Crowded Skies. More passengers and more planes are clogging the airways. The air traffic control system is `hanging on by its fingernails,' says a pilots union executive. Safety is a concern and the push is on for solutions

FLYING is cheaper. If you leave the driving to your friendly bus company these days, it will cost you $40 more to make the trip by tire from Chicago to Boston than by air on a bargain fare.

For speed and economy, Americans are flocking to the nation's airports and boarding planes at a record rate of more than 1 million people a day. The number of passengers is expected to increase by another 75 percent within the decade.

The increasingly busy skyways are good news for airlines, but they are straining the nation's 50-year-old air traffic control system. It is still rebuilding after the walkout and firing of most of its controller work force in 1981. Indeed, Federal Aviation Association (FAA) officials say air traffic is now 8-to-10 percent higher than before the strike. The system, says Air Line Pilot Association president Henry Duffy, is ``hanging on by its fingernails -- just staying slightly ahead of the traffic that's out there.''

Along the highways of the sky there are no ``do not enter'' signs. Under the traditional open-skies policy, any plane willing to wait in line gets a turn. It may be a luxury the system can no longer afford.

The Department of Transportation says 11 airports currently have a major congestion problem. The agency predicts the number will grow to 41 by 1990. ``There will come a time -- and it happens occasionally now -- when the system is saturated,'' admits FAA Deputy Administrator Richard H. Jones.

Those mulling over solutions to the traffic problem talk about everything from the need for more runways and technological aids to rationing of scarce physical space -- like reserving express lanes for carpoolers or charging higher landing fees during rush hour.

George Eads, dean of the University of Maryland's School of Public Affairs and a longtime advocate of fewer economic controls over airlines, predicts that passengers one day will pay more to fly to busy airports such as Chicago's O'Hare during peak travel periods. ``There have got to be some innovative ways of making more efficient use of the capacity you've got,'' he says.

But some are holding out hope that the system itself is still expandable.

The FAA, for instance, expects to make major capacity gains through its air traffic control modernization plan. The 20-year $11.7 billion program, launched a few years ago, aims, largely through improved technology, to handle more airway traffic with fewer controllers.

Much of the hardware has already been ordered. Early next summer, for instance, Seattle will get the first of a new generation of faster and more-reliable computers to be installed in all 20 FAA regional air traffic control centers by the end of 1987. By the mid-1990s increased automation may allow these computers to do much of the actual traffic control. Controllers would then be freer to become what FAA spokesman John Leyden calls ``systems engineers.''

Also on order are some 1,270 microwave landing systems, new surveillance radar technology, and improved electronic links between ground controllers and aircraft. The hope is that more of the weather, routing, and clearance information now radioed by controllers could be sent directly to the cockpit by computer.

FAA Administrator Donald Engen speaks often of the need to exempt the modernization plan and other safety chores of the agency from the expected across-the-board budget cuts of the Gramm-Rudman act.

But few aviation observers think the FAA or its plan can avoid the ax. They point to the continued refusal of the Reagan administration and Congress to release the estimated $3.3 billion surplus in the Airport and Airway Trust Fund. That money, which comes from user fees on tickets and fuel, is intended for improvements in the air traffic control system, including the modernization plan. But it is part of the general federal budget and anything in it not spent makes the deficit look that much smaller.

Congressional sources say the FAA modernization plan, already an estimated $1 billion behind schedule, could easily slip $2 billion and two years behind by fall of 1986.

But even if such technology and the hiring of many more skilled controllers were vastly to expand the capacity of the airways, it would do nothing to solve the problem of limited available concrete for landings and takeoffs.

There have been no major airports built in more than a decade. The planned $2 billion airport at Denver to replace Stapleton International will probably take another 10 years to build. Citizens tend to balk at the prospect of giving up more land close to major cities for new airports and coping with added noise.

The only growth at airports these days is the occasional new terminal (usually adding some gate space), new taxiways, and occasional new runways. ``It's amazing -- sometimes just building a couple of taxiways can be a big help,'' says Norbert Owens, the FAA's deputy associate administrator for air traffic control. ``I don't think we're at the limit as far as things that can be done.''

Leo Duggan, the technical vice-president of Airport Operators Council International, agrees but warns: ``There is no panacea. . . . We're just fighting to stay abreast of traffic growth right now -- we're not getting ahead.''

The FAA will never get ahead of traffic, in the view of some observers, as long as its budget remains so vulnerable to cuts. Daniel Henkin, vice-president of the Air Transport Association (ATA), suggests that the FAA's air traffic control function would fare better if siphoned off into a separate federal corporation. Funded by user fees and separate from the federal budget, it would allow more businesslike management and flexibility, he says. The ATA hopes Congress will consider its plan this spring when current trust-fund legislation is up for reconsideration.

Congressional sources say the advent of Gramm-Rudman cuts makes the ATA proposal look increasingly attractive. ``It may be an idea whose time is coming,'' says one Capitol Hill staff member.

At the very least Congress is expected to try once again to remove the aviation trust fund from the unified federal budget. The effort lost by a narrow margin last year. The proposal's proponent, Rep. Norman Y. Mineta (D) of California, chairman of the House Aviation Subcommittee, predicts that without some change the current deficit law threatens to tear apart the air safety system carefully built up over the last 50 years.