The numbers just don't add up.
The number of air traffic controllers: 9,500, a little more than half as a year ago. The number of flights they must track: soaring upward and doubling by the year 2000.
That adds up to a potential crisis for air travelers and for the Federal Aviation Administration (FAA), whose job it is to manage the nation's airways.
The continuing air controller shortage, brought on by last year's controller strike, has brought a special urgency to the FAA's master plan to revolutionize the way airplanes are directed as they travel the highways of the sky.
The FAA wants a new generation of air-traffic cops: advanced computers and other sophisticated equipment that it says will make for safe skies while cutting the need for a big increase in the number of air controllers.
The FAA claims that 12,500 controllers, plus 1,300 flight data specialists, could operate the new system. If the current air traffic system were simply updated, the FAA says it would need roughly 28,000 controllers.
The multibillion-dollar FAA plan has won widespread support among pilots, airlines, and airport operators and on Capitol Hill.
But the specifics of the proposal -- who should pay, how quickly can the change be safely made, and what should be done about the thousands of air controllers who'll lose their jobs -- are being hotly debated even among these backers.
Congress still must authorize the plan and approve financing. At least six different Capitol Hill committees are involved in various aspects of that venture.
Pressuring action on a new system is an FAA estimate that air traffic in the US will more than double by the year 2000. To best gear up for that increase, the FAA wants to buy and install a computer system by 1985 and hook it to the agency's present software system of instructions and programs. Eventually the FAA would also bring in an entirely new software system.
The overall FAA proposal includes:
* Introduction over the next 20 years of the sophisticated new computer system capable of handling much more traffic more reliably than today's 15-to-20 -year-old computers. Some components of the present system have broken down during peak periods of use, forcing controllers to turn to a manual backup system.
* Installation of an advanced radar system (called Mode S) that would require new equipment both on the ground and in most aircraft. These would yield much more precise information about a plane's position, identity, and the surrounding traffic situation. With the help of the advanced computer network, instructions on everything from the weather to suggested flight paths could be automatically transmitted to pilots.
* Gradual replacement of current instrument-landing systems with a more flexible microwave system. The microwave system makes bad weather landings easier and safer by allowing approaches from several directions rather than just one.
* Voluntary installation of traffic alert and collision avoidance systems in airplane cockpits. These would serve as a backup to any failure in the ground system to warn pilots of an impending air crash. The Air Line Pilots Association (ALPA) has long pushed for such a system. It wants the collision avoidance systems to be mandatory rather than voluntary, as the FAA now proposes.
The Office of Technology Assessment (OTA), set up by Congress to analyze such proposals in a nonpartisan manner, questions the rush to switch to the new computer system. Larry Jenney, director of OTA's aviation system review, says that in the past the FAA has overestimated traffic growth and that it again ''may be running to catch a train that isn't going to leave for another hour.''
He suggests another option for the FAA -- updating the current computer system for use in the next several years and later buying the new system and software together -- could prove cheaper and more efficient.
Mr. Jenney says the FAA proposed that option itself last January when agency administrator J. Lynn Helms first unveiled the plan. Jenney argues that the gain in flexibility -- freeing designers from having to keep ''one foot in the old system'' -- could be considerable. The FAA counters that IBM will only supply spare parts for existing FAA computers for another two years. Jenney suggests that two years is ample time to order enough parts to shore up the system for several more years.
''We'd (OTA) really like to see an honest comparison of the two options,'' he says.
An important feature of the new FAA plan allows each controller to work at a station of three display screens, rather than one, keeping watch on a broader section of the skies. Indeed, the FAA insists the job could be done under the new system with only 9,500 to 12,000 controllers instead of the 28,000 or more that would be needed by the year 2000 if the current system were kept. (There were about 17,500 air controllers before last year's strike. There now are about 9,500.)
Several backers of the change, including the ALPA and Ralph Nader's Aviation Consumer Action Project (ACAP), also seek a much more careful assessment of the impact of the shift from men to machines. The Rand Corporation, hired by the FAA to analyze a portion of the new plan, warns that care must be taken to leave enough decision-making with controllers to keep them alert and fully in control.
''The more passive their role, the more likely controllers are to become inattentive . . . (or) complacent,'' cautions the Rand study.
Both Rand and ALPA spokesmen say it is important for people -- whether controllers or pilots -- to know what factors go into computer decisions so they are better able to correct a situation in case of a failure in the system.
''It's not just a question of designing new machines but of the relationship between the machines and the human being,'' says Aviation Consumer Action Project director Matthew Finucane. ''The FAA has always had a hard time dealing with the human element, and we're very concerned for safety reasons that people remain machine managers and that the design be such that the human being will know what to do if something goes wrong.''
It is for that same reason that pilots, while agreeing that the FAA must remain the traffic cop in the airways, want to be sure that with all the changes they are given information as well as instructions. The pilot must be part of the decisionmaking process, insists ALPA spokesman John Mazor.
No one knows what the final price tag on the modernization plan will be. The FAA's only official estimate so far is $7.2 billion for the first five years. Whatever the final cost, FAA officials insist that the program will more than pay for itself by the $25 billion it is expected to save in fewer controllers, traffic control facilities, and the fuel savings from computer-suggested shorter flight paths.
The Reagan administration proposes that 85 percent of the costs be paid by airway users. Congress has been cool to user fees, and the question of which users should pay the highest share is also controversial. The administration asks that the current 5-cent passenger ticket surcharge be hiked to 8 cents and that the tax on fuel used by general aviation or private aircraft be raised from 4 to 12 cents a gallon.