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LOS ANGELES — Hundreds of airliners headed to or from the Southwest were grounded Thursday because of repeated failures of air traffic control radar, leaving airports gridlocked and thousands of passengers stranded or delayed. 

“I’ve got a grandma dying in New Mexico and I can’t get there,” said a frustrated Davis Hill of Boise, Idaho, as he waited for a connecting flight at Los Angeles International Airport. 

Federal Aviation Administration spokesman Jerry Snyder said a “national ground stop” was ordered after the computer in southern California handling air traffic control radar failed for a second time.  

The order meant airliners bound for the Southwest were stopped from taking off at their originating point. 

A backup system took over and there were no safety problems for aircraft in the air, Snyder said. Controllers at the Los Angeles Center, where the failure occurred, guide flights over a 100,000-square-mile area from Nevada and Utah out to sea 250 miles. 

The radar failure was the result of a computer software upgrade Wednesday night. 

“When the systems were being brought back up on line, it failed. It’s not accepting the software,” Snyder said. “It’s worked for 17 other systems but it’s not working in this one.” 

There were tense moments for FAA controllers. At one point, there was no more room on the ground at Los Angeles International Airport and flights from Australia, for instance, were sent to Las Vegas. 

“It’s been pretty crazy,” said Garth Koleszar, local vice president of the National Air Traffic Controllers Association. “Every flight in and out of our sector was affected.” 

Asked if there were any safety concerns, he said: “Any time you use a system that isn’t the best that we have, I feel there is a degradation.” 

The backup system doesn’t have the automatic feature that passes off planes from one regional controller to another, and requires a controller to manually type the flight information and pass along information by telephone. 

The first main computer outage lasted from 6:50 a.m. to 8:30 a.m., and led to what was called a Tier 1 ground stop, meaning Southern California-bound aircraft west of the Mississippi couldn’t take off. 

The computer went down again at 9 a.m. and a Tier 2 ground stop was ordered, halting planes on the ground nationwide. 

The computer was restored again at 11:15 a.m. using the old software program, and the FAA ordered the Tier 2 ground stop lifted incrementally throughout the day to meter the flow of aircraft. 

The radar system at the FAA’s Los Angeles Center, 60 miles north of downtown in the Mojave Desert community of Palmdale. 

“This was a standard upgrade. It was new software. Something is corrupting the system,” Snyder said, adding, “There was nothing wrong with the old software.” 

Southwest Airlines canceled 70 flights, airline spokesman Melanie Jones said from Dallas. Other flights experienced delays of up to four hours. 

San Francisco International Airport spokesman Ron Wilson said about 20 flights were grounded there during the first outage. 

United Airlines canceled at least 32 flights into and out of San Francisco.