Civil Aeronautics Board chairman Alfred E. Kahn said yesterday he is "not pleased " with President Carter's decision to give Braniff Airways, not Pan Am, a new route to Europe, but he won't resign over it.

He did, however, consider resigning, at least "fleetingly," he told reporters after yesterday's board meeting. "That's why I counted 24 [hours before commenting], he admitted. "I just wanted to take a little time."

He decided overnight to stay because, "I'm having much too much fun, and there's so many things that we're trying to do."

Nevertheless, Kahn, whom many believe to be the President's most distinguished regulatory appointee, didn't pull any punches about the President's decision to reverse the board on two key parts of a major transatlantic route case. "He's the President, he has the right to make those judgments. I presume, and I have the right to disagree, and I do," Kahn said.

In a decision Wednesday, the President approved a major expansion of U.S. air service to Europe, authorizing nonstop service from 11 additional American cities and letting three airlines fly across the Atlantic for the first time. The President adopted most of the board's recommended decision, but reversed it on two key matters. He gave Braniff Airways a desirable route between Dallas/Ft. Worth and London, instead of of Pan American World Airways, and he added Amsterdam and Frankfurt to National Airlines' route system.

"We based our decision, on a very thorough consideration of the record, and the result we reached because we though it would best serve the interests of travelers and shippers," Kahn said.

Referring to the President's stated reasons for the changes he wants, Kahn said, "I just emphasize that we are no less dedicated than the President to promoting competition in international aviation markets, and I am somewhat disappointed that he nowhere alludes to our thorough explanation of why we thought our decision was the one that would best promote effective competition."

Kahn said the board - like the President - also thinks it a good idea to promote the development of new transatlantic routes by strong regional carriers - cited as his reason for altering the board decision - but that the board felt there was also an interest of travelers and shippers in a strong that can be provided by Pan Am. central trunk round-the-world service "Our decision was inevitably a balance between them, and they are both elements in effective competition - in making alternatives available to travelers and shippers."

When someone suggested that the President's letter didn't make any mention of "balance," Kahn replied, "well, of course, he has an advantage over us. We have to explain what we did; he doesn't."

Kahn said he "very strongly" believes the President's decision on Braniff gives Pan Am an even stronger case for getting some domestic routes and more fill-up rights, a goal Pan Am has pursued on and off since 1945, with very little success until very recently. "Since we are having people nibbling at Pan Am in the international field, Pan Am ought to get a fair shot domestically," he said.

When asked, Kahn said he had no knowledge that politics played a role in the selection of Braniff over Pan Am on the route.