There are many reasons why flights might be delayed —sometimes its the fault of the airline and at other times its completely outside of their control. The Bureau of Transportation Statistics (BTS) has five categories for why aircraft are late. These are Air Carrier, National Aviation System, Weather, Late-Arriving Aircraft, and Security (the categories for cancellation are the same minus the late-arriving aircraft category). Here are some of the top reasons why aircraft are delayed .

1. Extreme Weather

• Extreme weather accounted for 5.2% of delay minutes

The BTS considers extreme weather to be "Significant meteorological conditions (actual or forecasted) that, in the judgment of the carrier, delays or prevents the operation of a flight such as tornado, blizzard or hurricane." This category only includes extreme weather that prevents flying; there is another category of weather in the National Aviation System category that only slows down aircraft operations (but does not prevent flying).

It is difficult to establish exactly how many flights are delayed by extreme weather, as aircraft in other categories (like arriving late) may have been caused by extreme weather at another airport. Airlines do not report the reasons why aircraft arrived late (but some will be delayed by bad weather). In 2023, only 5.2% of delay minutes were attributed to extreme weather (although weather's effect is greater).

2. National Aviation System (NAS)

• NAS delays accounted for 18.1% of delay minutes

National Aviation System delays and cancellations are "attributable to the national aviation system that refer to a broad set of conditions, such as non-extreme weather conditions, airport operations, heavy traffic volume, and air traffic control." The percentage of delay time categorized as NAS delays by the BTS has trended down over the last ten years. In 2004, its share of total delay minutes was 36.5% and by 2023, that was only 18.1%.

In 2020, 45.8% of days placed in the NAS category were due to adverse weather . These are weather delays that could have been reduced with corrective action by airports or the FAA (while bad weather may make some flight operations impossible, much of it can be handled with appropriate measures).

3. Late-arriving aircraft

• Late-arriving delays accounted for 40.0% of delay minutes

If an aircraft arrives late, this will often mean it will delay the next flight (domestic flights are typically turned around in less than an hour - with some airlines able to do it in just 25 minutes). Late arriving aircraft is a "previous flight with same aircraft arrived late, causing the present flight to depart late."

There are many reasons why an aircraft might be late, including every other reason on this list (e.g., extreme weather, security issues, and even the same aircraft was already late for its previous flight). The share of total minutes for flight delays has varied between around 30% and just over 40%, meaning this is a very large category used by the BTS.

4. Security

• Security delays accounted for 0.3% of delay minutes

Security is one factor affecting an airline's ability to operate its aircraft on time. It is "Delays or cancellations caused by evacuation of a terminal or concourse, re-boarding of aircraft because of security breach, inoperative screening equipment and/or long lines in excess of 29 minutes at screening areas." For reference, JFK Airport in NYC is considered to have the longest wait times in the United States , with travelers averaging around 40 minutes to clear security and passport control (around 15 minutes for TSA and 25 minutes for passport control).

Other major airports like Orlando International Airport and Chicago's O'Hare International Airport come a close second and third. According to the data provided by BTS, security delays consistently make up a fraction of 1% of the share of delay minutes. In many years, the percentage was only 0.1%.

5. Air Carrier: maintenance or crew problems

• Air carrier delays accounted for 36.4% of delay minutes

The other category for aircraft delays provided by the BTS is "Air Carrier." This is a large category that includes things in the airline's control and should be broken down by factors (although it is not possible to provide the statistics for the share of these issues). Aircraft that are delayed by maintenance issues are considered within the airline's control. Passengers are entitled to compensation for maintenance-related delays lasting more than 3 hours.

Maintenance issues could be problems with the aircraft's tires or error messages related to the engines or other systems. Even hot breaks can result in delays. For example, a Delta Air Lines A330-300 recently rejected takeoff in Amsterdam after pilots decided the breaks were too hot (aircraft that are turned around quickly may not have time for their breaks to cool down).

6. Air Carrier: Aircraft cleaning

• Aircraft cleaning takes 15 minutes to an hour

After every flight, the aircraft needs to be cleaned. This has been refined to a fine art, and they can be cleaned within minutes. Narrowbodied aircraft take less time to clean than widebodied aircraft—sometimes 15 minutes or less. Widebodied aircraft can take up to an hour (depending on factors like if it's the last flight for the day). Cleaning is often contracted out and the aircraft can be delayed if the contractors fail to clean the aircraft on time.

There are three types of cleaning: turn clean, overnight clean, and deep clean. Turn cleans are light clean as the airline unloads and reloads the next load of passengers as fast as possible, while an overnight clean takes longer when the aircraft has more down time. Deep cleans are done periodically, often removing seat areas and using specialized equipment.

7. Air carrier: baggage loading

• Alaska & Delta offer compensation for delayed baggage

Airlines don't load the baggage on the aircraft themselves, but still, this is classified by the BTS in the "Air Carrier" category, meaning the airline is responsible. On rare occasions, baggage can delay an aircraft even after it has taken off (e.g., the stench of 100 pigs in the cargo hold recently forced a KLM Boeing 787-9 to divert to Bermuda).

When it comes to baggage unloading, some airlines (like Alaska and Delta) offer compensation to passengers when their baggage takes more than 20 minutes to arrive on the carousel.

8. Air Carrier: refueling

• It takes around 12 minutes to refuel A320

Another reason why the airline may be delayed is refueling. No aircraft (other than the Air Canada Glimi Glider) is going to take off if it doesn't have enough fuel. It takes around 12 minutes to refuel an A320 (including five minutes of positioning and removing the tanker and seven minutes actually transferring fuel).

If there are delays with refueling the aircraft, then that will also delay the flight. In 2017, a burst pipeline cut the only source of fuel to New Zealand's Auckland Airport . This resulted in many flights being canceled or delayed. Some aircraft were instructed to fill up in Australia until the situation was fixed.

9. Disruptive passengers

• Unruly passengers can be fined tens of thousands of dollars

Sometimes flights are delayed by disruptive passengers. While most passengers are well-behaved, some are not. Occasionally, flights will not leave until security come and remove passengers from the flight. In recent years, the FAA has reported "a disturbing increase in incidents where airline passengers have disrupted flights with threatening or violent behavior."

Besides causing a flight to be diverted after it has taken off, unruly passengers can delay flights by refusing to comply with safety instructions (like fastening their seat belts or smoking). They may make threats to the crew or passengers or engage in riotous behavior (like screaming or kicking seat backs).

10. Miscellaneous

• Many unusual factors can delay flights

"Miscellaneous" is not a category provided by the BTS (and every delay needs to be placed in one of the five categories above). But there are many unusual reasons why aircraft may be delayed that are bundled into large categories. Some passengers may have a nervious fear of flying and freak out even before the aircraft has departed. There's always the risk of a medical emergency before take off.