A computerized system that guides arriving planes onto a runway at Los Angeles International Airport failed, marking the second serious problem to disable the world's fifth-busiest airport in the past three weeks, officials said.

The latest malfunction on Monday caused flight delays across the nation.

Aviation experts said the equipment failures raise questions about the nation's aviation system, which is straining under passenger loads that have rebounded to their highest levels since Sept. 11, 2001. LAX, for example, averages 1,800 daily flights and will serve an estimated 18.7 million passengers this summer, 200,000 more than last year.

"The FAA's complete instrument system, as we know, is somewhat accident prone," said Jack Keady, an aviation consultant who has followed the problems at LAX. "We need to do more what-if thinking,"

Monday's malfunctioning equipment, called a localizer, acts as a beacon to guide arriving planes onto runways. It is most crucial when it is foggy or hazy. It was foggy at the airport on Monday.

The problem was compounded because one of the airport's four runways was closed for a major construction project. To compensate, one of the departure runways was handling both departures and arrivals — and it was that shared runway that had the problem.

The localizer failed shortly before 9 a.m. PDT, the Federal Aviation Administration said. Airport authorities responded by reversing the direction of the runways so that the faulty equipment was no longer needed. The change reduced the number of landings, usually about one a minute, to 32 an hour, the FAA said.

Flights were forced to circle the airport, and some planes were ordered to remain on the ground at other airports, officials said. Arriving flights were held up to 90 minutes. Departures were also delayed, and several flights were canceled.

Technicians were able to repair the system shortly after noon PDT. Officials were investigating the cause of the equipment's failure.

The airport was hit with a major power failure July 18 that backed up flights across parts of the western United States and Canada.

That outage happened when a vehicle crashed into a utility pole, causing a power fluctuation that prompted the air traffic control center's backup generator to turn on. About an hour later, that generator failed.

The failures prompted Sen. Barbara Boxer, D-Calif., to demand that the FAA investigate the landing system's failure.

Air traffic controllers said the localizer flickered off intermittently Saturday, but that each time it came back up after a few minutes. Technicians found nothing wrong with the beacon after the outages, said Diane Aceves, an air traffic controller and local president of the National Air Traffic Controllers Association.

FAA spokesman Ian Gregor said it took more than three hours to fix the system Monday because the technician, who works at several airports in the region, was not at LAX at the time.