Members of the Civil Aeronautics Board spent all day yesterday hearing from airlines and others about how slots should be allocated at the nation's busiest airports. Generally, the airlines want restrictions lifted on the number of takeoffs and landings each is allotted.

The session was intended to discuss whether the board should continue to grant antitrust immunity to committees of airline representatives who meet to allocate the limited takeoffs and landings at Washington National, Chicago's O'Hare, and LaGuardia and Kennedy in New York, or whether the process is anticompetitive and should be replaced. But the limitations themselves, especially the Department of Transportation's "high density rule" limiting flights at National, came in the for most criticism.

"What really is at issue . . . is the validity of the high density rule," said Gerry Levenberg, attorney for New York Air. NYAir has been accused by some of the airlines of destroying the committee system by requesting too many slots to start its Washington-New York service, thus forcing other established airlines to give up service to let it in.

John W. Simpson, attorney for United Airlines, agreed, arguing that the slot quotas themselves have anticompetitive effects. Airlines can start up new services easily in 637 airports in the country, he said, but at the four restricted airports, "new entry requires cancellation of other services."

Many airline representatives said Federal Aviation Administration statistics show that there are more landings and takeoffs almost every day at National than can be allowed under its slot limits, especially by private planes. "What kind of system reserves runway space for planes with an average of 2.3 passengers each when it displaces 100 passengers in a 727?" Richard J. Fahy, Jr., American Airlines' attorney, asked.