The Civil Aeronautics Board has approved a revised Trans World Airlines proposal to put discount and full-fare coach customers in different sections of the airplane.

American Airlines already had received the CAB's okay for a similar plan; both plans will start Sunday.

They are designed to appeal to the full-fare - usually business - traveler by adding services the reduced-fare passengers won't receive such as special advance seat selection and ticketing procedures and separate check-in arrangements.

TWA's original plan was turned down by the CAB because it made no provisions for the passengers who booked discount tickets - and expected full service - before TWA announced plans for the segregated sections.

The CAB approved TWA's revised plan on short notice when the airline agreed to make advance seat selection and other coach amenities available - on request - to discount fare passengers who bought their tickets before Sept. 18 for flights after Oct. 15. TWA began advertising its three-class service fare plan Sept. 18. The CAB has urged discount passengers who feel they fall under the revised plan to request regular coach amenities when lowed TWA's and American's three-contacting TWA.

To date, no other carriers have fol-class plan. In fact, United Airlines has announced plans to improve service for all its coach passengers "whether they pay full fare or a discount," the carrier said. At the same time, United said it would try to lure more travelers to the first-class cabin by reducing fares and improving service there.

United has asked the CAB for permission to cut its first-class fares to a level 20 percent above coach fares for travel beginning Nov. 17: first-class fares are currently 30 percent higher than coach fares. Delta Airlines recently asked the CAB to reduce the mark-up on first class over coach to 25 percent on short trips and 20 percent on long trips (800 miles or longer).

In another action, the CAB approved fare increases of about 5 percent proposed by American Airlines for economy and some discount fares between the U.S. and Caribbean points. The agency rejected an increase in first-class fares on those routes on grounds that the first class fares are used as the basis for excess-baggage charges, and an increase would raise those charges excessively.