NEW YORK (AP) _ Starting Thursday, jetliners will trace new routes above the eastern United States, raising the likelihood of on-time flights but giving air traffic controllers new concerns over staffing and training.

The Expanded East Coast Plan, in the works for five years, is part of a grand plan the Federal Aviation Administration says will simplify air routes, improve monitoring, reduce departure delays and handle more flights.

''The EECP is only one piece in the mosaic of FAA's National Airspace Plan, which is designed to meet aviation's needs through the end of the century,'' said Joseph M. Del Balzo, director of the FAA's eastern region.

But some air traffic controllers in the New York area say they are understaffed and insufficiently trained for the new system.

Two petitions asking postponement of the plan were signed by 140 controllers and sent to FAA supervisors and federal and state legislators in the metropolitan area, served by La Guardia, Kennedy and Newark airports.

The FAA met with controllers last week to review the complaints. The agency maintains the system is ready, and staffing and training are adequate.

''They're doing it too quickly,'' said Stephanie Burzon, an air traffic controller at New York TRACON, or terminal radar approach control in Westbury, which monitors approaches and departures from the airports. ''We want it delayed by however long it takes to train the new people.''

The plan creates more flight routes for planes to leave airports - a ''four-lane highway'' instead of just one lane, Burzon said.

Peter Nelson, FAA eastern manager of planning and public affairs, said the plan ''isn't a panacea to resolve all air traffic problems and delays.''

''We will be able to reduce the amount of departure restrictions, because one runway will be feeding multiple departure routes,'' he said.

The plan covers airspace south from Maine to Miami and west to Chicago, an FAA announcment said. Phase I, starting Thursday and directly affecting the New York and Washington, D.C., areas, modifies 33 jet routes, sets up separate flight paths for slow planes and establishes more direct jet routes.

Though more takeoffs can be accomodated, said Burzon, ''what happens at the end of the funnel? There's still only so much conrete'' to land on.

Nelson acknowledged the concern, but said the FAA would maintain safe space between planes.

''The bottom line is we will not allow too many aircraft to arrive in New York metro airspace unless they can be accomodated without holding in airspace,'' Nelson said.

He said that as part of the plan, holding patterns are higher and farther away from airports.

Controllers generally do not oppose the plan, Burzon said.

''It could work very well,'' she said. ''We're just saying do it in a more responsible way.''

She added, however, that controllers always think ''safety first'' and will simply slow down plane traffic if a problem arises - leading to the delays the plan is designed to reduce.

''The first time you're faced with something you've never seen before, you stop the traffic,'' she said. ''If some people have to sit and wait at the airport, well, I have more important things to worry about, like not putting two planes together.''

Seven congressmen and a New York state Assemblyman whose district lies between La Guardia Airport and Kennedy Airport asked that the plan be delayed until staffing and training questions were resolved.

The FAA's goal was 122 controllers at TRACON to run the new system, 90 of whom should be fully trained, FAA Administrator Donald D. Engen told the congressmen. He said there will be 115 on duty Thursday, 90 fully trained.

Burzon said that as of last Friday, an understaffed TRACON had 102 controllers, including trainees, and 81 fully trained.

The FAA said supervisors would supplement regular crews.

''Nobody's trying to cram something down the controllers' throats,'' Nelson said. He said former controllers devised the plan and current controllers were constantly consulted.