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FAQ: Weather Delay

What is the largest cause of delay in the National Airspace System?

Which airports have the worst weather-related delay?

What type of weather causes the most delay?

What happens when en route flights encounter thunderstorms?

What happens if thunderstorms prevent landing at an airport?

How far in advance do traffic flow planners need weather predictions?

What is NextGen Weather providing to help reduce weather delay?


What is the largest cause of delay in the National Airspace System?

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Causes of air traffic delay in the National Airspace System.

By far, the largest cause of air traffic delay in the National Airspace System is weather. The pie chart shows that weather caused 69 percent of system impacting delays of greater than 15 minutes over the six years from 2008 to 2013, as recorded in the OPSNET standard "delay by cause" reports. While weather is the largest cause of delay because of too much demand for the impacted resources, volume alone, caused by too much demand even with unconstrained resource capacity, also accounts for 19 percent of delay. Equipment failure creates 1 percent, runway unavailability 6 percent, and "other" factors the remaining 5 percent of delay. These delay statistics include air carrier, air taxi, general aviation, and military classes of aircraft.

The portion of delay due to weather represented nearly 10 million minutes in 2013. Delays translate into real costs for the operators and passengers. Currently, the cost to the air carrier operators for an hour of delay ranges from about $1,400 to $4,500, depending on the class of aircraft and if the delay is on the ground or in the air. If the value of passenger time is included, the cost increases annother $35 per hour for personal travel or $63 per hour for business travel for every person on board.

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Which airports have the worst weather-related delay?

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These airports had the worst weather-related delay in 2013.

The bar chart shows that the combined delay at the three biggest airports in the New York City area (Newark, LaGuardia and Kennedy) is the highest in the country, with more than 57,000 significant delays of more than 15 minutes in 2013. The other top delay airports are in Chicago (nearly 26,000 delays in 2013), Philadelphia (almost 18,000), San Francisco (16,000) and Atlanta (nearly 12,000).

These seven airports with the worst weather-related delay experience many impacting weather events but weather alone does not necessarily lead to huge delays.

If an airport has much excess capacity, many delayed planes can be shifted to non-weather periods without overloading the system. However, airports with the most weather delays also tend to operate close to capacity for large parts of the day. System-impacting weather, combined with excess demand, means that delayed flights may have to wait hours to land or depart.

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What type of weather causes the most delay?

Chart showing total number of weather-related delays compared to the total number of delays for the combined three major airports in NY by month in 2013. The chart shows corresponding sharp increases in both types of delays in the summer and lesser increases through late fall and early winter. The least amount of both types of delays occur in February and September. The weather-related delays range approximately from 50-80% of the total delays over the year.
Weather-related delays compared to total delays at Newark, LaGuardia and Kennedy airports by month over the course of 2013.

The type of weather causing air traffic delay differs over a year, and also depends on the geographical area of the country.

For example, the graph shows combined total delays (dark orange) and weather delays (light orange) by month in 2013 at Newark, LaGuardia, and Kennedy airports. Weather delays and total delays peak in May, June and July (weather delays ranging from nearly 6,700 to more than 7,800; total delays ranging from 9,000-9,500), but weather delays are also considerable in March, April, October, November, and December (ranging from about 3,500-5,400).

The pie charts below show the type of weather leading to this combineddelay. In the winter, October through March, the combination of airport surface winds and low ceiling and visibility (C&V) conditions account for about 75 percent of delays; convective weather, winter weather, and a small amount of other weather conditions contribute to the remaining 25 percent.

However, during the summer months,April through September, when delays peak, more than 40 percent of the delay affecting airport arrivals is due to rain and thunderstorms. Low C&V conditions cause about 30 percent, airport winds create about 20 percent, and "other" account for 10 percent of summer delay.

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These types of weather contributed to delays at Newark, LaGuardia and Kennedy in winter versus summer in 2013.

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What happens when en route flights encounter thunderstorms?

Jet aircraft can safely fly over thunderstorms only if their flight altitude is well above the turbulent cloud tops. The most intense and turbulent storms are often the tallest storms, so en route flights always seek to go around them.

If a busy jet route becomes blocked by intense thunderstorms, traffic will reroute into the neighboring airspace, which can become overcrowded if the flow is not managed (see animation). In these cases, a planning team consisting of FAA personnel at the Air Traffic Control System Command Center who coordinate with the centers, select terminals, airlines, NAVCANADA, general aviation organizations and the military has several options, some of which are discussed below.

In case of a large scale weather impact, a severe weather avoidance plan may be put into place to relocate demand to another part of the country. The planning team's strategic placement of airspace flow programs with reduced hourly flow rates allow airlines to prioritize and plan which of their scheduled flights they will route through the restricted airspace. Ground delay programs are also used to temporarily hold aircraft at their departure airports to reduce the number of flights going into an impacted area.

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Aircraft shown as dots avoid New York area weather-impacted airspace September 11, 2013. Trailing lines show their recent paths.

As an example, on September 11, 2013, an approaching cold front caused a broad region of rapid storm development in New York Center airspace. An airspace flow program was set about 1 hour before the weather impact quickly increased to reduce flow, but West Coast traffic bound for New York was already en route.

As seen in the animation, few flights could get through the weather-impacted airspace, and many of them were routed northward to avoid the weather. However, the weather continued to progress northward, making for increasingly long reroutes. Although the New York airports remained clear of weather, flights bound for New York couldn't arrive on time.

Sixty-nine aircraft diverted to alternate airports and 72 aircraft taxied -back to the gate when there was no available airspace. In addition, 55 aircraft were assigned to a holding pattern, and almost 600 departures and arrivals were canceled.

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What happens if thunderstorms prevent landing at an airport?

As the arriving aircraft approaches its destination airport, the pilot will usually be asked to slow down or enter a holding pattern until the thunderstorms in and around the airport have cleared. As more planes arrive and holding continues, over-crowded airspace and running out of fuel can become serious issues. Landing these arrivals safely becomes the top priority.

Controllers can opt to use more of the available terminal routes for arrivals and fewer for departures. With fewer planes departing, remain occupied and airport grid lock can occur. Cases where passengers were stranded for excessive periods of time led the Department of Transportation to pass a rule prohibiting airlines from leaving planes parked for more than 3 hours without allowing passengers to disembark.

If thunderstorms persist, holding aircraft will divert to alternate airports, wait out the bad weather, refuel, and fly again later to the original destination. Diversions are undesirable because of the magnitude of passenger delay and cost to the airlines.

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Flights shown as dots and their trailing lines showing their paths are forced into holding patterns during thunderstorms in the New York City area July 2, 2014. Oval patterns show aircraft that were held in the air.

An example of thunderstorms temporarily preventing landing at airports occurred July 2, 2014, as growing thunderstorms moving eastward delayed several flights destined for the New York airports.

The animation shows that as storms approached the airports, aircraft were forced to hold around the airports until the weather cleared. In total, 177 flights were held, representing more than 5,000 minutes of delay. Ninety-seven aircraft diverted to alternate airports.

Once the storms began to dissipate and move away, regular flow of traffic resumed.

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How far in advance do traffic flow planners need weather predictions?

Unforeseen weather impacts on en route and terminal airspace can lead to long delays and ultimately be costly to the airlines and traveling public. If weather impacts are either short-lived or local, they can be mitigated by effectively using available airspace. All airborne and scheduled flights can be handled with only minor reroutes.

However, as the weather impacts become longer lived, affect larger regions of the country, or both, management of the demand must be planned strategically. In weather events requiring moderate to aggressive management, many scheduled flights will require new flight plans that do not intersect the weather impacted areas.

Some flights through the impacted airspace may originate at nearby airports, with only short intervals from departure to arrival, whereas other flights may cross the country and be airborne for hours. A severe long-lived weather impact will require management of short- and long-haul flights in order to effectively control the demand.

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Airborne time of flights arriving are shown for Newark, LaGuardia and Kennedy on a day with no weather delay, September 19, 2014.

For example, the bar chart combines all arrivals into the three New York airports — Newark, LaGuardia, and Kennedy — through an entire day with no weather delays, and categorizes them by their airborne time interval. Most arriving flights were airborne 1–3 hours (blue), with about 45–60 flights arriving each hour from 8 a.m. to 9 p.m. local time. About 10–25 arrivals per hour had airborne time intervals less than 1 hour (green) throughout most of the day (6 a.m. to midnight), reflecting the steady traffic between New York, Boston, Washington DC, and other nearby cities. Finally, many New York arrivals were airborne more than 3 hours (red), with 30–40 flights per hour landing daily between noon and 9 p.m. The noon–9 p.m. time period coincides exactly with the time period most thunderstorms occur in the summer.

Strategic traffic flow managers must plan hours in advance to influence long-haul flights. If the time needed for pre-departure planning and filing of amended flight plans is added to the airborne time intervals, predictions of convective weather impacts on airspace capacity are needed 4-8 hours in advance to influence long-haul flights and 2-6 hours in advance to influence shorter flights.

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What is NextGen Weather providing to help reduce weather delay?

NextGen Weather provides aviation weather products that support tactical and strategic management of air traffic during weather events, helping to minimize passenger delays as well as improve aviation safety.

Tactical Traffic Flow Management

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Aircraft wait in line for takeoff.

Managing traffic tactically uses available airspace resources to handle the normal traffic demand and requires accurate depictions of weather impacts in the 0-2 hour time frame. Traffic flow managers can take different steps, such as locally reroute traffic around weather, direct en route traffic to a weather-free arrival path when near the destination airport, and delay arrivals by placing them in holding stacks until the weather clears. Many of NextGen Weather's improved products support tactical traffic flow management.

Strategic Traffic Flow Management

To support strategic traffic flow management, operational planners require clear, high-confidence predictions of weather impacts on airspace capacity out to 8 hours so they can plan strategies such as traffic flow re-routes, flow rate restrictions, or both. They also need the shared situational awareness that is required for collaborative decision-making on strategic time scales. NextGen Weather provides the foundation for development of critically needed traffic flow management tools.

Detailed information is provided in the section on support for strategic traffic flow management.

Aviation Safety

Although improved efficiency of the National Airspace System (NAS) is NextGen Weather's primary benefit, the program also enhances aviation safety in several ways. The NextGen Weather Processor (NWP) provides aviation weather products with improved coverage, faster product update rates, and reduced artifacts. For example, the NWP growth trend product updates every 25 seconds, and indicates where thunderstorms are actively growing and airspace to avoid.

Additionally, Common Support Services – Weather (CSS-Wx) and the NWP Aviation Weather Display (AWD) enable access throughout the NAS to NWP products such as lightning and tornado detections. Broad and timely access to these products enhances awareness of serious ongoing safety hazards. Another safety improvement provided by CSS-Wx and AWD is the display of NOAA aviation-oriented icing and turbulence products, providing users with an indication of where and when in-flight icing and/or turbulence may occur.

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