SR SSD 2002-04

Technical Attachment

Importance of Accurate Forecasts and an Analysis of Air Traffic
in the Vicinity of Dallas/Fort Worth International Airport (DFW) as a Cold Front Moved Across North Texas

James Ott

This technical attachment will illustrate how a single wind shift associated with a cold front can affect air traffic around a major hub airport during a peak arrival and departure period. In addition, it will illustrate the role of the CWSU meteorologists, and how they interact with FAA traffic management. Finally, this attachment will illustrate how the CWSU can utilize the Flight Explorer Professional (1) software for case studies.

On the evening of January 13, 2002, a cold front was moving through the Southern Plains states. The cold frontal passage was fairly well forecast by the National Weather Service Forecast Office in Fort Worth (FWD WFO) and the Fort Worth Air Route Traffic Control Center (ZFW ARTCC) Center Weather Service Unit (ZFW CWSU). Timing the wind shift associated with the front to the minute is critical at major hub airports. Accurate wind forecasts can reduce air traffic delays, minimize air carrier and general aviation operations costs, and result in smoother and safer transition from one direction of flow to another.

Imagery produced by Flight Explorer Professional will be used to illustrate the impact of this event. Flight Explorer Professional is a PC- based program which displays aircraft flight information from around the world. The NWS Southern Region has purchased this software for ZFW CWSU for one year to test its utility in the CWSU environment. Images from Flight Explorer Professional have been used to illustrate the complexity of air traffic control (ATC) in transitioning aircraft from a south flow operation to a north flow operation at a major hub airport.

DFW TRACON Traffic Flow Patterns and ATC Concerns

Arriving and departing aircraft move with synchronicity around busy airports in major metropolitan areas. At DFW, arriving aircraft approach the area from four corner post (VORs) around DFW: northeast - BYP, southeast - CQY, southwest - JEN, and northwest - UKW (Fig 1). Each corner post is located approximately 60 miles from DFW airport. All arriving aircraft must fly over one of these corner posts before flying toward DFW or other satellite airports. All departing aircraft must leave the DFW TRACON (Terminal Radar Approach CONtrol) area on one of 16 routes, flying north, east, south or west.

With approximately 2,000 operations in a 24-hour period, DFW experiences several peak arrival and departure periods each day. These occur between late morning to mid-evening. Wind shifts at peak arrival and departure periods are always a challenge for ATC.

Wind shifts with wind speeds less than 5 kt are generally not an issue. The traffic flow can change from south to north, or north to south, when it is convenient for the control tower, TRACON or the ARTCC, based on the amount of aircraft in the area and their locations. Wind shifts greater than 5 kt are significant enough that departing and arriving aircraft are affected by the tail wind component and are forced to taxi or fly to the opposite end of the runway. It is important for FAA management to have the proper amount of ATC personnel in position prior to the implementation of a flow change so it can be executed safely and efficiently.

The Event

A dry stable airmass prevailed over the Southern Plains on January 13, 2002 (Fig 2). A weak cold front was moving toward the southeast at 15 kt across Oklahoma and north Texas. At the 2200 UTC briefing FAA management at ZFW and DFW TRACON were advised the cold front would move into the area around 0300 UTC. At 0030 UTC the CWSU meteorologist re-evaluated the situation and it became apparent the wind shift would occur between 0130-0145 UTC. At this time, the ZFW traffic management unit and the CWSU meteorologist briefed the affected ZFW corner post supervisors and DFW TRACON about the earlier arrival of the wind shift.

A fine line visible on KFWS WSR 88D and the DFW Terminal Doppler Weather Radar (TDWR) was being monitored by the CWSU and TDWR DFW TRACON (Fig 3). The cold front moved at an average speed of 16 kt. When the front was in the vicinity of Alliance Airport (AFW), which is located 15nm NW of DFW, the Weather And Radar Processor (WARP) computer system was used to access minute by minute AWOS reports from AFW. The wind shift occurred at AFW around 0045, and was expected to occur at DFW about one hour later, between 0130-0145 UTC. The Integrated Terminal Weather System (ITWS) was not available for this event.

Impact on ATC Operations

At 0138 UTC, DFW TRACON (still with a south wind) stopped accepting all arrivals landing at DFW from ZFW ARTCC. For a 12-minute period, ZFW ARTCC held all inbound aircraft while DFW TRACON transitioned from a south flow to a north flow. This resulted in all aircraft in the ZFW ARTCC area having to circle and hold outside the corner post VORs. DFW TRACON landed all arriving aircraft currently in their airspace, and quickly released other aircraft queued for departure from DFW (Fig 5 and 6). This hold occurred in the midst of a peak inbound arrival period (Fig 4).

DFW TRACON resumed arrivals at 0150 UTC (Fig 7). During this peak arrival time period, most arriving aircraft accrued 17-21 minute delays, with a maximum delay of 29 minutes for one aircraft. This is a complicated but common procedure for the ATC and, in this particular event, the transition was orderly. These delays do become costly for the airlines when averaged over the course of a year. However, if the wind shift occurred a few minutes later at a lull in activity, it would have cost significantly less.

With Flight Explorer Professional at the CWSU, the meteorologist can stay attuned to the needs of the traffic management unit by monitoring jet routes, arrival corner posts, and departure gates. With the combination of aircraft flight data and WSR-88D, the CWSU can also monitor the impact of thunderstorms on major jet routes with specific real-time flight information. Flight Explorer Professional also allows the CWSU meteorologist to construct a study of aircraft flight data for a post analysis of a weather event.

It is important for all NWS aviation forecasters to understand the impact of the timing of cold fronts and wind shifts to ATC operations. Any adjustments to the forecast of the passage of a wind shift should be timely so FAA traffic managers can plan accordingly. While wind shifts during peak arrival and departure periods cannot be avoided, accurate forecasts can enable the FAA to minimize delays and reduce the complexity of the transition.

1. Reference to any specific commercial product, process, or service by trade name, trademark manufacture, or otherwise does not constitute an endorsement, a recommendation, or a favoring by the NWS or the United States government.