Serving the CWSU Customers: Controllers and Pilots

Richard J. Kesslert
National Weather Service
Minneapolis Air Route Traffic Control Center
Farmington, Minnesota






Center Weather Service Units (CWSUs) are in the business of serving the safety and economic needs of the aviation community: pilots, passengers and especially Traffic Management Units (TMUs) at all Air Route Traffic Control Centers (ARTCCs). Uniquely situated near the user, and significantly focused on the "aviation" aspects of meteorology, each CWSU forecaster has numerous, daily opportunities to assist in the smooth flow of air traffic through their airspace (Felton 1993). Each shift presents situations for CWSU meteorologists to help the controllers and TMU specialists. Some days the meteorologist does everything "right"forecasts represent the actual evolution of "sensible weather", and timely, too. On other days, enough amendments are made to "fill-a-book" and an airport can get shut down with little notice. Passengers, pilots and controllers then express frustrations from Los Angeles, California (LAX) to O'Hare Field, Chicago, Illinois (ORD) to La Guardia Airport, New York City, New York (LGA). The latter are generally the result of delays, reroute and airport closings.


Years of forecasting weather events have proved to meteorologists that not every forecast will be correct. By learning from their mistakes, meteorologists can decrease the number of "bad" days and increase the number of "great" days (Tomlinson 1993). This paper will discuss a two-day event and how, given similar situations, the CWSU forecast helped controllers, TMU specialists, the airlines and their customers. This study will also examine what local issues need to be understood about airports and approaches to better serve the CWSU customers.



    A. FAA, Airport Arrival Rate and the Impact of Weather

Weather plays a major role in aviation delays and accidents, according to the National Transportation Safety Board (NTSB). Nearly 50 percent of air carrier accidents, and 80 percent of airline delays over 30 minutes, are attributable to weather. Weather-related operations and services are among the most often cited areas of dissatisfaction among the users of the Air Traffic System (Mandel 1989).

CWSU meteorologists are in a unique position to aggravate or relieve these weather related issues, depending upon the timeliness and accuracy of forecasts. At Minneapolis (MPX), and other major airports, the Airport Arrival/departure Rate (AAR) is determined by local weather conditions including cloud cover and cloud height, wind direction and speed, visibility and runway conditions. Each airport's Approach Area Manager (AAM), often working with the TMU Supervisor, determines the AAR.

The maximum AAR at MSP is 60 aircraft per hour, utilizing dual runways. At MPX, visibilities less than 6-8 statute miles and clouds below 3,000 to 4,000 feet Above Ground Level (AGL) usually require a reduction from the maximum AAR. The main decision concerning which runways to use for takeoffs and landings is determined by the wind direction and speed. To achieve the maximum AAR at MSP, the dual runways must be utilized.

At MPX active dual runways are 29L/R and 11R/L with single runways 22 and 04. In airport nomenclature, "L" and "R" refer to left and right runways (when there are dual runways), and 29, 22, 11, and 04 refer to the "compass directions" in which the runways are oriented. Therefore, runways 29 and 11 are the "northwest-southeast" runways and 22 and 04 are the southwest-northeast runways. The runways at airports are designed to take advantage of the prevailing wind directions.

Aircraft usually takeoff and land into the wind. When winds speeds are "light", usually below 10 knots, any of the runways can be used. To keep the AAR high, dual runways 29L/R and 11L/R are most frequently used. These runways can usually be in service when wind directions stay within around 70 degrees of the 29 or 11 designations. For example, if aircraft are landing on runway 29 the wind can be from any direction between 220 degrees and 360 degrees. In the situation of airplanes using the dual runways with a "strong" crosswind, and if a pilot indicates to the controllers that he or she experienced difficulty when landing, or occasionally taking off due to difficulty controlling the aircraft, the controllers will change to either runway 22 or 04. This is frequently the case when sustained or very gusty winds exceed 15-20 knots. Changing to either of these runways will cut the ARR in half--down to 30 or less. If planes are already in the air when this happens, delays will occur. The other factor figured into this "equation" is the number and experience of controllers working on any given shift.

Another consideration for the AAR is the upper level winds, especially if they differ in speed or direction from the surface wind. When pilots need to make large adjustments just before landing, or the wind differential causes difficulty controlling the aircraft due to turbulence or "wind shear", controllers will usually leave more space between landing or departing aircraft.

This is another way the AAR can be reduced. Even under the most ideal conditions, there are times when, due to airline scheduling, demand exceeds capacity. The primary function and responsibility of the CWSU Meteorologist ". . . is to provide meteorological advice and consultation to center operation personnel and other designated FAA Air Traffic Facilities, Terminal and Flight Service Station (FSS), within the Air Route Traffic Control Center's (ARTCC) area of responsibility" (Mandel 1989). CWSU meteorologists can work with FAA employees to help keep delays to a minimum.

The remainder of this paper will present an example of how the CWSU's meteorological input to controllers can maximize the AAR. This individual incident might not have made a significant impact in the smooth flow of air traffic. On the other hand, when meteorologists combine these little actions, they add up to produce a safe and efficient air traffic system.



    A. Meteorological Situation

On May 3, 1993, Minneapolis, Minnesota was under the influence of a high pressure ridge (MSLP field) that extended from north of Lake Superior, southwest into western Kansas and then to the panhandles of Oklahoma and Texas. A weak low pressure system was located over northeast Missouri and southeastern Iowa, and moving slowly to the north along a stationary front (Figure 1). The front had passed through the area the day before, and stratus clouds developed overnight Sunday. The clouds remained into the late morning hours, Monday, and "broke-up" during the early afternoon. While satellite airports (surrounding airports that handle private and some commercial aircraft, but not large commercial jets) to the west and northwest of the Minneapolis airport lost the "low" clouds by noon (1700 UTC), MSP was still reporting a 2,000-foot AGL scattered layer of stratocumulus at 4:00 p.m. (2100 UTC) (Figures 2 & 3).

Figure 1. Surface analysis (MSLP isobars) for 1200 UTC 3 May 1993. Visibility in statute miles; cloud heights in 100s of feet above ground level (AGL).

Figure 2. Weather depiction chart for 1200 UTC 3 May 1993. Visibility in statute miles; cloud heights in 100s of feet above ground level (AGL).

Figure 3. Weather depiction chart for 2000 UTC 3 May 1993. Visibility in statute miles; cloud heights in 100s of feet above ground level (AGL).

Surface winds were generally from the east to northeast and the active runway was 11. Planes approached the airport from the northwest, landing toward the southeast. The early morning AAR was 48, then increased to 52, then to a full 60 rate by 8:00 a.m. (1300 UTC). The improvement was due to clouds moving off to the east and southeast, thus allowing pilots to see the airport at a greater distance.

Tuesday, May 4, 1993, a weak high pressure was located over Minnesota. A stationary front stretched northeast from the low through central Wisconsin and the Upper Peninsula of Michigan into southern and eastern Canada (Figure 4). The Minneapolis weather again began with a stratus deck forming after midnight; around sunrise the ceiling was partially obscured 100 feet AGL to 300 feet AGL overcast (-X 001-003 OVC). Visibilities diminished from 2-3 miles, to between one half and one and one half (½-1½) miles in fog between 7:00 a.m. and 8:00 a.m. (1300 UTC-1400 UTC). Between 12:34 p.m. and 2:00 p.m. (1734 UTC to 1900 UTC) the ceiling improved from measured 1,500 feet overcast AGL (M15 OVC) to measured 2,300 feet AGL broken (M23 BKN) with 15 statute miles visibility. Within the next few minutes the sky condition went too scattered (SCT) (Figures 5 & 6). Wind conditions were generally similar during this time, with North to North Northeast winds at 7-10 knots. Due to the low ceiling combined with wind direction the AAR was 48, and would have remained at that rate the rest of the day. This is where input from the CWSU meteorologist was beneficial.

Figure 4. Surface analysis (MSLP isobars) for 1200 UTC 4 May 1993. Visibility in statute miles; cloud heights in 100s of feet above ground level (AGL).

Figure 5. Weather depiction chart for 1200 UTC 4 May 1993. Visibility in statute miles; cloud heights in 100s of feet above ground level (AGL).

Figure 6. Weather depiction chart for 2000 UTC 4 May 1993. Visibility in statute miles; cloud heights in 100s of feet above ground level (AGL).


Noticing the same general weather conditions present Tuesday as Monday, determined that conditions were better for a northwest arrival. Reasoning was based on 1) the clouds were moving off to the east and southeast, as they did the previous day, and 2), the current and forecasted wind speed was to diminish to less than 10 knots. The wind direction and speed were the deciding pieces of information on this day. Since the wind had switched to the northwest and then north Tuesday, the active runway was 29, meaning planes were landing from the southeast. This also meant pilots were encountering a scattered layer of clouds below 4,000 feet on the approach to runway 29 from the southeast. It was determined there was a strong likelihood that planes could land on runway 11, coming in from the northwest, therefore avoiding the clouds below 4,000 feet AGL.

The TMU and approach controllers and supervisors were told that the wind speed would continue to diminish, and remain below 10 knots for the rest of the day. Recommendations were made to the controllers and supervisors to change the active runway to 11, to take advantage of clearer skies to the west and northwest. This would be a way to increase the AAR to 60. Although the wind direction varied by more than 90 degrees from the runway, the speed of the cross winds would most likely not be a problem for pilots. FAA personnel were asked what they thought of the idea. They agreed the reasoning made sense and decided to try, and see how pilots would respond. No one had any problems with the crosswind, and planes continued to use runway 11. The rate went back to a full 60 AAR.


FAA controllers, supervisors and managers, whether at ARTCC, tower cabs or approach positions, rarely have a meteorology background comparable to that of the CWSU, or other National Weather Service meteorologists. The FAA personnel must rely on an outside source for their weather related decisions, or make a decision based on their limited knowledge (Kavoussi 1989). There are numerous times when the experience, knowledge, and creativity of the CWSU meteorologist can be employed to take advantage of the meteorological opportunities that will keep air traffic moving smoothly through the system.

With WSR-88D radars coming on line at new locations each month, meteorologists will have increased opportunities to work more closely with aviation interests. Working hand in hand with other meteorologists, CWSU staff can contribute more accurate and timely significant weather information to air traffic controllers (Mandel 1989). Wind and turbulence data are two areas where improvements in forecasting may be realized.

Meteorologists will continue, through research and improved technology, to have increased knowledge about how atmospheric conditions and specific local situations effect the movement of traffic through the national airspace. What they do with this information will be determined by the skill, ingenuity and commitment of the individual forecaster to improving the service given to their customers.

The continued impact to members of the aviation community by weather events dictates the importance of timely and accurate forecasts and updates. Private and many commercial aviation pilots and companies, depend on NWS forecasts to determine whether to carry extra fuel, declares an alternate airport or even to fly.

Once in the air, getting aircraft to their destination and on the ground by the least time consuming and safest possible means is the job of each controller. Meteorologists can help these controllers and while often help pilots save time and fuel. These savings in time and money translate into better service for their customers, at no additional cost to either the NWS or FAA (Felon 1993).


I want to thank members of the ZMP TMU and MSP Approach staff for their contributions to my understanding of the local MSP AAR. Controllers and supervisors patiently explained the variety of determining factors that go into the AAR. Thanks also to an ex-commercial pilot now working at the ARTCC for explaining fuel consumption formulas and costs.


Felton, D., 1993: NWSFO Seattle, WA, The Cost of "Occasional" at Seattle-Tacoma Airport. Preprints, Aviation Weather Workshop, Boulder, CO, 111-116.


Kavoussi, S., E. Keitz, and G. Louden, 1989: Weather Information for Air Traffic Controllers in the National Airspace System of the 1990s. Preprints, Third International Conference on the Aviation Weather System, Anaheim, CA, AMS (Boston), 437-442.


Mandel, E., K. Young, D. Panzer, and H. Ludwig, H., 1989: The Status of the FAA Central Weather Processor (CWP) Program. Preprints, Third International Conference on the Aviation Weather System, Anaheim, CA, AMS (Boston), 422-425.


Tomlinson, M., 1993: New Policies and Procedures To Improve Aviation Terminal Forecast Services. Preprints, Aviation Weather Workshop, Boulder, CO, 91-100.


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