aIncludes weather events that prevent flying. Other weather delays that slow operations are included under other categories.
Key: NAS = Delays attributable to the national aviation system (NAS) that refer to a broad set of conditions, such as non-extreme weather, airport operations, heavy traffic volume, and air traffic control.
Note: Percents do not add to 100 due to rounding.
Source: U.S. Department of Transportation, Research and Innovative Technology Administration, Bureau of Transportation Statistics, Office of Airline Information, available at www.bts.gov as of October 2013.