Federal officials said Tuesday they do not know what is causing a critical instrument landing system to fail at Los Angeles International Airport.

The system, a beacon that guides planes to one of the airport's runways, has shut off twice within a week, most recently for about 40 minutes Monday. That created delays of up to 45 minutes as controllers kept planes circling.

"We haven't found the cause of the problem," said Steven Zaidman, vice president of technical operations for the FAA's Air Traffic Organization. "At this point, we're maintaining technicians 24-7, full-time, to reservice the instrument landing system and reset it" whenever it shuts itself off.

The Federal Aviation Administration, which owns and maintains the equipment, has drawn fire from the head of the city airports department, politicians and the air traffic controllers' union.

"This is clearly a systemic problem with the equipment," Lydia Kennard, executive director of Los Angeles World Airports, said Monday. "We believe there's something fundamentally wrong with the equipment. It has to be fixed or replaced."

The malfunctioning equipment, called a localizer, is most crucial when it's foggy or hazy, as it was on Aug. 7 when the equipment flickered off for several hours, creating 90-minute arrival delays.

Zaidman, speaking in a conference call with reporters Tuesday, said 46 flights were delayed during last week's outage, and 13 delays were attributed to Monday's problem. The airport averages 1,800 daily flights.

In some cases when the instrument landing system isn't working, pilots can land by relying on their own vision, FAA spokesman Ian Gregor said. Other times, he said, they will pull up and circle until the system is functioning.

Zaidman said technicians have not been able to tell whether the problem lies with the localizer antenna array at the end of the runway, the computerized system next to the runway or the cables running between them.

Zaidman said any number of things could be causing the outage, including corrosion, dust or problems with the circuit board or cable pins. He said the technician that reset the localizer on Monday found one of the cable pins was pushed in too far, but added that officials do not believe that was the cause of the repeated outages.

The malfunctioning system is one of eight localizers placed at each end of the airport's four parallel runways, one of which is closed for construction.

On July 26, a system designed to alert controllers at the airport tower to potential collisions on the ground was partially disabled minutes before a turboprop plane narrowly missed a jet that had strayed onto its runway.