The nation's aging airline fleets, already struggling with high fuel costs and growing passenger demand, delivered another blow to travelers Wednesday as American Airlines and Delta Air Lines canceled hundreds of flights while planes were reinspected for compliance with federal maintenance rules. More disruptions are expected today.

Experts said the airlines' decision to voluntarily ground the planes didn't signal that they were unsafe. But the episode is the latest indication that a combination of aging aircraft and schedule reductions are leaving the industry with fewer options.

U.S. airlines, beset by financial woes that have forced them to postpone capital investments, operate some of the oldest fleets in the world, noted Richard Aboulafia, an analyst with the Teal Group.

Older aircraft need more upkeep, which can result in more down time.

"This isn't a safety issue," he said. "But reliability is becoming a concern due to the age of the fleet." And with fewer planes, the carriers also are hard-pressed to re-book passengers onto other flights when there are cancellations for inspections or other reasons.

American, which was forced to cancel more than 320 flights Wednesday as it performed maintenance checks on its MD-80 jetliners, said it may have to cancel more flights today as it completes wiring inspections and repairs.

Maintenance issues have been high profile at America's airports for weeks as carriers scramble to bring maintenance inspections up to date after a recent Federal Aviation Administration crackdown. On March 6, the FAA assessed a $10.2-million fine against Southwest Airlines.

Since then United Airlines, American Eagle, American and Delta have conducted voluntary inspections, and more may be ahead.

At Los Angeles International Airport on Wednesday, American canceled 10 of 92 domestic flights, affecting about 1,000 passengers.

"It's good to know that they're taking precautions, but I'm still nervous that there are problems in the first place," said Amy Isenberg of Los Angeles, who was waiting for an American flight to Nashville. American has more flights into and out of LAX than any other carrier.

American said it also canceled a flight at Ontario International Airport and four flights at San Diego International Airport.

The cancellations did not affect passengers at other local airports, including Burbank's Bob Hope and John Wayne in Orange County, the airline said.

American had inspected about 200 planes by Wednesday afternoon and found that 80 required being taken out of service while modifications were being made to wiring bundles near the landing gear, spokesman Tim Wagner said.

The cancellations represented about 13% of the day's schedule for the nation's largest carrier and between 25,000 and 30,000 passengers were affected as flight schedules were disrupted across American's domestic route system.

"This was not related to an incident or issue that would have endangered passengers," Wagner said, adding that the inspections were carried out in cooperation with the FAA.

American said 95 MD-80s that flew normal schedules Wednesday still needed to be inspected Wednesday night and today. Those checks probably would require an unspecified number of flight cancellations today, the airline said.

Delta added to travelers' woes when it said it would have to cancel some flights today while it inspects the wiring bundles on its fleet of 133 MD-80 and MD-90s. Delta said it hoped to re-book passengers when necessary. The inspections are expected to be completed by Saturday.

Among big U.S. carriers, American operates the second-oldest fleet, with an average age of 14 years, according to a new study by AirlineForecasts, a Washington-based consulting firm. Many of American's 300 MD-80s were built in the late 1980s, according to the study.

Northwest Airlines has the nation's oldest fleet of planes, with an average age of about 20 years, the study found.