation Safety Specialists union that represents FAA inspectors.
In testimony prepared for the hearing, Brantley details maintenance and safety issues at United, Continental Airlines Inc., Northwest Airlines Corp., Hawaiian Airlines Inc. and elsewhere where the carriers were given great leeway by the FAA to correct problems that inspectors on the ground said merited more serious attention. Financial penalties for infractions suggested by inspectors against United and other carriers also were ignored or significantly reduced by the time they were assessed, he added.