Without proper security, there is nothing to stop a terrorist from taking over a control center at an airport, preventing aviation and law enforcement authorities from tracking a plane, said Ruth Marlin, executive vice president of the National Air Traffic Controllers Association.

"We are that last line of defense," Marlin said Tuesday. "If someone needed to make sure the military didn't find out where the plane was going, all you have to do is physically interfere with the controller's ability to track them. That's why physical security is a very real concern for us now."

Federal Aviation Administration spokesman Fraser Jones said the agency is continuing to review security needs at the airport towers.

"We are in the process of placing additional guards at facilities and are undertaking other security improvements," Jones said.

Following the Sept. 11 terrorist attacks, the FAA posted armed guards at all 325 air traffic control facilities, Marlin said. Since then, guards have been removed from 180 facilities.

Nor are there barriers preventing a would-be terrorist from crashing a truck full of explosives into the control tower at many airports, Marlin said. At Fort Lauderdale-Hollywood International Airport in Florida, for example, someone can drive right up to the tower without being stopped, she said.

NATCA is seeking armed guards at all facilities, and wants new tamper-resistant identification cards for controllers.

"It is vitally important that the FAA make extra efforts to guard air traffic control facilities because our national security is at stake," union president John Carr said. "Air traffic controllers function as front-line troops in the domestic war against terrorism."

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On the Net:

National Air Traffic Controllers Association: http://www.natca.org

Federal Aviation Administration: http://www.faa.gov