Why US Flights Run Late: How the Five Delay Causes Work

The departure board shows your evening flight as delayed, yet the sky outside is clear and the gate is quiet. The agent at the desk says the plane is still on its way in. That short explanation matches how the United States counts delays. The Bureau of Transportation Statistics (BTS) is the federal agency that publishes airline performance data. It does not file each late flight under a single reason. Airlines split the late minutes among five causes. One cause exists only to record delay handed down from an earlier flight on the same aircraft. Weather is spread across three of the five.

What counts as late

Every published on-time or delay rate rests on a few federal definitions. They come from the federal on-time reporting rule, quoted here from the current eCFR text (2026):

• On time: a flight that “arrives less than 15 minutes after its published arrival time.”
• Late: a flight that “arrives at the gate 15 minutes or more after its published arrival time.”
• Cancelled: “a flight operation that was not operated” but was listed in the carrier’s reservation system within seven calendar days of the scheduled departure.
• Diverted: a flight “operated from the scheduled origin point to a point other than the scheduled destination point.”

The lateness test is applied at the arrival gate, not at departure. A flight can push back late and recover time in the air. It still counts as on time if it arrives less than 15 minutes behind schedule. An on-time rate is the share of flights that pass this arrival test.

The five delay causes, side by side

When a flight arrives late, the carrier reports which causes produced the delay. BTS uses five categories, set out in its guide to reporting the causes of flight delays and cancellations:

Definitions: BTS cause-reporting guide, archived 2013 version. The last column summarizes those definitions.

The same categories apply to cancellations, with one exception. Late-arriving aircraft is not used for a flight that never operated. The five causes also mix two kinds of labels. Air Carrier, NAS, Extreme Weather and Security describe where a problem arose. Late-arriving aircraft records only that a problem came from somewhere else, carried by the plane itself.

Why one late plane delays the evening flight

BTS defines the category in one sentence: “A previous flight with same aircraft arrived late, causing the present flight to depart late.” It does not describe a new problem. It records an old one moving forward through the schedule.

For example, if a flight is scheduled to arrive at 2:00 p.m. and reaches the gate at 2:40 p.m., it is 40 minutes late. It fails the on-time test. Suppose the carrier traces 25 of those minutes to the aircraft arriving late from its previous trip. It traces the other 15 minutes to air traffic restrictions at the destination. The record would show 25 minutes of late-arriving aircraft delay and 15 minutes of NAS delay, for 40 minutes in all. This split is assumed to show how the categories fit together.

Now follow the same plane to its evening flight. If it cannot recover the time on the ground, that departure starts late as well. Its delay is coded as late-arriving aircraft again. The original trouble may have been a morning storm or an airline problem several flights earlier.

BTS notes that airlines “do not report the causes of the late-arriving aircraft.” The category shows that a delay was inherited, but not where it started. Because of that gap, the five-way table alone cannot settle who deserves most of the blame for a day of late flights: weather, the airline or the air traffic system.

How weather is spread across three categories

Weather appears in the BTS system in three places:

1. Extreme Weather holds only significant conditions, actual or forecast, that the carrier judges will delay or prevent a flight.
2. NAS includes non-extreme weather, meaning conditions that slow operations without stopping flights. BTS reported that 75.5 percent of NAS delays in 2011 were weather-related. That share shows how much weather can sit inside a category not labeled weather.
3. Late-arriving aircraft carries weather too, because some earlier flights ran late for weather reasons. Airlines do not report those causes, so BTS estimates them. It uses the proportion of weather-related delays in the other categories. That proportion sets weather’s allocated share of late-arriving aircraft delay.

BTS’s measure of weather’s total share adds the three parts. They are Extreme Weather minutes, the weather portion of NAS minutes, and the allocated portion of late-arriving aircraft minutes. A chart that shows only the Extreme Weather column captures one of the three. It understates how often weather sits behind a late arrival.

Delays and money: US refunds versus UK compensation

In the US, the passenger’s remedy turns on how much a trip changed, not on the 15-minute line. Part 260 defines a “significantly delayed or changed flight” by its new schedule. The scheduled arrival moves three or more hours later on a domestic itinerary, or six or more hours on an international one. A departure moved that many hours earlier also counts. Some triggers have no time threshold: a different origin or destination airport, more connection points, or a downgrade in class of service. Crossing one of these lines is what makes a US passenger eligible for a refund. A 40-minute delay like the earlier example falls far short of them.

The UK text of Regulation 261/2004, retained from EU law, ties help and fixed payments to lateness itself. Article 6(1) requires assistance once a delay reaches two hours on flights of 1,500 km or less, three hours on flights between 1,500 and 3,500 km, and four hours on longer flights. A passenger who reaches the final destination three hours or more late is owed compensation under Article 7: £220, £350 or £520, depending on distance.

The airline does not owe that payment if it proves “extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken.” Flights governed by the EU’s own version of the regulation follow that text instead.

What the five-cause split leaves out

The cause definitions quoted above come from the archived 2013 version of the BTS guide. Current BTS pages may use different labels for the same categories. The names in a recent table may not match this one word for word.

The 75.5 percent NAS weather figure comes from the same archived page and describes 2011. It is not a current share.

Weather’s share inside late-arriving aircraft is a BTS estimate, not an airline report. It assumes inherited delays had weather causes in the same proportion as the delays airlines did explain. A “total weather share” built on it is therefore modeled, not reported.

Both federal rules quoted here can be amended. The eCFR links above show their current text.