LOS ANGELES — The U.S. is enjoying an unprecedented period of aviation safety — there have been no major plane crashes in nearly four years and runway violations are down.

But you would never guess that from the frosty relations between the controllers union and the federal government, which are in contract negotiations.

The National Air Traffic Controllers Association has ratcheted up its criticisms of the Federal Aviation Administration, saying insufficient staffing and equipment failures are jeopardizing safety in the skies and on the runways.

This week, the union claimed that severe personnel shortages were to blame for a series of controller mistakes, including two in the past six days, that caused planes to fly dangerously close over California.

"You're really dealing with people who are overworked, low morale, stressed out," said Hamid Ghaffari, the union representative at the center in Palmdale that handles high-altitude aircraft in Southern California and parts of Arizona, Nevada and Utah. "Boy, that's not a good mixture for air traffic controllers."

The FAA says that the recent close calls resulted from human error unrelated to working conditions. The union's claims are nothing more than a negotiating ploy, said FAA spokesman Greg Martin.

The current contract expires Sept. 30, though it would still be in effect so controllers don't stop working.

The federal government and controllers union have a history of discord dating back decades. President Reagan fired more than 10,000 controllers who illegally walked off the job in 1981.

Controllers' complaints may be contract-related, but their warnings of faulty equipment are legitimate, according to aviation analyst Mike Boyd.

"By and large, when they say things are not as safe as they need to be, take it to the bank," said Boyd, president of Colorado-based The Boyd Group. "I can understand their frustration."

The numbers suggest that the nation's aviation safety system is in good shape.

No major airplane crashes have occurred in the U.S. since November 2001, when American Airlines Flight 587 lost its tail and plunged into a New York City neighborhood, killing 265 people. Runway safety violations during the first 10-plus months of the 2005 fiscal year totaled 277, compared with 295 during the same period in 2004, preliminary FAA statistics show.

At the same time, those data show that "operational errors" — for example, when two airplanes get too close in the air — have increased over the same span from 988 to 1,308.

Air traffic safety statistics may be unreliable, however, because in many cases, controllers report their own errors.

Controllers say that the numbers do not tell the whole story.

A highly touted anti-collision system failed to warn of a near-collision recently at New York's Kennedy Airport. The system also hasn't worked properly in Boston because, controllers point out, it shifts into limited mode during bad weather.

The FAA's Martin said the anti-collision system, which has been in place for four years at the nation's top 34 airports, is just one component of the runway safety system — with pilots and controllers remaining the backbone.

That's the point, according to controllers who complain of understaffing.

A prime example is Los Angeles Center, where the two safety problems occurred during the past week, said Ghaffari, the facility's union representative.

The control center is authorized to employ 310 controllers but has just 217 certified personnel and 48 trainees, including 21 who can do very little because they are too new to the job, according to Ghaffari.

Both of the recent close calls resulted from mistakes made by overworked controllers when the facility was understaffed, he said.

On Sunday, Ghaffari said, a small Learjet flying from Wyoming to Burbank came within 100 vertical feet and 3 horizontal miles of a Frontier Airlines jet that had departed from Los Angeles International Airport for Denver.

On Aug. 12, a commuter jet heading to Los Angeles from Salt Lake City came within 800 vertical feet and 1 horizontal mile of a King Air jet flying out of Palm Springs, Ghaffari said.

FAA officials said that the Palmdale center was properly staffed when the incidents occurred and that neither amounted to any imminent danger of a collision.

"This is a case where two controllers made mistakes and now they're trying to blame the FAA," said agency spokesman Donn Walker.