The Civil Aeronautics Board gave tentative approval yesterday to the proposed merger of Western Airlines and Wien Air Alaska. The merger is to be considered by Western's shareholders at the company's annual meeting today in Los Angeles.

But the board tentatively decided that Western's chairman, Neil G. Bergt, could have no dealings with his wholly owned Alaska Air International for 18 months. Citing a possible potential competition problem within Alaska, the board held out the possibility that Bergt would have to divest himself of AIA at the end of the period.

The board also decided that Wien will have to relinquish its federal subsidy if the merger goes through.

A spokesman for Western said yesterday that the airline is "pondering the decision."

A CAB administrative law judge had recommended to the board that it approve the Western-Wien merger, and also permit the merged carrier and AIA to come under the common control of Bergt, on grounds that the proposed transaction would not lessen competition substantially.

Bergt applied to the CAB in January to purchase all of Wien's stock from Household International for $50 million through his own company, Eagle International Corp. Following the acquisition, Wien would be merged with Western, assuming the approval of Western's shareholders. If completed, the transaction would give Bergt, through Eagle, 48.9 percent of the common stock of the combined Western-Wien.

In another action, the CAB decided to recommend to the Federal Aviation Administration that it reallocate some landing rights of the private aviation sector to commercial airlines until their operations nationwide are back up to 95 percent of what they were before last year's air traffic controllers strike.

CAB members also said they strongly oppose an FAA proposal to give preference to airlines whose operations are down at certain airports. Because many airlines are restructuring their route systems and others have added flights at some airports to compensate for losses at others, CAB members said the FAA plan would cause "distortions," administrative difficulties, and a potential for "mischief."