aIncludes weather events that prevent flying. Other weather delays that slow operations are included under other categories. bDelay resulting from a previous flight with the same aircraft arriving late.
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. Percents do not add to 100 due to rounding.
Sources:U.S. Department of Transportation, Bureau of Transportation Statistics, Airline On-Time Performance, available at https://www.bts.gov/.
Last Updated: April 2018