LOS ANGELES - In at least five separate cases, planes passed closer than standards allow during a radio failure in the West that was caused by a missed routine maintenance check and a faulty backup system.

Two of the flights "were almost near-mid-air collisions," said Hamid Ghaffari, local president of the National Air Traffic Controllers Association.

But Federal Aviation Administration spokesman Greg Martin said the planes were never in danger of crashing.

On Wednesday, FAA officials announced that human error caused the radio breakdown at the Los Angeles Air Route Traffic Control Center, which controls airspace for a vast region that encompasses California, Arizona, Nevada and parts of Utah.

"A required 30-day maintenance check on the primary radio and voice communications system was not performed," the FAA said in a prepared statement. "This system turns off if this check is not performed."

A backup system also failed because it "was not configured properly to ensure its availability in the event of the primary system's failure," FAA officials said.

The agency noted the problems could have been avoided if workers had followed procedures.

Planes were grounded for about three hours, delaying flights and frustrating passengers who were stalled at airports. Air travel returned to normal Wednesday.

Under FAA "separation standards," planes flying higher than 29,000 feet must remain at least five miles apart horizontally, and no less than 2,000 feet apart vertically.

In the first 13 minutes after radio contact failed, five pairs of planes flew closer to each other than permitted under safety standards, FAA records showed.

Ghaffari said that during the radio blackout, planes traveled dangerously close to one another as controllers watched radar screens in disbelief.

"We couldn't do anything," he said. "If there are no communications, you are helpless."

Three workers filed on-the-job injury claims after becoming traumatized by watching the flights on radar without being able to intervene, Ghaffari said.

FAA officials said at one point, two planes came within slightly less than a mile of each other while separated by 1,400 vertical feet. Martin said it was "breathless hysteria" to suggest a collision was imminent.

United Parcel Service spokesman Mark Jiuffre said the captain of one of its cargo jets, headed from Louisville, Ky., to Santa Ana was alerted by on-board safety equipment that another plane was flying too close.

The pilot "climbed about 1,000 feet, and when they were climbing they did see a business jet in the area," Jiuffre said. "The flight continued without event."