The FAA said developmental controllers were closely supervised by fully certified controllers and were never assigned duties for which they weren't qualified. FAA Administrator Randolph Babbitt told Congress this month that the agency always made sure that "the right number of trained controllers are in the right place at the right time."

But Forrey, the president of the controllers union, said the FAA was "jamming the system with a bunch of trainees."

"It means that there's a whole lot less experience going on out there working the airplane," he said.

Busiest radar site could be overwhelmed The level of inexperience is "particularly critical" at the Southern California radar facility in San Diego, or SCT, the nation's busiest radar control center, the Transportation Department's inspector general reported in April.

Because of new hires, there is no official staffing shortage at SCT, where the overall number of controllers has held steady. But the number of those controllers who are fully certified has plummeted - from 236 in 2004 to 161 in January, the end of the study period. Seventy-six controllers were working without full certification. And 52 of them - or 68 percent - were new trainees with no previous certification, the report said.

By the end of this year, controllers still in some stage of training will make up more than 40 percent of the site's active staff, the inspector general projected, which could "overwhelm the facility's training resources."

Sen. Dianne Feinstein, D-Calif., said the certification pace was unacceptable. Because STC "handles some of the most complex airspace in the United States," she insisted in a letter to Transportation Secretary Ray LaHood, it "should be staffed with our most experienced controllers, not more than 100 controllers who are yet to receive full certification."