The Most Common Reasons Flights Get Delayed

Air travel can be incredibly stressful today, in large part because of the potential delays. These can cause late arrivals, which is hardly ideal for a person's travel plans, but they can also lead to missed connections. Given that many passengers connect at a hub airport to reach their final destination, this can lead to chaos.

What's more, communication about the reason behind delays from airlines can be opaque, leading to confusion and frustration. Air travel is, of course, highly complex and relies on a tight web of interconnected systems to function, which means that many factors can influence flight schedules and cause delays.

In the US, the FAA splits airline delays into five broad categories: late-arriving aircraft, carrier-caused delays, security, National Airspace System delays, and weather. These are just the groups that the FAA has created, but flight delays themselves can occur for a wide variety of reasons.

Security Delays

Security delays can include a number of different events, including a terminal evacuation, an uncleared person entering the aircraft, inoperative screening equipment, and significant delays at the airport's TSA facility. Notably, however, delays caused by disruptive passengers are not classified under this category, but are rather considered carrier delays.

This is because the decision to delay or divert a flight to remove a passenger is one made by the airline, even if there may be a security risk. Ultimately, these delays constitute by far the smallest proportion of overall delays, although they're not unheard of. In February and March 2026, the partial shutdown of the DHS halted pay to TSA workers, who were still required to show up due to being classified as essential workers. A record number of callouts and attrition created severe staffing shortages at airports.

This resulted in lines that lasted hours. The FAA's security delay threshold is set at over 29 minutes, so any departure that was impacted by these long wait times was placed in this category. Other notable events that fall under this group include the March 2026 evacuation of DCA Airport's Concourse D after a suspicious package was discovered.

Elsewhere, delays were caused by a trespasser breaching DEN Airport's fence and walking across the airfield earlier in October 2026. Because these events are so rare, they make up less than 1% of all airline delays in the US, and an incident that would result in security delays is notable enough to make headline news when it does occur.

Hazardous Weather Conditions

Severe weather conditions can pose significant dangers to aircraft. Winter storms can bring snow and ice, which can impact not just aircraft but the airfield as well. Low visibility conditions, typically as a result of fog, require increased separations during instrument approaches and can restrict departures as well.

Meanwhile, much of the US can be susceptible to severe thunderstorms, particularly during the summer months, and these can severely disrupt airline operations due to the hazards that they bring. Thunderstorms can move far quicker than the naked eye might perceive, and the size of the largest thunderstorm clouds can make it difficult for airliners to maneuver around them.

The heavy rain that often accompanies these storms reduces visibility, but more concerningly, they bring windshear, intense updrafts and downdrafts, severe turbulence, microbursts, and possible lightning. Therefore, aircraft take every effort to avoid them, but this is more difficult for departing and arriving flights if a storm is near an airport. Airports often have to issue a ground stop if a thunderstorm is near the airfield.

Arriving aircraft are then usually placed in a holding pattern or may even need to divert in some cases. Even when a ground stop is not in effect, thunderstorms can decrease the number of departures permitted while also requiring inbound aircraft to hold or divert due to reduced capacity, further causing delays. Hurricanes bring much of the same dangers as thunderstorms but on a larger scale, and they impact a higher number of areas for longer periods of time.

National Airspace System Delays

The National Airspace System is essentially the US's entire network of air traffic control facilities, airspace, airports, regulations, and more to allow for safe air travel within the country. NAS delays typically result from airport congestion or reduced staffing at an ATC tower, as well as ground stops, airport construction, or a system/software issue at the facility.

In other words, this is a delay resulting from factors on the airport/airspace side, rather than from weather, the airline, or a security risk. ATC staffing shortages have become a hot topic in recent year, and the ever-increasing volume of flights is overwhelming air traffic facilities across the country, as they continue to be chronically understaffed.

In 2025, approach controllers managing arrivals into Newark lost their data connection for 90 seconds, resulting in severe delays and cancellations. In January 2023, a failed software update to the FAA's NOTAM system caused a nationwide ground stop, the first since the September 11 attacks, delaying over 10,000 flights. Issues with the NAS have become more prominent as the infrastructure ages, and disruptions become more severe.

As such, problems like ATC staffing shortages and crumbling infrastructure have become major political topics, with the DOT and the FAA both pledging to invest more heavily in the US's airspace and airport infrastructure, while also increasing hiring efforts for ATC towers. The effectiveness of these efforts remains to be seen, but it's clear that there are fundamental issues with the nation's airspace system as is.

Carrier Delays

Delays categorized under this group are those occurring for reasons within an airline's control. This includes factors like aircraft cleaning, aircraft damage, late connecting passengers and luggage, catering, and more. These are arguably the most important statistics to track for an airline, since they indicate which areas of their own operations need to be improved to directly influence on-time performance.

This is hugely important to passengers and can influence customer decisions, while also helping or hurting an airline's image. Delta Air Lines prides itself on its historically excellent reliability, and even trademarked the name 'On-Time Machine' for advertising purposes. The airline is well-known for having a premium image with the general public; reliability is a major component of that.

To this day, it remains the most profitable airline in the US, with only United Airlines coming close, a carrier that has also significantly improved on-time performance. Meanwhile, customers tend to book away from carriers with higher cancellation rates. While total on-time metrics take into account all delays and cancellations, investments in back-end systems contribute to improvements in reliability, which benefits passengers and the airline.

This becomes especially worthwhile during major operational disruptions, such as large storms, because they affect how quickly an airline can recover. Initial delays and cancellations may be due to the event, but the number and magnitude of subsequent delays are a reflection of an airline's operations.

Late Arriving Aircraft

Aircraft rarely spend much time on the ground, since they only make money when flying. Widebody airliners typically operate longer flights and have more ground time, which means that it's less common for them to experience cascading delays, but narrowbodies often operate multiple flights each day with minimal time on the ground. This decreases tolerance for delays.

As such, when one flight experiences a delay for a different reason, this can affect the rest of an aircraft's flights for the day. The FAA refers to this as 'delay propagation', and this is by far the largest cause of airline delays in the US. An inciting event, such as disruptive weather, can impact a huge number of flights within a given time period. However, aircraft are usually scheduled to always be flying.

​​​​​​​As such, this then has a ripple effect throughout the rest of the day, or even longer. Because of the shorter time and ground times that come with short-haul flying in particular, it can be difficult to make up the lost time, and depending on the aircraft's schedule, the delays may even increase as the day goes on. This is where the benefits of a hub-and-spoke operating model shine.

When almost all flights arrive or depart at an airline's hub, this means that aircraft usually operate 'out-and-backs'. If a flight experiences a delay en route to the spoke, the return to the hub may also experience a delay. At the hub, however, the airline can substitute another aircraft to operate the next flight that the delayed aircraft was originally meant to fly, reducing or eliminating the delay. This is much harder to do with flights that operate between two spokes.