Airlines operating at Washington's National Airport have been given federal approval to discuss the possibility of shifting all flights on some routes to underutilized Dulles International Airport or to Baltimore/Washington International Airport.

The airlines were given discussion authority and antitrust immunity for the discussions by the Civil Aeronautics Board in response to a petition from four airlines operating at National: Midway, Piedmont, Republic and USAir. A similar petition filed earlier by the Dulles Policy Task Force was turned down by the CAB on grounds that it wants to use its authority to grant immunity for discussions only to airlines and airline groups. The task force is a group composed of Virginia state and local bodies, chambers of commerce and citizens groups.

The unanimous board said it believed the airline discussions wouldn't be adverse to the public interest and could lead to an agreement that would alleviate some of the scheduling problems at National Airport, an outcome that would benefit the traveling public.

Any agreement reached by the airlines must be submitted to the CAB for its approval. When it is, the board will consider whether any plan formulated by the carriers would be anticompetitive, the agency's members said. Although the task force originally asked that the airlines be allowed to discuss the transfer of all--or partial--service in some markets out of National, a CAB source said yesterday that the board expects the airlines to consider the total transfer of all nonstop flights on some routes from National to the other airports, not just some flights. The shifting of some flights by agreement would be considered an illegal carving up of the market, he said.

Also, it probably wouldn't work. One of the reasons Dulles has failed to attract some service is that airlines feel they are at a competitive disadvantage if another airline's flights to the same destination remain at National.

Although no one was making any predictions yesterday, some expressed optimism. According to airline and government sources, one of the markets the airlines might agree to shift out of National would be Florida destinations. It's believed by many that Florida-bound travelers are generally taking longer vacation trips, not short business trips. Because of that, they may be more willing to spend extra time getting to the airport for their trips than would the business travelers who are taking hour flights from National for one-day trips.

CAB members took no position on the way in which the airlines should proceed, other than to require that the discussions take place at a meeting open to the public. Their discussion authority runs out in 90 days.

In another development, representatives of almost 50 airlines agreed on a mechanism to be used to trade slots--takeoffs and landings--at the 22 airports whose operations have been reduced by the Federal Aviation Administration because of the undermanned air traffic control system. The idea is to allow the airlines to improve their scheduling to get better aircraft utilization.

The representatives decided to ask the Civil Aeronautics Board to sanction a procedure similar to one that has been used since 1968 to allocate slots at four restricted airports, including National. Under the airlines' proposal, airline scheduling committees, made up of representatives of the airlines operating at the airports, would meet openly to work out offers by airlines to trade a slot at one hour for a slot in another. If more than one airline wanted a particular hourly slot and a voluntary solution wasn't reached, it would be settled by lot.

Under the tentative arrangement, no airline would leave the meeting with any more or fewer slots than it entered with.