Southwest may become No. 5

Southwest Airlines Co. is poised to displace Northwest Airlines Corp. as the fifth-largest U.S. carrier by traffic.

Southwest’s edge over Northwest in miles flown by paying passengers widened to 2.1% through May, based on airline data. Delta Air Lines Inc. is buying Northwest in a deal they expect to close this year.

Among the biggest airlines, No. 3 Delta posted the largest gain in traffic on its main jet operations, 5%, as higher demand for international flying offset U.S. declines. Continental Airlines Inc.’s traffic increased 1.1% and Northwest’s advanced 1%. The biggest carriers, AMR Corp.’s American Airlines and UAL Corp.’s United Airlines, led declines in May with 1.1% and 4.1% drops, respectively. Southwest’s traffic for May rose 6.5%.