Delta Air Lines' net increased 11% to $59.1 million for the third quarter, compared to the same quarter last year. Operating revenue totaled $1.632 billion, up 48% from the 1986 quarter. The Atlanta-based company said the results reflect the acquisition of Western Air Lines. Piedmont Aviation Inc.'s income rose 35% to $33.9 million for the third quarter, compared to the year-ago period. Revenue for the quarter grew to $583 million, up 22.5% from last year. The Winston-Salem, N.C.-based firm credited the results to industrywide fare increases and the introduction of first-class service.