Five airlines are "systematically overcharging" passengers who have been re-routed because of a two-month-old pilots strike that has grounded Northwest Airlines, according to a suit filed by the Civil Aeronautics Board yesterday.

The unprecedented action in U.S. District Court here charged United Air Lines, Continental Air Lines, Ozark Air Lines, Frontier Air Lines and North Central Airlines with forcing passengers who have Northwest tickets to pay as much as $100 in additional fees, an action the CAB says is in violation of tariffs filed by those airlines with the board.

According to the filing, "each defendant has filed a tariff with the Board which, in substance, provides that passengers holding confirmed reservations on another airline will be carried at no additional charge when scheduling irregularities cause them to be re-routed onto defendants' lines."

The agency claimed the airlines are not following this practice and is seeking a preliminary injunction to prevent a continuation of "unlawful" activities. At the same time, the CAB is seeking refund payments to consumers who have been subjected to the alleged overcharges.

The five airlines in question also have refused to file amendments to their CAB tariffs which could change their obligation. "We have the idea why they won't do that," said CAB attorney Robert Seldon.

The situation has been aggravated, according to CAB sources, by a Northwest policy of continuing to sell tickets for flights more than 14 days after the sale. But since the airline then cancels all flights 14 days before flight time, the customers generally are told that Northwest has re-routed them on other airlines.

When the passengers arrive at the new airline gate, however, they are told that they will have to pay as much as $100 more to be allowed on the flight, because the trip usually requires re-routing through additional cities served by airlines other than Northwest.

In one case cited in the complaint, two passengers had to pay $250 more each to board a North Central flight from the Fargo, N.D., airport.

Many of the overcharges have been reported at Fargo, Minneapolis, and Madison. Wis., areas normally dependent upon Northwest service.

The CAB filing states that the airlines' decision to routinely overcharge re-routed passengers "is not accidental or sporadic, but is the product of executive decisions of management made in the face of instructions by the Board that such practices violate the (Civil Aeronautics) Act."

Sources at the board said that Western Airlines avoided being named as a defendent by applying to the CAB for an amendment to its tariff that state that it would not accept such passengers unless they are willing to pay the added charges.

CAB staffers said the government could not file charges against Northwest because it was acting within the law, even though the airline is forwarding passengers to airlines that it knows will then hit the consumers with extra charges.

The CAB action comes in the wake of dozens of consumer complaints. Despite CAB telegrams to each of the airlines involved reminding them of their obligations, return telegrams from the airlines indicated that they planned to continue the overcharges.

Since April 29, Northwest's pilots have been on a strike that has brought the airline's passenger service to a virtual halt.

But under the CAB-approved mutual aid pact, the other major airlines make payments to Northwest to help defray the costs. According to CAB sources because Northwest has low overhead costs, it is likely that the airline is actually making money during the strike period. This has been the case during earlier strike periods, the sources said.

The mutual aid agreement currently is under review by the CAB.