A U.S. District Court judge ordered Northwest Airlines yesterday to pay $52.5 million in back pay and interest to a group of stewardesses who sued the airline 12 years ago for discriminating against them. It is the largest amount ever awarded in an employment discrimination suit, according to lawyers in the case.

Chief Judge Aubrey E. Robinson Jr., who has presided over the case since it began in 1970, formally approved the compensation figures agreed upon by lawyers for the airline and for the 3,364 stewardesses in the long-delayed case.

Robinson ruled in 1973 that the airlines had discrimated against the stewardesses in several ways, including paying more money to men who were doing similar jobs, forcing women to adhere to rigid weight requirements but not having requirements for men, and requiring women to share hotel rooms but allowing men to have single rooms.

The case was delayed for five years by appeals by both sides of aspects of Robinson's rulings. It then took the lawyers another four years to work out compensation schedules for the stewardesses under a formula laid down by the judge. Some of the stewardesses stand to get as much as $50,000 as a result of Robinson's final order yesterday.

The company will appeal Robinson's order, according to an attorney for the airline.