The five delay reasons airlines report, and which they control

US airlines report every flight that arrives 15 minutes or more late under five causes: air carrier, extreme weather, national aviation system, late aircraft and security. Late aircraft is the largest, at 40% of delay minutes in the 12 months to August 2026. Airlines’ own problems are next at 33%.

Only one of those, air carrier, is what the DOT calls within the airline’s control, and that word is about to change meaning. A final rule published September 3, 2026 moves ten kinds of event out of the air carrier category. It was due October 19, 2026, and the DOT has since pushed it to February 1, 2027. Some articles still give the old date.

The five, and what they’re made of

Here’s each cause’s share of reported delay minutes, from public flight records for about 7 million US domestic flights, September 2025 to August 2026. The categories are the DOT’s. The arithmetic is mine.

About 23% of flights that weren’t cancelled or diverted arrived 15 or more minutes late. Those flights account for 1.9 million hours of delay over the year.

Cancellations look different, and late aircraft isn’t a category there. Weather causes 61% of them, airlines 22%, the aviation system 17%. Weather that closes a runway cancels flights. Airline problems more often delay them.

The list under “air carrier” is longer than you’d guess

BTS’s technical directive gives airlines a long list of what goes here, and the Federal Register quotes it in full. Beside maintenance and late crew, it includes “slow boarding or seating”, “stowing carry-on baggage”, “last minute passenger”, “gate congestion” and “awaiting gate space”. So the passenger who took six minutes to find a bin can count as the airline’s fault, in the reports at least.

Security has its own surprise. BTS includes long lines of more than 29 minutes at screening.

Weather is bigger than 6%

The extreme weather row undersells weather, and BTS says so. Bad-but-flyable weather sits in the aviation system category: BTS says 45.8% of those delays were weather in 2020, the year its page uses. Some of the late aircraft row is weather too, passed down the line by an earlier flight, and airlines don’t report what made the inbound late.

You can’t rebuild a true total from the five numbers. If someone tells you weather causes 6% of delays, the honest reply is that it causes at least that.

Who decides

The airline does. Each one assigns the cause for its own flights, and BTS’s definition of extreme weather turns on “the judgment of the carrier”.

It matters because the DOT’s public dashboard is built on the word “controllable”, and it lists what each airline promises when a delay is its own doing. The dashboard calls a controllable delay “essentially” one caused by the airline, and gives maintenance, crew problems, cabin cleaning, baggage loading and fueling as examples. The reporting code and the dashboard are separate things, but they use the same idea, and that makes a borderline case worth arguing about. What US airlines owe you has the commitments.

What changes on February 1, 2027

Section 511(b) of the FAA Reauthorization Act of 2024 says ten kinds of event “shall not be included” in the air carrier code. The DOT’s rule puts them in a new category of their own, called Section 511(b). They are:

• Aircraft cleaning after a passenger’s death
• Aircraft damage from extreme weather, foreign object debris or sabotage
• A bag or cargo delay from an outage in a system the airline doesn’t control
• Cybersecurity attacks, if the airline met the applicable rules
• An unexpected shutdown or failure of a government system
• Overheated brakes after an emergency procedure
• Unscheduled maintenance that shows up outside the scheduled program and can’t wait
• A medical emergency through no fault of the airline
• Removal of an unruly passenger
• An airport closure for volcanic ash, wind or wind shear

Unscheduled maintenance is the one to watch, since maintenance is the example everyone gives of an airline’s fault. Airlines asked for the delay to February 2027, so that reporting under the new categories starts with January 1 flights. The DOT agreed.

From early 2027, expect the air carrier share to fall. Flights won’t have got better. Some delays will have moved to a new label. If you compare years, compare with care. A second change is on the way too: Section 511(a) requires a new category for delays from air traffic control instructions, and the DOT says it will handle that in future rulemaking.

The reports have gaps

The percentages are shares of delay minutes reported by the airlines for US domestic flights by large carriers and their regional partners. A late flight can list several causes, and only flights 15 or more minutes late get causes at all. The rule’s practical effect depends on how airlines code things, and I haven’t seen any 2027 data.

The most useful of the five, for a passenger, is the top one: late aircraft can be checked from your phone. For how delays pass from flight to flight, see does the first flight of the day leave on time. Trackers and airlines also disagree on the times themselves: why the tracker and the airline disagree.

When Skycap can tell why your flight is late, it says so in one line, such as low clouds at the airport you’re landing at, or the plane still on its way in. When it can’t tell, it doesn’t guess. How delays work in the app