Turnover in airport towers and at regional radar facilities and flight service centers is so rapid that experienced controllers are leaving faster than their replacements can be hired and certified. That means controllers still in training are often guiding planes on takeoff, in the air and on landing.

Meanwhile, a satellite-based air traffic management system, designed to automate more efficient flight paths, remains years off, even as current facilities are reaching and surpassing their expected useful lifespans.

'Trainees' can be highly experienced According to staffing records of the FAA and the Transportation Department, its parent agency, nearly 4,000 air traffic controllers in training were on the job at the beginning of the year without having completed full certification. That's about 27 percent of a current professional staff handling the 50,000 or so flights that travel the nation's skies each day.

By itself, that figure isn't unusual; in fact, it's below the FAA's own ceiling of 35 percent. Air traffic controllers complete their training on the job, so trainees are always going to make up a significant proportion of the working staff.

"The only alternative is to not replace controllers who retire, and that simply is not an option," said Ian Gregor, a spokesman for the FAA.

Controllers in training aren't necessarily inexperienced controllers. They can be veteran controllers who are simply learning the specifics of new assignments: retired military controllers, certified controllers who have transferred from other facilities or retired FAA controllers working as contractors.

But the clear majority of the trainees in the FAA's recent hiring wave are new to the profession. The FAA is hiring so quickly that "developmental controllers" - those with no previous experience - make up about two-thirds of those in training and slightly less than a fifth of all federal air traffic controllers.