Why Is My Flight Delayed? The 5 Real Causes, Explained by a Pilot
Matt — Founder & Airline Pilot. Published October 31, 2026. 7 min read.
Figures checked against BTS on 2026-10-31.
Why is my flight delayed? On US domestic flights, the largest share of delayed flights from January 2023 to December 2025 was a late-arriving aircraft, at 36.7%, then the airline itself at 31.8%, then the National Aviation System at 27.5%. Extreme weather was 3.8% and security was 0.2% (BTS, January 2023 to December 2025). A delayed flight in these figures arrived 15 or more minutes late.
The rank tells you where to look. It is not a promise that your flight will leave late, or by how much.
Short Answers
Why is my flight delayed? Late-arriving aircraft was 36.7% of delayed US domestic flights, the largest share (BTS, January 2023 to December 2025).
Why are flights delayed? Five BTS causes, led by late-arriving aircraft at 36.7%, the airline at 31.8% and NAS at 27.5%.
Why are flights being delayed? The same five causes, so check the inbound airplane first, then FAA status at your origin and your destination.
What causes flight delays? Late-arriving aircraft, air carrier, NAS, extreme weather and security, in that order for January 2023 to December 2025.
How do I find out why a flight is delayed? Check the inbound aircraft, then FAA NAS status, then ask the gate agent if the hold is crew or maintenance.
What Causes Flight Delays? The 5 BTS Causes, Ranked
Airlines that report on-time data to the Bureau of Transportation Statistics (BTS) also report why each delayed flight was late, using five causes. The table shows each cause as a share of delayed flights.
| Cause | Share of delayed flights (BTS, Jan 2023 to Dec 2025) | What it covers |
|---|---|---|
| Late-arriving aircraft | 36.7% | The previous flight with the same aircraft arrived late. The airline does not report what made that earlier flight late. |
| Air carrier | 31.8% | Within the airline's control, such as maintenance, crew problems, aircraft cleaning, baggage loading or fueling. |
| National Aviation System (NAS) | 27.5% | Non-extreme weather, airport operations, heavy traffic volume and air traffic control. |
| Extreme weather | 3.8% | Significant weather that delays or prevents the flight, such as a tornado, blizzard or hurricane. |
| Security | 0.2% | A terminal or concourse evacuation, re-boarding after a security breach, inoperative screening equipment, or screening lines over 29 minutes. |
BTS, January 2023 to December 2025. When several causes are assigned to one delayed flight, BTS splits that flight across the causes by delay minutes. A flight that waited on a late inbound and then on a ground delay program is not dropped entirely into one row.
What is an NAS delay?
An NAS delay is a National Aviation System delay. BTS uses that name for non-extreme weather, airport operations, heavy traffic volume and air traffic control. It is not an air-traffic-control-only label. A ground delay program or a ground stop is one way it shows up at the gate: the FAA holds flights because the airport or the airspace cannot take the scheduled flow. Those two programs are different tools. See ground stop vs ground delay program.
That 27.5% is NAS as a share of delayed flights. A separate count, on DelayGuard's air traffic control delays by airport page, is the share of completed departures with any NAS minutes: across these 39 airports, 11.1% of 14,323,349 completed departures had NASDelay greater than zero from January 2023 to December 2025. Fort Lauderdale (FLL) was highest at 16.1% and San Jose (SJC) was lowest at 6.5%. The sample is those 39 airports, and NAS is not ATC-only.
What each cause means for you
Late-arriving aircraft (36.7%). The airplane for your flight is still on a previous leg, or it arrived late and the turn is running long. The label stops there. Airlines do not report what made that earlier flight late, so you cannot tell from the category alone whether the root was weather, the airline or the NAS. Find the inbound flight. Where your plane is coming from shows what the airline app and the flight status page list.
Air carrier (31.8%). Something inside the airline's own operation: a crew that timed out or missed the trip, a maintenance item, or a slow turn for cleaning, bags or fuel. FAA status will not name this. The gate agent can, and crew or maintenance is the phrase to listen for.
NAS (27.5%). The system is metering flights. Your departure airport can be in clear weather while you sit at the gate because the destination cannot accept arrivals yet.
Extreme weather (3.8%). BTS keeps this bucket for significant weather that delays or prevents the flight, such as a tornado, blizzard or hurricane. Ordinary rain, wind and low cloud are not in this line.
Security (0.2%). Uncommon. A concourse evacuation, re-boarding after a breach, screening equipment that is down, or a screening line over 29 minutes. The useful response is extra time at the airport, not a guess at a new departure time.
Why Weather Looks Small in the Data (and Isn't)
Extreme weather is 3.8% of delayed flights because BTS keeps that category narrow. The weather category is only extreme weather. Ordinary weather sits inside NAS. BTS says that during 2020, 45.8% of NAS delays were due to weather. Weather also sits inside late-arriving aircraft, because the airline does not report what made the earlier flight late. Weather's real share of delayed flights is larger than the 3.8% line. This table does not support a single combined weather figure, so none is printed here.
How to Find Out Why Your Flight Is Delayed
The airline's first message often says only "delayed." You can still narrow the cause.
- Find the inbound aircraft. In the airline app or on the flight status page, open the airplane assigned to your flight and read the leg before yours. If that flight is late, late-arriving aircraft is the likely cause. Where your plane is coming from shows where that inbound is listed.
- Check FAA NAS status for your origin and your destination. FAA NAS status lists active ground stops, ground delay programs, general delays and closures by airport, with a stated reason such as volume, a runway closure or weather. Read it as a description of the operation, not a new departure time.
- Ask the gate agent which category it is. If the inbound is on time and NAS status is quiet, the likely cause is inside the airline's own operation. Crew and maintenance are air carrier delays. Cleaning, baggage loading and fueling sit in that same bucket.
- See what is already posted, then look at one flight. Flight delays today shows FAA status at 39 US airports. To check the delay risk for a specific flight, use that flight's origin, destination and scheduled time. The result is a probability, not a guarantee the flight will leave on time or late. Will my flight be delayed? explains how to read a risk before you travel.
A Pilot's Take
The first thing I check is the inbound aircraft. If that airplane is still at the last airport, the departure waits on the turn, even when the sky over the gate is clear. A ground delay program at the destination can hold you at the gate in good weather for the same structural reason: the FAA keeps the flight on the ground so it does not arrive before the airport can take it. I read both as probabilities that move as the inbound and the program update. Neither one is a departure time I would hand a passenger as a sure thing.
The same checks, in more detail, are in how pilots predict flight delays.
Data notes
Shares in this article come from the BTS delay-cause table for all reporting carriers, national, January 2023 to December 2025 (BTS cause of delay). The flights are US domestic. A delayed flight arrived 15 or more minutes late. On that page each cause is a percentage of all scheduled flights by reporting carriers. This article does not reprint those rates. It shows each cause as a share of delayed flights, labelled BTS, January 2023 to December 2025. When several causes are assigned to one delayed flight, BTS splits the delay across causes by minutes. Definitions are from BTS cause definitions. FAA NAS status is a live board. This article is not a live FAA status. Figures checked on 2026-10-31.
Which day you fly is a separate question from which cause hit a flight that is already late. That comparison is in the best day to fly to avoid delays.