Two separate human errors caused a breakdown in radio communications that brought Southern California's major airports to a near-stop Tuesday and led to at least five instances in which planes came too close, Federal Aviation Administration officials said.
"A loss of communication is a serious matter, and it should not have occurred," Rick Day, a senior FAA official, said Wednesday.
On Tuesday, FAA officials had insisted that the more than three-hour system shutdown posed no safety risks. But they acknowledged Wednesday that they were investigating five incidents in which planes lost the required separation distance during the first 15 minutes of the communications breakdown.
In two cases, large airliners -- a UPS cargo plane and a Northwest Airlines flight bound for Southern California airports -- came much closer to small corporate jets than federal guidelines allow, requiring at least one pilot to take corrective action. FAA officials repeated Wednesday that they did not believe lives were ever at risk.
The agency's radio system in Palmdale shut itself down Tuesday afternoon because a technician failed to reset an internal clock -- a routine maintenance procedure required every 30 days by the FAA. Then a backup system failed, also as a result of technician error, officials said.
The radar system in Palmdale, contrary to what some FAA and union officials had said Tuesday, did not shut down.
The radio system that crashed about 4:30 p.m. Tuesday at the Los Angeles Air Route Traffic Control Center in Palmdale is a high-tech touch-screen tool that allows air traffic controllers to quickly communicate with planes in transit. Controllers at the Palmdale facility communicate with cruise-altitude air traffic across Southern California and most of Arizona and Nevada, an area of about 178,000 square miles.
FAA officials said they had known for more than a year that a software glitch could shut down radio communications and were in the process of fixing it. In the meantime, they required manual resetting of the communications system -- a process they described as similar to rebooting a personal computer.
The problem so far has been corrected only in Seattle, one of 21 FAA regional air-traffic control centers that have used the system since the mid-1990s.
The radio failure rippled throughout the nation's airports, grounding hundreds of commercial flights and forcing controllers working from other centers to divert hundreds more to locations outside Southern California.
