Allegheny Airlines must pay consumer activist Ralph Nader $15,010 because it "bumped" him from his reserved seat on Flight 864 from Washington to Hartford, Conn., nearly six years ago, a federal judge here ruled yesterday.

U.S. District Court Judge Charles R. Richey's order is the fourth major legal ruling to come out of the lengthy litigation concerning Nader's being bumped from the flight. It reduces by $10,000 an earlier award Richey had made to Nader arising out of the same bumping incident, but still reflects a finding by Richey that Allegheny's policy of overbooking its flights in 1972 was "wanton . . . and callous."

Richey's first ruling in the case came in 1973, qhen he ordered Allegheny to pay $25,010 to Nader and another $25,051 to the Connecticut Citizen Action Group, the organization to which Nader was scheduled to speak when he was bumped from his flight.

Of the damage amounts awarded in that ruling, $500,000was in punitive damages and the other $61 was in actual costs to Nader and the group. Yesterday's ruling includes $15,000 in punitive damages to Nader and $10 in actual costs to him, but deletes any award to the group.

The U.S. Court of Appeals had reversed Richey in 1975, saying airlines were allowed to sell more seats than are actually available on a flight and deny seats to passengers with confirmed reservations.

Then, in 1976, the U. S. Supreme Court ruled in Nader's favor in the same case, and reinstated his suit that accused Allegeny of fraud in concealing its booking practices from customers.

When the case came back to Rihey, he was under orders to determine whether Allegheny had a "good faith" defense to the charges by Nader.

The airline company then basically argued that it believed the Civil Aeropnautics Board had indirectly approved Allegheny's overbooking policy in a 1967 CAB ruling.

Richey rejected that argument, saying allegheny's failure to tell Nader and other passengers of their overbooking policy "was the result of a conscious and deliberate policy" by the firm.

Richey added that Allegheny should pay Nader $15,000 in punitive damages because "the mere fact that a fraudulent practice is commonplace in a particular industry in no way deprives that practice of its outrageous character."