Airline Price-Fixing Suit Now Class Action : Antitrust: A federal judge’s decision means that up to 12.5 million passengers can join in the litigation against the major air carriers.

A federal judge, charging nine of the nation’s major airlines with price collusion, expanded a multimillion-dollar antitrust suit Wednesday into a class action covering up to 12.5 million passengers.

U.S. District Judge Marvin Shoob certified that passengers who flew between Jan. 1, 1988, and the present on American, Continental, Delta, Midway, Northwest, Pan American, TWA, United and USAir are entitled to sue them as a group in an antitrust lawsuit pending in his court here.

Because of the millions of people covered, lawyers said the case could become the most massive civil case in the history of the U.S. court system.

The suit charges that the airlines used their affiliation with Airline Tariff Publishing Corp., a Washington-based computerized fare-listing service, to communicate with one another on proposed fares before those fares went into effect.

The suit resulted from nearly 50 individual suits filed across the nation against the airlines, mostly by business passengers, according to persons familiar with the case.

A judges’ panel consolidated them in Atlanta earlier this year.

Pitts Carr, an Atlanta lawyer for the plaintiffs, said possible damages could total “hundreds of millions of dollars” if the class-action suit succeeds.

Northwest and TWA, he said, had agreed to out-of-court settlements for undisclosed sums earlier this year, pending the outcome of Shoob’s decision on making the suit a class action.

Since Continental, Pan Am and Midway have filed for bankruptcy protection from creditors, he said claims against them would have to be presented to federal bankruptcy courts.

Actual trial of the class action against remaining airlines--which have denied the claims--would not be expected until mid- to late 1992, Carr said.