Federal Aviation Administration chief Lynn Helms, attempting to close out a years-long debate, is circulating details of a multibillion-dollar proposal to replace aging computer equipment and further automate air traffic control. The 20-year program is designed to foster safety and fuel savings while eventually cutting technical personnel by about a third.

Helms' plan, scheduled to be released formally later this month, calls for the "highest practical level" of automation. Many messages--weather reports, for instance--that controllers now give to pilots by voice would be sent electronically in written form to cockpit computer screens.

The FAA's approximately 200 airport radar rooms and "en route" centers would be merged into about 50 facilities. About 1,200 of 3,000 ground radar, radio and beacon installations would be eliminated as newer equipment is installed.

Preliminary copies of Helms' plan, which his staff began working on shortly after he took office last January, are now circulating in federal offices. An FAA spokesman declined to comment pending the document's release. A copy was examined by The Washington Post.

Though minor changes could be made before release, Helms said recently that he had made the major decisions of the program.

Entitled "National Airspace System Plan," the volume contains few surprises but lays out in detail for the first time the FAA's plans for handling expected growth in aviation in the next two decades. FAA projections show flights growing from 134 million in 1980 to 212 million in 1990 and 290 million in the year 2000.

The study does not lay out the costs of implementing Helms' plan. But in the past FAA officials have talked in terms of $10 billion. Spending on that level would rank the program among the federal government's largest-ever nonmilitary procurements.

While Congress has not yet appropriated the money, many aviation specialists feel lawmakers would be amenable to big spending despite Reagan administration budget-cutting, due to concern over the air controllers' strike and the 1960s computer technology that now holds the system together.

Spurred by directives from Capitol Hill, the study was already well under way when the strike began on Aug. 3. The firing of 11,500 of 17,000 controllers, however, gave new urgency to a search for ways to reduce staffing needs. Decertification of the controllers' union, if upheld on appeal, also would give the FAA a freer hand in implementing changes.

The plan does not significantly alter FAA thinking about where to center air control. "The air traffic control system will continue to be fundamentally ground-based," the study says. Pilots and some aviation consultants had favored cockpit radar screens that would let crews see oncoming planes and assume some of the ground controllers' functions.

The plan centers on installing advanced, standardized equipment and taking many functions out of human hands. Though staffing at airport and en route centers would fall significantly, new equipment would let fewer people control more airplanes with greater safety, the study promises.

Currently, the system relies on 1960s-vintage IBM 9020 computers, which have attracted much concern on Capitol Hill. Under the plan, they would be replaced by around 1985 with new computers that have greater capacity and speed.

Individual consoles that controllers work at would get computing abilities of their own, to reduce disruption caused by the failure of central computers. Backup systems and alarms that activate when planes are on headings that could take them too close to each other would be upgraded.

Currently controllers spend much time handling paper strips with flight data on them. The strips system would be replaced by computer displays of the information, decreasing controller staffing requirements. The computers also would be able to expand an individual controller's sector on command, allowing a smaller work force during off-peak hours, the study says.

By 1990, planes flying higher than 12,500 feet would have to carry special equipment allowing ground radar to take readings on them individually. That would yield more accurate position data and let controllers put planes closer to each other, the study says.

The same equipment would allow written messages on weather, flight plans and other matters to be beamed to cockpit computer screens. At present, this communication is by garble-prone voice radio.

After 1990, the FAA would phase in a computer service to analyze individual flight plans and select routes that were the most fuel efficient and had the fewest conflicts with other planes. Currently, planes waste fuel by flying diversions ordered by controllers who at the last minute see the potential for conflicts ahead.

Overall, the plan says, enhanced safety and fewer delays would result from the innovations, due to the more accurate equipment and less "down time." The study claims that "system errors," inadvertent violations of separation rules, could be reduced by 80 percent between 1983 and 1985.

The document claims that the innovations would mean a $6.7 billion operating savings between 1981 and 1990 over spending that would be necessary if the present system is retained, and $17.8 billion between 1991 and 2000.

Other facets of the plan:

* The 20 en route control centers in the continental United States would be consolidated into 16 by 1990. The more than 180 radar facilities at individual airports, which guide planes on final approach, would be merged by the year 2000 into about 30 hub facilities, each serving more than one airport.

* Instrument landing systems serving U.S. airports would be replaced with the more advanced and flexible microwave landing systems.

More than 300 flight service stations, which accept flight plans and provide weather information, would be absorbed into about 60 automated facilities. Pilots would be able to call the stations' computers directly and get readouts on cockpit screens of weather and other flight data.

* Collision warning devices scheduled to come into use in the mid-1980s would continue to operate independently of the ground.