On July 8th, Southwest Airlines (NYSE: LUV) updated shareholders on flight activity in the month of June -- and the news was good.

Southwest airplanes carried passengers a total of 10.8 billion "revenue passenger miles" in June 2015, up 6.9% year-over-year. Meanwhile, available seat miles (ASM) increased only 6.7% year-over-year to 12.5 billion.

A revenue passenger mile (RPM) equals the number of revenue-paying passengers Southwest carried times the distance they traveled. An available seat mile (ASM) is the number of plane seats the company had times the distance the planes flew, but basically, more "available seats" means more planes in the fleet.

With passenger rolls growing somewhat faster than the number of seats available to carry them, this resulted in a slight increase to the company's "load factor," which expanded by 10 basis points to 86.2% in June -- a number the company says was a new record for the month.

What does this mean to investors?
In a nutshell, Southwest ran its fleet more efficiently this past June than in any other month of June ... ever. It kept growth in the number of airplanes it is buying in line with the demand for air travel from its customers, and as a result, maximized the rate at which it was able to keep seats filled, squeezing more money out of every flight.

Indeed, the best news of all may be that, according to Southwest management, load factors for the second quarter of 2015 and for the entire first half of this year were both at record levels for the company: 84.6% and 82.5%, respectively. That bodes very well for the possibility of an "earnings beat" when Southwest reports its second quarter on July 23rd.

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Rich Smith has no position in any stocks mentioned. 

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