PAN AM'S <PN> MAIN UNIT TO REPORT SMALLER LOSS Pan Am Corp said its largest division, the Atlantic, will report a first quarter loss that will be substantially lower than that reported a year ago. In the 1986 1st qtr, Pan Am reported a net loss of 118.4 mln dlrs, which included foreign exchange losses of 19.3 mln. The company did not say how much the Atlantic division lost. Pan Am also said it is encouraged by passenger traffic results so far this year as well as booking trends. In January and February, the airline said total systemwide revenue passenger miles were up five pct and 11 pct, respectively, over the same months last year. It also said that based on current trends, its largest division should report vastly improved second-quarter traffic compared to last year, which resulted in a load factor of 45.7 pct for the second quarter of 1986. Pan Am also said its principal subsidiary, Pan American World Airways, had a 1986 net loss of 469.3 mln dlrs, compared with a profit of 38.7 mln dlrs in 1985, which included a gain of 341 mln dlrs for the sale of its Pacific Division. It said the airline's operating loss for the year was 325.5 mln dlrs compared with a operating profit of 196.5 mln dlrs in 1985.