For United (NASDAQ:UAL), May wasn't exactly a month for the record books. The company released its metrics for the month on Monday, and they were generally down on a year-over-year basis.

Revenue passenger miles slipped by 0.8% from May 2012's figure to 17.9 billion. Available seat miles also saw a decline, dropping to 21.2 billion, or 1.7% lower than the year-ago period's 21.6 billion. The figure for total onboard passengers also fell a bit, declining 1.4% on a year-over-year basis to just over 12 million.

Not every measurement for the month saw a drop. United's load factor (i.e., the average "occupancy" of its planes) was 86.5% in May for domestic flights, which was higher than the 86.2% recorded in the same month of 2012. That figure for international flights increased from 81.5% in May 2012 to 82.7% in May 2013.

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