The Civil Aeronautics Board yesterday adopted new air charter travel regulations which eliminate most of the complex restrictions travelers had to endure in the past in order to purchase low-fare air tickets and travel packages.

Under the new rules, the board of charters with a single "public charter" free of most of the traditional charter restrictions.

The new charter from requires no advance purchase and no minimum stay: it drops the minimum group size: and eliminates restrictions on discount pricing except for those designied to prevent discrimination.

While the new rules permit one-way charters, they do not permit open-ended round-trip charters. A traveler wishing flexibility on a return trip is supposed to purchase one-way tickets each way.

Also under the new rules, a charter operator cannot cancel a charter less than 10 days before departure.

In the works since the winter, the rules permit "public charters" to be organized immediately but will allow the other types of charters to run through the end of the year.

By making it almost as easy to purchase a ticket on a charter flight as a regularly scheduled flight - because they by law must be different, the board retained the restriction that charter tickets be sold through four operators, not directly to the public - the board hopes to make it easier for charter airlines to compete with the scheduled carriers.

As a practical matter, the charter carriers used to be the only source for low-fare air transportation for most Americans. As the scheduled carriers began to offer discounts,the board worried that the charter carriers might not be able to retain enough passengers to stay in business without a loosening of their operating restrictions.

At its meeting yesterday, the CAB set in motion a proceeding that is designed to protect charter participants from tour operator abuses. The proposed rule-making, which is expected to be formally approved next week, would require tour operators to disclose to potential travelers more information about the trip, such a which airports the charter will utilize from and what hotels participants will stay in. The rules would require refunds when there are changes if the consumer desired.

Also at yesterday's meeting, the board began grappling with its consideration of several mergers in the airline industry it has been asked to approve.

Although the board made no decisions during a two-hour discusson of how to approach the proposed Southern Airlines-North Central Airlines merger and Texas International Airline's attempt to take over National Airlines, there were indications board members are inclined to adopt strong tests for the mergers using traditional antitrust doctrines.

With Chairman Alfred E. Kahn leading the discussion, the board members appeared willing to adopt language laying out the scope of the proceedings that recites many concerns board members have with approving mergers in the industry in a transition period from heavy regulation to a more competitive environment.

Kahn said he felt proponents of mergers have "a heavy burden of proof" to make a showing that benefits outweight disadvantages when the merger eliminates actual or potential competition. Until there is free entry into the system - Congress had not yet enacted a bill and the board's own policies are being challenged in the courts - Kahn indicated a reluctance to approve what might mean a permanent dimunition of numbers of airlines in the business.

Kahn acknowledged, however, that effectiveness of competition is not measured exclusively by numbers of competitors and that the board would be open to a showing that a reduced number of airlines that were more efficient and more vigorous could enhance competition.

At member Elizabeth Bailey's urging, board members agreed to eliminate passages in the proposed documents which might suggest a prejudgment against the specific mergers while laying out their difficulties with approving mergers generally during this period.

In the TXI-National case, the board's proceeding - besides deciding whether the merger is in the public interest - will decide whether TXI violated the law by acquiring a 9.2 percent interest in National without the board's prior approval.Prior CAB approval is required before anyone takes "control" of an airline, and "persumed" to be 10 percent, it could be less as well, board lawyers say.

The board tentatively decided it would make its decisions on the mergers by March 1, which would mean a seven-month period altogether, compared with the three-and-a-half year minimum of past merger cases.

In other airline developments yesterday:

Pan American World Airways said it would ask government approval to institute a new class of service, Clipper Class, to provide full-fare economy passengers with a distinctive level of service in the air and on the ground. Effective Oct. 29, Clipper Class passengers on flights between the U.S. and London (and worldwide later) would receive preferred seating, complimentary cocktails, wine and stereo headsets.

William H. Waltrip Pan Am executive vice president, said Pan Am wants to "increase the satisfaction of normal economy fare passengers, which include many business travelers and others who are unable to take advantage of excursion fare discounts, by offering a higher level of service at no additional charge."

American Airlines proposed substantial fare reductions on travel between Puerto Rico and Washington or New York. American asked CAB approval to drop fares by as much as 39 percent between New York and Puerto Rico and by as much as 28 percent between Washington and Puerto Rico. Effective Oct. 1. consumers would only have to purchase their tickets 14 days in advance.

Trans World Airlines yesterday files specific proposals to simplify its fare structure for travel beginning Oct. 1 by replacing a plethora of plans to a smaller number.