Why flights are delayed: the five DOT causes

When a US airline’s flight arrives 15 minutes or more late, the airline assigns the lost minutes among five causes and reports them to the Department of Transportation. Knowing what each category covers, and what it leaves out, changes how the totals read, most of all for the largest one, which is not a cause in the usual sense.

The five categories

Under 14 CFR 234.4, the operating airline codes a delay only when the flight’s arrival delay is 15 minutes or more, and it reports each cause that accounts for at least five of those minutes. The categories, in the rule’s terms:

• Air Carrier. Circumstances within the airline’s control. The rule’s own examples, given for the matching cancellation code, are a lack of flight crew and maintenance.
• Extreme weather. Significant meteorological conditions, actual or forecast, at departure, along the route or at arrival, that prevent the flight or later flights from operating under the regulatory standards or in the airline’s judgment.
• National Aviation System (NAS). A broad set of conditions inside the air traffic system: non-extreme weather, airport operations, heavy traffic volume and air traffic control.
• Security. Malfunctioning screening equipment, a security breach that forces an evacuation, or the need to re-screen passengers.
• Late-arriving aircraft. The result of a late incoming aircraft from the previous flight.

The weather rule needs a second look. Only extreme weather has a category of its own. Ordinary rain, wind and low cloud that slow traffic fall under the NAS, so the weather line in these totals understates what weather does. For that reason FlyerIntel measures weather separately, from the observation at the departure airport in the hour a flight was due to leave, in the “What weather costs” section of an airport page. The research on how much of each season’s lateness is the weather uses the same observation-based measure.

What “late-arriving aircraft” means

An aircraft flies several legs a day, so a delay can travel with it. Suppose, as an illustration, that an aircraft lands 40 minutes late and its next flight is scheduled to leave 50 minutes after it lands. That leaves little time to unload, clean and board again, and the next flight leaves late even though nothing new went wrong. The airline codes that second delay as late-arriving aircraft.

The category records how a delay reached a flight, not why the first flight was late. The storm or the fault that started it is upstream, and is coded on the earlier flight if that flight also arrived 15 minutes late. This is why the label tells a traveler less than it seems to. A late aircraft is the mechanism by which one problem becomes many. FlyerIntel’s research on what a late aircraft costs the next flight follows each aircraft’s previous flight in the DOT file instead of relying on the code.

Where the minutes went

Across 3,847,216 arrivals that were 15 minutes or more late on departures from the covered airports, July 2023 – July 2026, the airlines attributed 268,547,004 delay minutes to the five causes.

These are shares of minutes, not of flights, so a few very long delays weigh heavily. Late-arriving aircraft is the largest share, ahead of air carrier. By airport, late-arriving aircraft is the largest category at 52 of the 64 covered airports and the airline’s own category at 12 of the 64.

The mix also changes through the day. On departures scheduled for 5–9 AM, late-arriving aircraft was 20.2% of the attributed minutes; on departures scheduled for 6–9 PM it was 48.5%. That is the pattern the mechanism predicts: an aircraft has had more chances to fall behind by the evening than it had at the first departure. The guide to choosing a flight turns this into a practical rule.

What the categories cannot tell you

• The codes are the airline’s own filing, not an independent finding. Two carriers in the same weather at the same airport can code the same event differently, so the totals describe what was reported, not who was at fault. The methodology page says the same of the cancellation codes.
• Only flights that arrived 15 minutes or more late carry cause minutes. A flight 14 minutes late contributes nothing, so the totals describe delayed flights, not all flights.
• Cause minutes belong to a flight’s arrival delay but are filed under the airport it left from. On an airport’s “Why flights run late” section, weather that was met on the way or at the far end can appear in the airport’s own figures.
• The categories are not fixed. A DOT final rule published September 3, 2026 (91 FR 56588) adds a sixth reporting category for ten kinds of event, such as aircraft cleaning after a passenger’s death or a cybersecurity attack, and bars airlines from coding those as Air Carrier. DOT has postponed its effective date to February 1, 2027 (notice of October 15, 2026). The totals above cover earlier months and use the five categories.

Looking up your own case

Every airport page has a “Why flights run late” section with its own split of the minutes, for example the one on the Atlanta page. The late-aircraft ranking shows which airports inherit the most delay from earlier flights. For the cancellation side, which uses four codes instead of five, see FlyerIntel’s report on why airlines cancel. The reports on the delay you inherit and on what causes delay at big and small airports go further into the figures above.

What this is based on

The delay totals are the minutes the airlines attributed on departures from the 64 airports FlyerIntel covers, July 2023 – July 2026, summed from the airport records behind the airport pages. The cause definitions are quoted from the regulation, not from FlyerIntel’s own classification.

• US Department of Transportation, 14 CFR Part 234, Airline Service Quality Performance Reports Sections 234.2 (definitions) and 234.4 (what carriers report, and the cause codes).
• Bureau of Transportation Statistics, Reporting Carrier On-Time Performance, field list The fields of the file FlyerIntel reads, including the 15-minute arrival indicator and the five delay-cause minutes.
• Federal Register, 91 FR 56588, Cause of Airline Delay and Cancellation Categories Under Section 511(b) of the FAA Reauthorization Act of 2024 Published September 3, 2026.

An error in a figure or a definition can be reported on the corrections page. How FlyerIntel decides what to publish is set out in the editorial policy.

More guides

• How on-time performance is counted. The 15-minute rule, scheduled against actual times, and what happens to cancelled and diverted flights.
• How to read an airport’s reliability profile. What each section of an airport page means, and how to compare airports without fooling yourself.
• Choosing a flight to avoid delays. Hour, day, airport, route and connection: what the record says to check, in order.