MEMPHIS, Tenn. — The Federal Aviation Administration is realigning the duties of air traffic controllers in some cities, a move that critics say will mean less training for the people responsible for the safety of the flying public.

Faced with a nationwide shortage of controllers, the FAA says it wants to streamline training by dividing the job of air traffic controllers into two specialties. In January, controllers in Memphis and Orlando, Fla. — now trained to work in their airport towers as well as companion radar centers — will be restricted to one job or the other.

"It's simply focusing their training to do precisely what they're going to be doing," FAA spokesman Paul Takemoto said.

Some lawmakers and the controllers' union say the change will allow the FAA to certify controllers with fewer training hours than the current standard.

"It masks their staffing problems," said Victor Santore, regional vice president of the Air Traffic Controllers Association.

The union also argues that the new job descriptions will cut controllers' salaries by 4 percent to 8 percent and limit staffing flexibility in emergencies.

Radar centers called TRACONs, for Terminal Radar Approach Control, direct aircraft for landings and takeoffs up to 50 miles from their airports. Towers handle planes when they're within five miles of an airport or on the ground. More than 40 percent of the FAA's 315 air traffic control facilities have towers with companion radar centers.

John Wallin, president of the union local in Memphis, said training controllers to work in both airport towers and radar centers improves coordination between the groups as they work to keep planes safely spread out over busy airports.

He called the FAA's move dangerous.

"Controllers who work in the tower will no longer have the experience that radar controllers have, and that could lead to a disaster, because they're not going to know what each other is doing," he said.

The FAA's move to split the tower and radar center jobs is not new. More than 20 of the busiest airports in the U.S., including those in Atlanta and Chicago, already operate that way.

But Wallin said those airports get the most experienced controllers, many of them with both tower and radar experience earned in smaller cities like Memphis.

The FAA also has looked into splitting the controllers' job functions at cities including Cleveland, San Antonio, Pittsburgh, Cincinnati, Tampa, Fla., and Charlotte, N.C. Moves to split the work in towers and radar centers at Miami and Philadelphia were recently scaled back following complaints from members of Congress and others who argued the plans needed more study and input from outside the FAA.

Sen. Robert Casey, D-Pa., joined other Pennsylvania lawmakers in opposing the plans, arguing that "any action that would dilute staff would dilute safety," his office said.

The FAA hired most of its 14,800 controllers within a few years of a 1981 strike that ended when President Ronald Reagan fired the strikers.

Rep. Jerry Costello, chairman of the House subcommittee on aviation, said the FAA has failed to lay the groundwork to replace so many experienced controllers, and has caused early retirements by refusing to negotiate since 2006 on a new work contract.

Costello, D-Ill., said the agency must get more controllers on the job, but any moves to lessen certification requirements will draw a congressional review, "if in fact that is taking place."

Nationwide, about a fourth of air traffic controllers are in training, meaning they need on-the-job supervision, and the transportation department's inspector general says that may increase to 30 percent over the next four years as more new controllers are hired.

Memphis has 62 controllers, 45 of whom are fully certified. The rest are in training, and up to 10 more trainees are expected next year. Though several of the older hands are likely to retire soon, Wallin said, the number of controllers deemed fully certified will increase after the job split.

"They can go to Congress and say, 'Look, we fixed Memphis,"' he said. "'We now have 54 or 55 fully certified controllers, some in radar and some in the tower."'