The five delay-cause categories

Since June 2003, carriers that report on-time data to the Bureau of Transportation Statistics have also reported the cause of each arrival delay of 15 minutes or more, splitting the delay minutes across five categories. The categories, in BTS's own terms:

Air carrier: the cause was within the airline's control, such as maintenance or crew problems, aircraft cleaning, baggage loading or fuelling.
Extreme weather: significant meteorological conditions, actual or forecast, that in the carrier's judgment delayed or prevented the flight, such as a tornado, blizzard or hurricane.
National Aviation System (NAS): a broad set of conditions in the aviation system, including non-extreme weather, airport operations, heavy traffic volume and air traffic control.
Late-arriving aircraft: a previous flight with the same aircraft arrived late, causing this flight to depart late.
Security: evacuation of a terminal or concourse, re-boarding because of a security breach, inoperative screening equipment, or screening lines longer than 29 minutes.

Who assigns them

The carriers do, under BTS rules. A delay can be split across categories, for example 20 minutes carrier and 15 minutes late aircraft. Cause minutes exist only for flights that arrived 15 or more minutes late; a flight that was 10 minutes late has no cause record.

Why weather looks small

Only extreme weather goes in the weather category. Ordinary bad weather that slows arrivals, closes runways or triggers ground stops is counted under the National Aviation System, because the delay comes from how the system responds. Late-arriving aircraft is also a knock-on category: the original reason for the earlier flight's delay is recorded on that earlier flight, not carried forward.

Cancellations

Cancelled flights carry a single reason code instead: A for carrier, B for weather, C for the National Aviation System and D for security. Airport and airline pages show the split.

All reporting carriers, the 12 months to July 2026

BTS cause category	Share of delay minutes	Minutes
Air carrier	33.3%	37,248,839
Late-arriving aircraft	39.7%	44,441,449
National Aviation System	20.7%	23,133,857
Extreme weather	6.1%	6,837,243
Security	0.1%	161,776

111,823,164 delay minutes on 1,542,495 flights that arrived 15 or more minutes late and carried a cause report. Carriers assign the categories; see what each category means.