In what was clearly a trend for American carriers, Southwest Airlines (NYSE:LUV) saw gains in its key operating statistics in May. Revenue passenger miles grew by just over 4% on a year-over-year basis, to 9.3 billion during the month. Available seat miles also headed north at a 3.4% clip, to reach 11.4 billion. A slightly higher growth rate (3.6%) was recorded in the total number of revenue passengers carried, which came in at 9.9 million against May 2012's 9.6 million.

Southwest's load factor (i.e., the average "occupancy" of its planes) also improved, but not by a significant amount. This past May, it stood at 81.9%, less than a percentage point higher than the year-ago month's 81.3%.The trips flown by the company, on the other hand, declined by 2.3% over that one-year period, to land at 115,713 last month.

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