The Civil Aeronautics Board began yesterday an intensive nationwide effort to determine what level of sir service the federal government should guarantee to small and isolated communities during the next 10 years.

Under the recently enacted Airline Deregulation Act, the federal government is committed to subsidize "essential air service" when the federal payment is necessary to ensure scheduled services to eligible cities.

The points initially eligible are all communities in the states and territories that were listed on some airline's certificate when the act become law, whether or not the authorized service was being provided.

The board, in addition, has descretion to add two more groups of communities to the subsidy program: after one year, cities that were deleted from airlines, certificates during the past 10 years can be added, and two years later, any point in Alaska or Hawaii can be added.

At a press conference yesterday, CAB Chairman Marvin S. Cohen said the board would hold nine hearings across the country in January and February to hear the views of the regional, state and local authorities as well as private citizens on what kind of subsidy program the board should draw up. The board will asking citizens to tell them specifically what level of air service and quality the federal government should be expected to guarantee, mow the board should go about deciding what additional cities may be subsidized, and how the board would decide how much should be paid and how it should process subsidy applications.

"Even if law did not require it, we want to consult with the communities in this undertaking," Cohen said. "We in Washington cannot know or feel what a community in Arkansas or South Dakota needs until we talk about them, and we cannot meaningfully accomplish that by sitting here in Washington."

At a minimum, the board must find under the law that essential air service is at least two flights a day, five days a week to eligible communities, or the level of service the community was receiving in 1977.

The board also announced it has established three regional offices - in Boston, Kansas City, Mo., and San Mateo, Calif. - to assist individual communities in originating and maintaining adequate commercial air service.

Although the subsidy program eventually will result in a decrease in the subsidies that have been paid in the past to keep air service in smaller communities - last year the federal government paid about $70 million - CAB officials note that the new law requires the board to keep paying essentially the same amounts to the same airlines for the next four years unless the airline in question decide to pull out of some of the subsidized services.

After the current program is phased out, however, it is believed the subsidy payment may go down drastically. Instead of a subsidy program that "fills in deficits of companies," as CAB member Elizabeth Bailey put it yesterday, the subsidy program will be designed to aid specific communities' needs, allowing the CAB to subsidize services by aircraft that are suited to smaller communities.