The Air Transport Assn. reported that domestic carriers flew 30.9 billion revenue passenger miles in August, up 11.9%over the same month a year ago. The previous monthly traffic record was 30 billion revenue passenger miles in July. Total available seat miles in August were up 9.2%. The passenger load factor was 66.7%compared to 65.1%in August, 1984. In domestic service, August traffic gained 11.2%over August, 1984. Available seat miles rose 6.3%. Passenger load factor was 65.4%, compared to 62.5%last year. In international service, traffic climbed 14.3%.