The debate between the Transportation Department and Congress over the air traffic control system flares perennially, as lawmakers and the agency dicker over how many controllers should be hired and when.

As the polemics neared their usual spring crescendo, the department announced abruptly Wednesday that it plans to hire 955 new controllers, supervisors and air traffic management specialists next year. The Senate Commerce Committee already had called for the hiring of 1,000 new controllers.

Meanwhile, the debate over this year's numbers rages on.

Congress passed a law last year that requires the Federal Aviation Administration to have 15,000 controllers working by Oct. 1. More important, the law requires that 70 percent of them be fully trained by that date.

That spawned a new debate: What is an air traffic controller?

The FAA said 15,132 controllers were working as of April 30. But the count includes 1,467 "air traffic assistants" who have clerical jobs and do not control planes. "They're like candy stripers at a hospital. They go and sharpen pencils," a controller said. "They do not have any control functions whatsoever."

The FAA count also includes students in training, even those attending their first day of class at the training academy. They are listed on the FAA account ledger as being in the "developmental pipeline." As of April 30, there were 2,474 in the "developmental pipeline," including 825 academy students.

That leaves 11,191 controllers controlling most of the country's air traffic. Of those, 9,563 are fully trained to work every radar position. (The others have been certified on at least two radar positions but are not fully trained.)

At times, the department's number-keeping takes on an "Alice in Wonderland" quality, which critics say camouflages the seriousness of staff shortages at some facilities.

For instance, the agency counts its entire air-control staff to tally its total work force but subtracts clerical workers from the total before calculating the percentage needed to be fully trained. In other words, 70 percent of 15,000 is 10,500. But subtract the air traffic assistants from 15,000, and 70 percent of the resulting 13,533 is only 9,471.

Thus, at 9,563 controllers, "we're well in advance of the requirement," Deputy Transportation Secretary James H. Burnley said.

Rep. Norman Y. Mineta (D-Calif.), chairman of the House Public Works aviation subcommittee disagrees. "What kind of double bookkeeping system has the secretary of transportation got?" he asked. "You can't say you're going to count these people as part of the total and then, to get at the 70 percent, deduct them. You can't have it both ways."

Burnley defends his math, saying the law's language defines controllers quite specifically.

Even a straightforward head count is complicated, however. When the General Accounting Office tried to tally fully trained controllers at major facilities, it came up with three sets of numbers -- two from the FAA, one of its own. Controllers in Boston and Los Angeles complain retirees no longer at work are carried on staffing rolls. In Chicago, the FAA once divided some airspace in half, and tried to count each controller as two if the controller was qualified to work in both halves. That has since been revised.

"This argument about numbers is one of my biggest frustrations," Sen. Frank R. Lautenberg (D-N.J.), chairman of the Senate Appropriations transportation subcommittee, complained. "A number is a number is a number. And we can't seem to agree on that."