DALLAS (AP) – Southwest Airlines Co. said a key measure of passenger revenue rose as much as 8 percent last month as it benefited from fare increases even as its planes carried fewer people.

Airlines measure how much money they collect for each seat flown one mile. Southwest estimated that figure rose an estimated 7 percent to 8 percent compared to April 2011. Southwest combined data from both years with AirTran, which it bought on May 2, 2011.

Airlines have been raising fares, resulting in higher passenger revenue. The higher fares don’t guarantee profits, since airline fuel bills have jumped, too.

Traffic on the combined airline fell 2.1 percent compared to April 2011. Southwest cut flying capacity by 1.9 percent. Because traffic and capacity fell by similar amounts, Southwest’s planes were about as full as they were a year ago — 79.8 percent, down by 0.1 percentage point from a year ago.

For the first four months of the year, Southwest traffic has fallen 0.7 percent, while flying capacity has risen 0.4 percent. Occupancy has fallen 0.8 percentage point.

Shares of Dallas-based Southwest rose 23 cents, or 2.8 percent, to $8.33 in midday trading. They are still near the low end of their 52-week range of $7.15 to $12.44 per share.

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