Scenario A¿Widespread Delays as a Result of Small Localized Problems 19:35 Zulu (Z) Time (Figure 7. For a larger view, click on the images): On a clear weather day, the capacity for the Newark International Airport runway configuration in use is 45 aircraft per hour. Five more aircraft than the capacity limit arrive causing a demand/capacity imbalance. The terminal radar approach control (TRACON) in New York and the New York ARTCC begin to hold aircraft in their airspace. They have limited room to hold aircraft and when that limit is reached at 19:40Z, New York Center notifies Cleveland Center that it cannot accept any more Newark arrivals until the existing queue is reduced.
 | | Figure 7. Local event: Demand imbalance at Newark. System response: Request to hold inbound aircraft. | 19:40Z (Figure 8): When Cleveland Center is notified by New York Center that it cannot hand off Newark traffic to New York, it must put those aircraft into holding patterns. Because Cleveland Center has no room to accommodate additional Newark traffic, it immediately notifies Indianapolis and Chicago Centers that it is no longer accepting Newark bound traffic. The Newark arrivals are held in Cleveland, Indianapolis, and Chicago Centers at their en route cruise altitudes.
 | Figure 8. Aircraft in holding
patterns propagate. | 19:45Z (Figure 9): Nine Newark arrival aircraft are being held in Cleveland Center and three are being held in each the Indianapolis and Chicago Centers. As the airspace fills up, the holding affects not only the Newark bound aircraft at the holding altitudes, but also all other aircraft in those sectors and surrounding sectors.
 | | Figure 9. Holding aircraft block airspace other aircraft plan to use. | 19:50Z (Figures 10 and 11): Chicago Center stops accepting additional Newark arrivals from Minneapolis Center. As time goes on, other centers (e.g., Kansas City and Atlanta) will be affected as the impact ripples to the west coast in a short time. All of this occurs in clear weather conditions. As a result of a very small demand/capacity imbalance, the short five to 10 minute no-notice hold in a congested airspace (Newark's), impacts an estimated 250 aircraft in other airports across the country in less than 20 minutes.
 |  | | than 250 aircraft within 20 minutes. | Scenario B¿Congestion Due to En Route Severe Weather This scenario begins with the identification of a problem with the New York area arrival flow facing a storm line that crosses Cleveland Center (Figure 12. For a larger view, click on the images).
 | | Figure 12. Local event: New York area arrivals cannot penetrate severe weather. | The New York area arrival flow is rerouted south into Indianapolis Center behind the most severe part of the thunderstorm line (Figure 13).
 | | Figure 13. System response: Reroute New York area arrivals to the south. | In order to address the resulting congestion, New York area arrivals receive restrictions to reduce the flow (Figure 14).
 | | Figure 14. Resulting traffic needs to merge with DC and New York arrivals from the south. | Additional reroutes occur for other traffic flows. Washington, DC, arrivals from the south and west are rerouted out of Indianapolis Center into Atlanta Center (Figure 15).
 | | Figure 15. DC Metro area traffic is routed south. | Atlanta Center already had a full airspace with traffic flying their normal routes. The rerouting of Washington, DC, arrivals adds to congestion over the Atlanta airport and begins to affect departures. Washington, DC, traffic departing from Dallas and Houston are then restricted to balance the traffic mix across the airspace among the competing traffic flows (Figure 16).
 | | Figure 16. Atlanta airspace full, inhibiting Atlanta departures. | In addition to showing the domino effect that rerouting major traffic flows has on the NAS, this scenario illustrates how weather delays can occur for flights that have clear weather along their routes. The weather in Atlanta, for example, is clear; but the rerouting of traffic has created departure delays at Hartsfield¿the world's busiest airport. People on the ground are being told of delays caused by weather, when what they see are sunny skies. The result: increased frustration and loss of confidence. Scenario C¿Capacity Reduction Due to Airport Weather Conditions In this final scenario, the capacity of the San Francisco airport is severely reduced as a result of fog. The conditions result in arrival/departure throughput constraints, which in turn translate into en route congestion in the surrounding airspace (Figure 17. For a larger view, click on the image).
 | | Figure 17. Capacity reduction at San Francisco due to fog. | Related Links:
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