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Understanding the Reporting of Causes of Flight Delays and Cancellations

How do we know the reason for a flight being late or cancelled?
Since June 2003, the airlines that report on-time data also report the causes of delays and cancellations to the Bureau of Transportation Statistics. Reported causes of delay are available from June 2003 to the most recent month.

Which airlines report on-time data?

Do the airlines report the exact cause of the delay?
The airlines report the causes of delay in broad categories that were created by the Air Carrier On-Time Reporting Advisory Committee.  The categories are Air Carrier, National Aviation System, Weather, Late-Arriving Aircraft and Security.  The causes of cancellation are the same, except there is no late-arriving aircraft category.

How are these categories defined?


What have the airline reports on the causes of delay shown about flight delays?

Is it true that weather causes only 4 percent of flight delays?
That category consists of extreme weather that prevents flying. There is another category of weather within the NAS category. This type of weather slows the operations of the system but does not prevent flying. Delays or cancellations coded "NAS" are the type of weather delays that could be reduced with corrective action by the airports or the Federal Aviation Administration. During 2017, 63.3 percent of NAS delays were due to weather. NAS delays were 25.1 percent of total delays in 2017.

How many flights were really delayed by weather?
A true picture of total weather-related delays requires several steps. First, the extreme weather delays must be combined with the NAS weather category. Second, a calculation must be made to determine the weather-related delays included in the "late-arriving aircraft" category. Airlines do not report the causes of the late-arriving aircraft but an allocation can be made using the proportion of weather related-delays and total flights in the other categories. Adding the weather-related delays to the extreme weather and NAS weather categories would result in weather's share of all flight delays.

Why aren’t all weather-related delays reported as a single number?

The Air Carrier On-Time Reporting Advisory Committee created this reporting system. The committee believed that separating the extreme weather delays from the weather delays that could be fixed through improvements to the system, would provide a truer picture of the extent of weather delays.  The separate reports would provide the system’s managers with better information for making improvements.

Is more information available on the Air Carrier On-Time Reporting Advisory Committee?



Is more information available on the causes of delays and cancellations?
Cause of delay numbers can be found on the BTS website at  This page allows searches by time period and by airline or airport.  It also has calculations for weather’s share of flight delays and a breakdown of NAS delays by category.  More data can be found on the TranStats database at Also, the Department’s monthly Air Travel Consumer Report includes a summary of causes of delay numbers reported by each carrier for the most recent month.  The report can be found at