The Civil Aeronautics Board yesterday cut to sit the number of cities under consideration for new low-fare air connections with Chicago's little-used Midway Airport.

Although the board has consolidated the counter-proposals of 10 other airlines into the one proceeding, service in and out of Midway at fares up to half the comparable fares from busy O'Hare was proposed by two corporations outside the regulated airline industry.

One application, filed by a new Midway Airlines, headed by Irving T. Tague, former president of Hughes Air West; proposed low-fare service between Midway and Minneapolis/St. Paul, Detroit, St. Louis, Cleveland, Kansas City and Pittsburgh - the six cities that CAB yesterday said it would include in its investigation.

The other application filed by a newly formed subsidiary of the successful Texas in carrier, Southwest Airlines, proposed almost-half-price service between Midway and 15 other cities.

In its order defining the scope of its proceeding, the board yesterday that limiting its probe to six cities - the six largest markets proposed by the applicants - was necessary to make the case "manageable. . . .

"Because of this case may have a profound impact upon the air transportation system, the board has the responsibility of assuring the issues are explored in sufficient detail for it to make a reasoned-final decision," the four members said. "Nevertheless, there are limits as to how much can be accomplished in one proceeding."

The CAB said carriers participating in the proceeding should focus upon the effect the low-fare service would have on the rest of the regulated air system and the board's fare policies.

The next step is the appointment of an administrative law judge who will schedule evidentiary hearings and preside over them. Although the ALJ has full discretion over the course of the proceeding, the board "earnestly" requested the cooperation of the parties to expedite the matter. Delays in many of the agency's past proceedings have been legendary.