U.S. air travelers have endured longer lines, more delays and the loss of amenities like meals and blankets. Now they are getting hit with a wave of schedule disruptions caused by airlines scrambling amid increased regulatory scrutiny to ensure that the expanding air transport system stays safe.

The latest complication came Wednesday, when United Airlines temporarily grounded dozens of Boeing 777s to test their cargo fire-suppression systems.

United said it canceled more than 30 flights and delayed dozens of others as it carried out work on the long-haul jets after a review of maintenance records showed that a test on a bottle in the fire suppression system hadn't been performed.

The move affected thousands of passengers around the world, as United's 777s mostly fly international routes from its major hubs. Among those grounded was a 777 used by many members of the White House press corps, who were traveling with President Bush in Romania - though it wasn't set to fly again until Friday.

Schedule foul-ups due to inspections have been commonplace since early March, including hundreds of flights canceled last week by American Airlines and Delta Air Lines as they checked wiring bundles on some planes. Stepped-up inspections began when the Federal Aviation Administration ordered a check of maintenance records at all domestic carriers after revelations surfaced about missed safety inspections at Southwest Airlines Co.

The FAA said Wednesday that four U.S. airlines are under investigation for failing to comply with federal aviation regulations, but would not name the carriers. Officials said three airlines had missed inspection deadlines and that penalties could be levied, though it would be several months before the probe was complete.

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.

United, a subsidiary of UAL Corp., said Wednesday that the 777s have "intuitive" self-diagnostic systems that would have detected any malfunction with the fire suppression system. The company said it alerted authorities after the missed test was discovered.

Testing on all 52 of United's 777s - 11 percent of its overall fleet - was expected to be completed sometime Thursday. The Chicago-based carrier said 22 of the jets had been inspected and cleared to fly by midday Wednesday. The planes fly mostly from Chicago O'Hare, Denver, Washington Dulles, San Francisco and Los Angeles.

United carried out unscheduled maintenance on seven of its Boeing 747 jets last month but said it found no safety-related issues.

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AP Business Writer Dan Caterinicchia in Washington contributed to this report.