Understanding flight delays

Every delay of 15 minutes or more gets attributed to one or more of these categories:

• Air Carrier Delay — Something within the airline's own control — maintenance, crew, baggage loading, fueling, aircraft cleaning.
• Late-Arriving Aircraft — The same plane arrived late on its previous flight, so your flight starts behind. This is the knock-on effect that ripples through the day.
• National Aviation System (NAS) — The shared system: airport operations, heavy traffic volume, air-traffic control — and non-extreme weather.
• Extreme Weather — Severe conditions, actual or forecast, that prevent flying — tornadoes, blizzards, hurricanes.
• Security Delay — Terminal evacuations, re-boarding after a breach, broken screening equipment, or screening lines over 29 minutes.

Weather does not live only in the “Extreme Weather” bucket. Non-severe weather is folded into the NAS category, and weather also hides inside Late-Arriving Aircraft (when an earlier flight was itself weather-delayed). Because of this, BTS publishes a separate combined figure for weather’s true share of total delay — and it is consistently larger than the Extreme Weather category alone suggests.

A delay only enters cause reporting once it reaches 15 minutes — the same threshold as the on-time definition. When several things go wrong at once, the delay minutes are split across the contributing causes rather than pinned on a single one.

Cause data is reported using the airline’s own judgment, and the Late-Arriving Aircraft bucket in particular hides the original reason. Because per-flight cause attribution is opaque, we explain the categories here but deliberately do not paint a precise cause-by-cause breakdown for individual routes. We would rather show you the honest on-time and delay figures than over-claim aboutwhy a specific flight was late.