The Civil Aeronautics Board yesterday issued final rules requiring airlines to pay up to $400 - twice the current maximum - to passengers who are "bumped" involuntarily from overbooked flights.

However, in an attempt to cut down on the number of passengers now bumped off flights against their will - 150,000 a year - the agency mandated the airlines to seek volunteers who will agree to be bumped in return for a payment of the airline's choosing.

If there are too few volunteers, however, the airlines may involuntarily bump passengers according to a priority seating plan approved by the board and available at all ticket counters.

Passenters bumped against their will, beginning Sept. 3, will receive more compensation. Passengers bumped against their will must be given the full value of their ticket up to the destination or first stopover, from a minimum of $37.50 to a maximum of $200, even if they are booked on another flight within minutes.

If the airline cannot provide alternate transport that brings the bumped passenger to his or her destination within two hours of the originally scheduled arrival time, the airline must double the compensation to the passenger, for a minimum of $75 and a maximum of $400.

The current rule provides compensation ranging from $25 to $200 but is not payable to bumped passengers who are re-routed to their destination within two hours. The current rule also provides that the airline only has to provide the passenger with the face value of the first ticket coupon. Because being bumped from one leg of a continuous trip often causes a passenger to miss connecting flights, the board said passengers must now be compensated for their entire one-way ticekt.

The CAB declined to outlaw deliberate overbooking of flights by the airlines which contend they need the freedom to give "confirmed" reservations to move passengers than can be accommodated on any given flight to take account of the people who make reservations, don't cancel them, and then don't show up.

The CAB yesterday acknowledged that overbooking offers tangible benefits to passengers by filling seats that would otherwise be left empty by no-shows and late cancellations, and keeping fears lower than they might be without it. If involuntary bumpings are not sharpely reduced under the new rules, however, the CAB warned that it will reeaxmine its decision.