The Civil Aeronautics Board gave interim approval last night to Eastern Airlines to operate most of Braniff International's South American routes for up to 15 months.

The board also said it would consider Eastern's proposed agreement with Braniff, made earlier in the day, to operate the routes for six years.

Eastern's proposal successfully headed off a plan under which rival Pan American World Airways would have obtained the Braniff routes for four years for $30 million.

The CAB had denied interim approval of that deal on April 16 and had urged Braniff to seek a deal that would avoid having a single U.S. carrier serve virtually all of the South American routes, as would have been the case had Pan Am got the Braniff routes.

The CAB met last night to discuss an appeal by Braniff and Pan Am for emergency reconsideration of their plan and the Eastern offer.

"At this point, Braniff's situation is critical," Braniff told the CAB, saying either the Pan Am or Eastern plan would meet its needs. "The board must act at once to approve one of these two agreements, at least on an interim basis."

In unanimously granting interim approval to the Braniff-Eastern agreement on an emergency basis, the board said the arrangement would help Braniff, preserve its route network and retain two U.S. airlines in South America.

Under the Eastern six-year proposal, cash-short Braniff would receive the same amount of money, $30 million, it would get under the Pan Am plan--$18 million this year and $12 million in installments over three years starting in 1983.

Braniff would continue to maintain its services to South America until June 1. Throughout May, however, Eastern would provide funds to help support Braniff's operations. Last week, the Dallas-based airine told the CAB that its South American traffic has fallen off and that it could be in a "negative cash position" as early as today.

The accepted proposal calls for Eastern to operate Braniff's routes in Argentina, Bolivia, Chile, Colombia, Equador, Panama, Paraguay and Peru. Not included are Braniff's routes to Venezuela, which it wants to keep, and Brazil, which was excluded because of restrictions in the bilateral air agreement between the United States and Brazil.

Under the agreement, Eastern would pick up, as Pan Am was prepared to do, about 800 Braniff ground employes who are based in Latin America.

Although Braniff has lost money on the routes for a couple of years, Eastern said it expects the routes to improve its profitability. Eastern said its major gateway south would be Miami, bolstered by nonstop flights from New York and New Orleans.

Ten days ago, the CAB said it couldn't make a decision on the Braniff-Pan Am proposal until the end of July, after consideration of the complex competitive issues. At the same time, however, the CAB had urged Braniff to consider other arrangements with other airlines--arrangements that wouldn't present the same competitive problems.