The Civil Aeronautics Board approved yesterday proposals by most of the nation's airlines to expand "Super Saver" discount fares to most domestic routes.

The fare plan generally offers discounts of 40 percent from regular roundtrip economy fares for travel Monday through Thursday, and 30 percent Friday through Sunday. The discounts apply on roundtrip travel between seven and 45 days in length, not counting the day of departure. Reservations must be confirmed at least 30 days before departure.

Depending on the airline, the fares become effective from March 18 to April 1 and are good through May 1979. The airlines are allowed to fill a maximum of 35 percent of the seats of any given flight with passengers paying the "Super Saver" fare.

American Airlines first introduced the fare last year and was followed quickly by United and Trans World Airlines. Until now, however, the fare had been limited to major transcontinental routes.

In seeking to extend the fares, the carriers argued that they had been enormously sucessful in attracting passengers who might otherwise not fly. American officials said, for instance, that 47 percent of the 435,000 Super Saver passengers it carried last year (between April and December) said they would not have flown at all if the discount fares had not beenavailable.

In addition to allowing the three original carriers to expand the fares, the CAB granted the applications of 12 other carriers to offer them.

Meanwhile the board is also taking steps that will make it easier for charter airlines to compete with the scheduled carriers.

Although the board has been liberalizing the regulations under which charters operate in recent years this week it instructed the staff to draw up new rules which would eliminate up new rules most of the remaining restrictions.

The board proposed eliminating all advance-purchase requirements, dropping the minimum sizes for groups, thus allowing one person to book a charter; and ending the requirement that charter passengers book roundtrip travel.

The board would retain the restriction that charter tickets be sold through tour operators, not directly to the public.