What the Five DOT Delay Causes Mean

When a flight runs 15 minutes late or more, the airline files why — and the reason isn’t free text. The Department of Transportation sorts every delay minute into one of five categories (BTS delay-cause reporting). Knowing what each one means — and which are within an airline’s control — changes how you read an airport’s or a carrier’s delay profile.

Air carrier delay

Causes inside the airline’s own operation: maintenance, crew availability, fuelling, baggage loading, aircraft cleaning, waiting on a connecting passenger. This is the bucket the airline most directly owns. A carrier with a persistently high air-carrier share on a route is telling you something about how tightly it runs its ground operation.

Late-arriving aircraft — usually the biggest single cause

Your plane was late getting in from its previous flight, so it left late on yours. This is typically the largest category, and it’s the most important one to understand: it means delay begets delay. A single morning delay ripples through every subsequent leg that aircraft flies, which is why late-arriving-aircraft delay compounds as the day goes on and why the first flight of the day is so often the most reliable one.

National Aviation System (NAS)

The broadest category: air-traffic control decisions, airport congestion, heavy traffic volume, and non-extreme weather — the routine haze, wind, and low cloud that slow an airport without shutting it. NAS delay is largely about where you’re flying, not who you’re flying. Congested hubs in the Northeast carry high NAS shares no matter the airline.

Extreme weather

Only significant weather that prevents operating — blizzards, hurricanes, major thunderstorm systems. Note the split: ordinary weather that merely slows things is filed under NAS, and only severe, operation-stopping weather lands here. That’s why the “weather” share can look smaller than intuition expects; a lot of weather’s real effect is hiding inside NAS.

Security

The rarest bucket: evacuation of a terminal or concourse, re-screening after a breach, or inoperative screening equipment. On most airport profiles it’s a rounding error, which is itself worth knowing — if you see a meaningful security share, it usually points at a specific incident in that month.

How to read the split

The categories separate what an airline controls from what the airport and the system impose. Air-carrier and late-arriving-aircraft delay follow the airline; NAS and extreme weather follow the place and the season. So a high-delay airport driven by NAS is a congestion story you can’t book around by switching carriers, while one driven by air-carrier delay might be.

The Airport Delay Profile shows this split month by month for any U.S. airport, in both directions — so you can see whether a field runs late because of the weather it sits under, the traffic it carries, or the airlines that fly it.