Peter Banfield resigned as president of Martin Aviation Inc., a fixed-base operator at John Wayne Airport, to sail the French Mediterranean coast. Robert H. Volk, owner and chairman of Martin Aviation, said he would assume the title of president. Marvin Toumala, previously vice president of maintenance, has been promoted to executive vice president and will manage the firm's day-to-day operations. Volk said that Banfield's resignation, tendered last August, came as a surprise to him. He said that Banfield, 51, explained that he and his wife had decided to fulfill a longtime dream. Banfield could not be reached for comment.