Source: U.S. Bureau of Transportation Statistics  

Release: Transportation Services Index and Seasonally-Adjusted Transportation Data  

Units:  Thousands, Not Seasonally Adjusted

Frequency:  Monthly

Data from all U.S. air carrier domestic and international flights, scheduled and unscheduled service (all service).

This data is collected by the U.S. Department of Transportation, Bureau of Transportation Statistics (BTS), Office of Airline Information, T-1: U.S. Air Carrier Traffic And Capacity Summary by Service Class (REV_PAX_MILES_140) available here.