Dynamic Resectorization in 

Air Traffic Control: A Human 

Factors Perspective 

Earl S. Stein, Ph.D., ATO-P 

Pamela S. Della Rocco, Ph.D., ATO-P 

Randy L. Sollenberger, Ph.D., ATO-P 

December 2005 

DOT/FAA/CT - TN05/19 

This document is available to the public through the National 

Technical Information Service (NTIS), Springfield, Virginia 22161. A 

copy is retained for reference by the William J. Hughes Technical 

Center IRC. 

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document format (PDF). 
Technical Report Documentation Page 

1. Report No. 

DOT/FAA/CT-TN05/19 

2. Government Accession No. 3. Recipient’s Catalog No. 

5. Report Date 

December 2005 

4. Title and Subtitle 

Dynamic Resectorization in Air Traffic Control: A Human Factors Perspective 

6. Performing Organization Code 

ATO-P 

7. Author(s) 

 Earl S. Stein Ph.D.,Pamela S. Della Rocco, Ph.D., Randy L. Sollenberger, Ph.D. 

8. Performing Organization Report No. 

DOT/FAA/CT-TN05/19 

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Federal Aviation Administration 

NAS Human Factors Group, ATO-P 

William J. Hughes Technical Center 

Atlantic City International Airport, NJ 08405 

11. Contract or Grant No. 

13. Type of Report and Period Covered 

Technical Note 

12. Sponsoring Agency Name and Address 

Federal Aviation Administration 

Human Factors Research and Engineering Division, ATO-P 

800 Independence Avenue S.W. 

Washington, DC 20591 

14. Sponsoring Agency Code 

ATO-P 

15. Supplementary Notes 

16. Abstract 

The NAS is a highly structured environment. Structure provides benefits including predictability for the decision-maker, the air traffic 

controller. When something is unusual, controllers can identify the event as out of the ordinary given their inherent and trained 

capacity for pattern recognition. The expertise in pattern recognition does not develop quickly. In the current system, it takes En 

Route controllers an average of about three years to certify as Certified Professional Controllers (CPCs, formerly Full Performance 

Level). In general, CPCs must learn and checkout on at least six different sectors to certify. There are situations, however, when the 

usual structure is reduced and the typical patterns do not work. This can happen with weather events and systems outages as examples. 

Dynamic resectorization offers a tool in these situations to increase the options and promote flexibility. In our current system, traffic 

managers can resectorize in a very systematic structured way to balance the load and increase the level of structure for controllers. The 

system of the future may include several types of resectorization supported by automation tools. Limited dynamic resectorization is 

similar to what is done now but may see more widespread use. Unlimited dynamic resectorization represents a leap into the future with 

under lying technology that does not exist today. Both approaches raise human factors questions, which should be approached 

systematically in a proactive manner. The more flexible the system becomes the more dynamic the options will be. Operators will 

need solid anchors if they are going to be able to efficiently and expediously maintain safe separation between aircraft. 

17. Key Words 

Air Traffic Control, Aviation Human Factors, Computer-Human 

Interaction, Dynamic Resectorization, Sector Boundary Adjustment 

18. Distribution Statement 

This document is available to the public through the 

National Technical Information Service, Springfield, 

Virginia, 22161 

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

32 

22. Price 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized 

iii

Table of Contents 

Executive Summary.........................................................................................................................v 

1. Introduction..................................................................................................................................1 

1.1 Background............................................................................................................................1 

1.1.1 The Current Airspace System .........................................................................................2 

1.1.2 The Concept of Dynamic Resectorization - Studies .......................................................2 

1.1.3 Current DR Practices at FAA Facilities..........................................................................5 

1.1.4 Human Factors Issues .....................................................................................................9 

1.2 Discussion............................................................................................................................16 

References......................................................................................................................................21 

Acronyms.......................................................................................................................................24 
iv
v

Executive Summary 

Dynamic Resectorization (DR) is a concept designed to increase flexibility in the National 

Airspace System (NAS), and increase capacity while reducing delays. The concept involves 

dynamically moving airspace boundaries to accommodate traffic flow constraints such as 

weather, equipment outages, or active special use airspace. There have been a number of 

proposals found in the FAA’s planning documents, including the Flight Plan 2004-2008, the 

Target System Description, and documents from the Joint Planning Organization. In addition, 

Eurocontrol proposed adaptive airspace management techniques like DR as an alternative to 

ground delays or reroutes (Eurocontrol, 1998). NAS controllers do some limited DR in today's 

system. However, there remain a number of questions about how NAS might implement the 

concept of unlimited DR and whether unlimited flexibility would be ideal. 

Current resectorization regularly occurs when sectors are combined or decombined depending 

based on traffic demand within all ARTCC areas. There are also numerous examples of 

controllers delegating airspace to another facility for specific uses. As a follow on to the MIT 

DR conceptual reports, the MITRE Corporation’s Center for Advanced Aviation System 

Development (CAASD) developed a casebook describing situations in which DR was already 

being used in FAA facilities within the limits of the Host computer (MITRE, 2000). They 

identified clear procedures that were established for the transfer of predefined airspace. These 

included new maps provided to the receiving facility and, when appropriate, even produced 

flight progress strips for the receiving facility prior to the transfer. Although this indicated 

considerably flexibility under some conditions much of the day-to-day activity in the NAS 

depends on organization and repeatability. 

Fundamentally the NAS is a highly structured environment. Structure provides benefits 

including predictability to the decision-maker, in this case, the air traffic controller. When 

something is unusual, it is easily spotted as out of the ordinary, because of the brain’s capacity 

for pattern recognition and identification of pattern changes. Expertise in pattern recognition 

does not develop quickly. In the current system, it takes En Route controllers an average of 

about 3 years to certify as Certified Professional Controllers (CPCs, formerly Full Performance 

Level). In general, Enroute CPCs must learn and checkout on at least 6 different sectors to 

become CPCs. 

There are situations, however, when the expected structure is lost and typical patterns cannot be 

achieved. These situations include but are not limited to the impact of weather systems and 

equipment outages. DR offers a tool in these situations to mitigate the loss of the decision 

makers' usual options. In our current system, those DR options themselves have been highly 

structured, adding the benefits of that structure back to the controller’s strategy options. 
1 

1. Introduction

Dynamic Resectorization (DR) is a concept designed to increase flexibility in the National 

Airspace System (NAS), and thereby increase capacity while reducing delays. The concept 

involves dynamically moving airspace boundaries to accommodate traffic flow constraints such 

as weather, equipment outages, or active special use airspace. Proposals can be found in a 

number of the Federal Aviation Administration’s (FAA’s) planning documents, including the 

Flight Plan 2004-2008 (2004), the Target System Description (FAA, 2003), and from the Joint 

Planning and Development Office. In addition, Eurocontrol proposed adaptive airspace 

management techniques like DR to replace ground delays or reroutes (Eurocontrol, 1998). Some 

limited DR is practiced in the NAS today. However, there remain a number of questions about 

how NAS might implement the concept of unlimited DR and whether unlimited flexibility would 

be ideal. Answers to these questions have human factors implications for air traffic controllers. 

This report reviews existing literature on DR with a view to identifying the human factors issues 

that need to be considered as we move toward realizing the concept. This report also examines 

several alternatives to an unlimited implementation of DR. 

1.1 Background

Air Traffic is projected to continue increasing in the foreseeable future (National Academy of 

Sciences, 2003). In the summer of 2000, delays were substantial enough that the FAA’s 

Administrator established meetings with industry and developed a plan to reorganize airspace to 

relieve nine “choke points” in the system. As of July 15, 2004 FAA’s Air Traffic Organization’s 

performance statistics revealed that two capacity indicators, on-time gate arrivals and operational 

availability, were below expected performance, indicating continuing capacity problems. 

The FAA’s Flight Plan 2004-2008 notes that the challenges for aviation call for nothing less than 

transforming the system (FAA 2004, p. 8). DR is a concept designed to provide a tactical tool, 

which theoretically may improve the situation. DR represents a radical change from today’s 

operations. 

The concept is found in a number of the future plans for the NAS. In the RTCA Operations 

Concept, planners presented DR as flexible airspace to match the dynamics of demand. In the 

late 1990s, the FAA’s Air Traffic Airspace Management Program (ATA-1) lead a DR 

Workgroup with representatives from headquarters, MIT Lincoln Laboratory, MITRE Center for 

Advanced Aviation System Development (CAASD), and the FAA William J. Hughes Technical 

Center (WJHTC) Human Factors Group (Wilhelmsen, et al., 1999a). 

The authors of the MIT reports noted the following: 

"Limited DR permits airspace management and reconfiguration based on a limited set of 

predefined airspace modules. Unrestricted DR allows unrestricted airspace 

reconfiguration. The limited DR concept is inherent in the current Host en route 

automation system. Enhancement of the Host's limited DR capability maybe possible in 

the near term. Unrestricted DR requires a new automation system and is therefore not a 

near-term product-improvement option. Limited DR is a functional subset of unrestricted 

DR (Wilhelmsen et al., 1999a, p. ii).” 
2 

While unrestricted DR seemed a distant possibility, authors focused on the present and near 

future in which they could conceive of Limited DR or LDR. The key word here was "limited" 

implying that considerable structure was likely to continue as long as technology took its current 

form and controllers continue to function in similar roles as they do today. 

The current NAS is the baseline for all future modifications and improvements. A full 

understanding of DR requires an in depth knowledge of the system as it exists today. 

1.1.1 The Current Airspace System

Current airspace is generally composed of rigid, predefined structures, or volumes. One 

operational volume is called a sector. One controller or a team of controllers is responsible for 

safe and efficient flow of traffic through that sector. The airspace staff within the ARTCC 

defines sector sizes and boundaries. Sectors are based upon historical patterns of traffic and 

geography to ensure appropriate distribution of workload. 

The air traffic system developed with route structures based on navigational aides as pilots 

navigated from one to another. Fixes were identified along the routes where controllers 

documented an aircraft’s position by recording time over the fix. A fix could be a Navigational 

Aid (NAVAID), named or unnamed intersections of airways, or other location. The system 

prepared flight progress strips for each of these fix posting areas to keep track of each aircraft’s 

progress. With automation of the NAS in the Host computer in the early 1960s, these fix posting 

areas (FPA) became the building blocks of the airspace adaptation database. Together, all of the 

FPAs in the airspace adaptation uniquely define the entire airspace within an ARTCC. 

Sectors are comprised of one or more FPAs and are predefined in the adaptation database. Based 

upon staffing levels to accommodate traffic flows, however, operational needs commonly require 

combining or decombining sectors. This process is accomplished in the Host with a resector 

command (CS) in real time. In addition, the CS command allows reassignment of specific FPAs 

to another sector. Thus, the Host offers a limited resectorization capability. However, it is 

defined and limited by the FPA geometries and building blocks (Wilhelmsen, et al., 1999a). 

Wilhelmsen and his colleagues pointed out that a substantial number of sectors (e.g., 60% of all 

ZKC sectors) are comprised of only one FPA and many sectors are very large. So, while the 

current system provides some DR flexibility, the flexibility is restricted by the automation’s 

structural requirements (i.e., FPAs) and the logistics of changing the adaptation database. There 

have been a number of research studies concerning the concepts underlying resectorization. 

1.1.2 The Concept of Dynamic Resectorization - Studies

1.1.2.1 The Honeywell and Wyndemere Studies

Goldberg and Eberlin (1997) used fast-time modeling simulations to investigate the concept of 

DR. Using Salt Lake City ARTCC airspace, they adjusted sector boundaries to accommodate 

representative traffic flows from actual traffic patterns. They concluded that using adaptive 

boundaries allowed for more user-preferred routing, which resulted in fewer delays and better 

aircraft fuel efficiency. 
3 

In 1997, Wyndemere Corporation completed a study under the joint National Aeronautics and 

Space Administration (NASA) and FAA Advanced Air Transportation Technologies (AATT) 

Program. They conducted a human-in-the-loop simulation to evaluate sector complexity with 

more flexible, user-preferred flight paths (Pawlak, Bowles, Goel, & Brinton, 1997). Controllers 

were responsible for two adjacent high-altitude sectors from Cleveland ARTCC airspace. The 

researchers explored continuously changing boundaries versus a change to optimize airspace 

configuration at 15-minute intervals, as well as limited and unlimited resectorization. The 

authors found that they were able to duplicate modeling results for aircraft traffic density. 

However, they warned that controllers rely on existing sector/air route dependencies for 

maintaining traffic awareness and safety. Wyndemere began developing a tool to assist with 

more flexible flight routing entitled “Dynamic Resectorization and Route Coordination 

(DIRECT) System.” This system could in theory facilitate increased information sharing among 

Traffic Management Units and Airline Operations Centers (AOCs). It would be designed to 

include prediction of airspace complexity and impact on controllers. 

In an evaluation of the DIRECT System, Wyndemere reported the following results: 

• “Procedures for changing sector boundaries should be formalized to ensure that 

transitions proceed smoothly. 

• Certain automated enhancements can be used to minimize the amount of controllercontroller coordination needed to accommodate a sector change. 

• The frequency with which sector boundaries can change will be constrained by the 

complexity of traffic situations as well as the complexity associated with making each 

boundary change. 

• Unless significant ATM system changes are made the magnitude of boundary 

changes will probably be restricted by current radio frequency limitations and 

controller specialization in certain areas of airspace. 

• New sector configurations may need to be limited to a pre-defined set so that 

controllers can receive appropriate training for each configuration. 

• Although more difficult to implement, added flexibility to accommodate weather 

systems or unusual traffic patterns may also prove beneficial.” 

Given the conclusions drawn by the developers of the DIRECT, ,system readers may conclude 

that there are advantages with more flexibility in resectorization. However, the same authors 

behind the development of the system sounded a note of caution. They concluded that despite 

potential flexibility, resectorization has limitations. It reduces the structure inherent in the 

current system, which allows it to work as well as it does. 

1.1.2.2 Massachusetts Institute of Technology Lincoln Laboratory Reports

In 1999, the En Route Integrated Product Team (IPT) requested an examination of a concept of 

DR and associated issues from the Massachusetts Institute of Technology (MIT) Lincoln 
4 

Laboratory (Wilhelmsen, et al., 1999a). This effort resulted in four reports and was the seminal 

effort to date. 

The authors noted that their idea arose out of a significant need expressed by personnel from 

oceanic and offshore ATC facilities (in a previous project) to have more flexibility in 

management of their airspace. The goals of the MIT DR program were to establish a common 

definition of the DR concept and to understand operational implications of implementing DR 

(Wilhelmsen, et al., 1999a, p. 4) 

The MIT authors defined DR as “a tactical airspace management capability that permits Air 

Traffic Control (ATC) sector boundaries to be adjusted in response to changing traffic flow 

patterns” (Wilhelmsen, et al., 1999b, p. 1). The DR concept is that of a flexible and wellcontrolled process of on-line sector boundary adjustment. They also distinguished between 

limited (capabilities within reach of near to mid-term) and unrestricted DR (capability possible in 

a longer-term timeframe). Limited DR provided for reconfiguration of airspace based upon a 

limited number of predefined airspace boundaries. Air space planners would define these areas 

in advance of traffic demand changes. This concept provides flexibility when traffic patterns 

vary but display recurrent properties (e.g., specific time of day increased traffic demand). 

Because the boundaries are predefined, the resectorization is predictable. Unrestricted DR

would, in concept, allow unconstrained boundary reconfiguration, which would handle recurrent, 

as well as unpredictable traffic flow demands. The MIT definition, however, did not mean an 

automatic, continuous, real-time boundary movement. The authors postulated that optimally 

reassigning airspace could result in improved workload balance, reducing coordination 

workload, and preventing sector saturation. 

The MIT reports reviewed current NAS computers and sectorization, current practices of limited 

DR, human factors, and technical issues associated with the operational implications of 

implementing DR. Their operational concept “denotes an unrestricted but well-controlled online process of sector boundary adjustment. It does not imply continuous sector boundary 

movement in real time in response to changes in traffic flow--that is, DR is not automatic and it 

is not continuous.” They envisioned a process that ensured positive control during 

resectorization. Specifically, their operational concept proposed a process in which the 

following occur: the developing traffic situation is predicted and monitored continuously; sector 

plans are developed and validated to balance sector demand and not exceed capacity; activation 

time is determined and pre-briefed; and finally, the new sector plan is activated. 

Wilhelmsen, et al. (1999b) identified issues associated with the DR concept. These included 

both human factors and technical. Among the human factors issues, they identified the 

following: sectorization planning, situation awareness, sector capacity, demand and workload, 

and training and certification. Technical issues involved predicting traffic and weather, 

automation and airspace constructs, communication, and surveillance. 

1.1.2.3 Human-in-the-loop Simulation

As a follow on to the MIT work in 2000, the WJHTC NAS Human Factors Branch conducted a 

human-in-the-loop simulation to examine the human factors of DR (Hadley & Sollenberger, 

2002; Hadley, Sollenberger, D’Arcy, & Bassett, 2000). The researchers simulated adjacent 
5 

sectors in different ARTCCs using generic airspace to investigate two candidate applications of 

DR for severe weather and traffic. The study compared fixed boundary sectors to preplanned 

boundary adjustment. Twelve current, non-supervisory, full performance level ATCl Specialists 

from several different FAA facilities participated in the study. Researchers examined system 

effectiveness, controller workload, and subjective measures. 

The researchers’ approach to DR was to predefine specific airspace that could be dynamically 

allocated from one ARTCC to the other based upon the traffic situational demands (high traffic 

or severe weather). Subject Matter Experts provided airspace design and procedures, as well as 

briefed the participants on areas of responsibility during DR. In the weather scenario, weather 

patterns dictated a region of airspace to allocate from one sector to the other so that the controller 

receiving the new area could accommodate deviations around the weather. Researchers 

hypothesized that this would decrease the handoff, point out and communication workload for 

the controller. In the high traffic scenario, projected traffic loads in the north sector prompt a 

boundary adjustment to allow the south sector controller to assume responsibility for traffic on a 

jet route through the north sector and thereby balance controller workload. 

Researchers provided controllers with a 2-minute notification prior to the boundary adjustment. 

They had to complete all handoffs and communication transfers during these 2 minutes. At the 

end of the 2 minutes, the resectorized maps appeared on the radar display. 

Resectorization significantly decreased the number and duration of landline communications 

between the sectors compared to baseline. On post run NASA TLX measures of workload, both 

resectorized conditions resulted in significantly lower ratings than baseline. On the real-time Air 

Traffic Workload Input Technique (ATWIT) (Stein, 1985) ratings of workload, the weather 

resectorization resulted in significantly lower workload ratings. Controller subjective self ratings 

of their own performance were higher for the resectorized scenarios for both weather and traffic 

density. Subjective situational awareness ratings were significantly better for the weather 

scenarios after resectorization. 

Researchers observed that this was just an initial evaluation of DR in dynamic human-in-theloop simulation under two ideal conditions. They cautioned that a number of additional 

questions needed answers. These included: when resectorization should occur, how long it takes 

the controller to adapt, how often airspace can or should be resectorized, and can it be 

incremental. The authors pointed out that resectorizing too early may waste resources. Waiting 

too long may adversely affect the controllers’ ability to maintain safety and efficiency. Finally, 

the researchers raised the question of the situation specific character of the benefits and the need 

to identify where it might not be appropriate. 

1.1.3 Current DR Practices at FAA Facilities

Current resectorization regularly occurs when sectors are combined or decombined depending on 

traffic demand within all ARTCCs. There are, however, other numerous examples of airspace 

being delegated to the control of another facility for specific uses. The MITRE Corporation’s 

CAASD developed a Casebook describing situations in which DR was already being used in 

FAA facilities within the limits of the Host computer (MITRE, 2000) as a follow on to the MIT 

conceptual reports on DR. These situations employ a number of structured, preplanned 
6 

activities. Facilities establish the airspace boundaries and procedures, a priori, and incorporate 

them into the facilities’ policies and training. The receiving facility gets new maps and when 

appropriate even flight progress strips. CAASD’s casebook focused on six specific examples of 

DR implementations including equipment outage, weather, special use airspace, airport 

configuration change, traffic volume, and oceanic track change. The examples involved 

transferring control of predefined volumes of airspace to another facility. The examples are 

summarized here. 

Equipment outages can result in reduced service to particular areas. CAASD’s casebook 

(MITRE, 2000) presented an example of an area in Miami ARTCC’s (ZMA) airspace with radar 

coverage by one long-range radar. When the radar is out of service due to scheduled or 

unscheduled maintenance, aircraft in parts of the airspace are not visible under ZMA radar 

coverage. In this condition without resectorization, controllers must apply non-radar separation 

procedures, which require greater spacing between aircraft and cause delays. Jacksonville 

ARTCC’s (ZJX) radar provides radar coverage for the affected ZMA airspace. To avoid the 

non-radar control condition and ensure radar service, both ARTCCs defined a volume of airspace 

to accommodate north and southbound traffic flows for which they could conduct a limited DR. 

Designated the HOBEE area, this rectangular airspace covers between FL240 and 600. When 

the ZMA radar is out, control of the HOBEE airspace is delegated to Jacksonville ARTCC. ZJX 

controllers provide radar separation to the aircraft transitioning HOBEE and handoff to the next 

ZMA sector. The ZMA controller accepts the handoff when he/she sees the aircraft has been 

acquired by another radar covering that Miami airspace. This arrangement provides seamless 

radar separation rules for flights instead of reverting to non-radar rules or rerouting around the 

area. 

In the current HOST automation, ZJX only needed to develop new maps in their airspace 

adaptation database to show HOBEE. When HOBEE is delegated to ZJX, they just display the 

maps and traffic at the appropriate controller position. CAASD documents that both ARTCC 

Traffic Management Units (TMUs) coordinate the delegation based upon procedures precisely 

specified in a Letter of Agreement (LOA). When the radar is taken off-line in a scheduled 

outage, the TMUs coordinate with the affected areas and determine a time for the change over. 

If the radar outage is unscheduled, controllers may coordinate with each other before the official 

switch occurs. ZJX already has flight progress strips for the aircraft transiting HOBEE. 

Automated features like automatic handoff work across facilities and will flash the target to the 

next ZMA controller. 

Thus, through predefined airspace, plans and procedures, training, airspace can be dynamically 

resectored using current automation to accommodate traffic demands. In this arrangement, 

however, everything is clearly structured, planned, and trained. ZJX controllers are certified like 

for any other airspace to work the HOBEE airspace prior to being assigned to work traffic. 

CAASD notes that the guidelines for static airspace design are easily adapted to those dynamic 

procedures. Also there is obviously more to airspace planning and management than traffic 

demand alone. 

Weather provides one of the most compelling cases for unlimited DR capabilities. Weather, 

alone, accounts for seventy percent of the delays in the NAS (Ahlstrom & Della Rocco, 2003). 

Air traffic controllersl must regularly reroute traffic flows around weather to avoid storms or 
7 

turbulence. From day to day, flights request routing to minimize or maximize effects from 

winds. Unlike the previous case of radar outage, the unusable airspace changes and is less 

predictable. 

The CAASD Casebook presented an example again from Florida airspace. Florida is notorious 

for the summer thunderstorms that build in the afternoon. Jacksonville and Miami Centers have 

defined airspace, MALET, on their boundaries, which can be reassigned from ZMA to ZJX to 

accommodate Orlando arrivals and departures. As storms pass north of the airport, controllers 

move the departure flow toward the east, and this may encroach on an arrival stream from the 

northeast. Without DR, moving the arrival stream to the east to make room for the departures 

requires rerouting the arrivals through ZMA’s airspace. For reroutes, controllers must coordinate 

the new routes with other controllers, as well as enter the new route into the HOST computer, 

creating a large workload if traffic volume is high. The MALET airspace sits east of Orlando 

airport and on the north side of ZMA airspace. When ZMA allocates the MALET to ZJX, 

controllers can route traffic through it using vectors instead of the formal reroute and 

coordination. As was the case with a radar outage, the airspace could be predefined in this case 

because the pattern of storms was recurrent and predictable enough. Traffic management then 

entered MALET in the HOST airspace adaptations and maps at both facilities. A negotiated 

Letter of Agreement (LOA) defined the procedures for delegation of the airspace. Specifically, 

ZJX requests MALET when storms are present. The ZMA TMU sets an appropriate time for the 

transfer of control based upon their traffic situations. CAASD authors noted that ZJX and ZMA 

controllers may informally coordinate prior to the formal transfer. Along with weather issues 

special use airspace generates additional opportunities for resectorization. 

Special Use Airspace (SUA) is a predefined airspace with restrictions on its use. Examples are 

Military Operations Area (MOA), Restricted and Warning Areas (Aeronautical Information 

Manual, 2004). A MOA is airspace established to indicate areas where the military is 

conducting activities, such as air combat, aerobatic, or low altitude tactics, and separate 

nonhazardous military activities from IFR traffic or identify where activities are occurring for 

VFR traffic. Restricted areas limit the use of the airspace and usually are established to protect 

traffic from the existence of unusual, often invisible, hazards to aircraft such as artillery firing, 

aerial gunnery, or guided missiles. Warning areas are predefined airspace over domestic or 

international waters identifying an area where there are activities that could be hazardous to 

nonparticipating aircraft. Many of these are jointly used until they go active. When these SUAs 

become active, traffic that would otherwise transit the airspace must be moved. CAASD 

presented the example of a situation created by a Kennedy Space Center launch. For certain 

types of rocket launches, the airspace over the Atlantic Ocean east of the launch site is closed. A 

large number of warning areas allocated to the Department of Defense (DOD) along the Atlantic 

Coast complicate this because the normal over water routes are east and above the DOD warning 

areas. Controllers reroute air traffic back over land north of the warning areas causing an 

increase in workload for northern ZJX controllers as the ZJX sectors become congested. To 

alleviate this situation, ZJX, ZMA and DOD negotiated ZJX access to a corridor of airspace 

through a warning area and including a small piece of ZMA airspace. 

Airport configuration changes present another example in where controllers can effectively use 

resectorization to improve efficiency. The CAASD authors described the example of wind shifts 

that cause reconfiguration of an airports and change traffic flows in overlying airspace. Sector 
8 

boundaries may be extended off the departure end of the runway to allow for reduced 

coordination for climbing aircraft. CAASD presented the example of shifts in airport 

configurations at Atlanta and Charlotte airports. Reconfiguration of these airports affects the 

traffic flow of departures into the Atlanta ARTCC (ZTL) airspace. Depending upon the 

operation, east or west, aircraft departing an airport to the opposite direction of their filed route 

would need to turn back. The system can hold aircraft at lower altitudes in Terminal Approach 

Control (TRACON) airspace until the turn was completed to minimize the controller 

coordination required. The lower altitudes interrupted the aircraft’s climb and increased fuel 

consumption. ZTL designed the airspace to accommodate departures relative to the 

configuration of airport operations by resectorizing. Charlotte had a north-south operation and 

similar issues. Because Charlotte’s airspace is next to Atlanta’s, the resectorization included 

consideration of both airports’ configurations. ZTL designed 4 new sets of maps for each 

combination of Atlanta and Charlotte operations. The new airspace involved changes in 

boundaries that included airspace from different areas of specialization. The maps clearly 

indicated the controller’s airspace, the neighboring airspace and other key information so that no 

confusion resulted for the controllers. Switching to the new configuration involves coordination 

among all affected sectors and facilities prior to switching. 

The fifth example of application of DR CAASD presented in the Casebook was due to 

fluctuations in traffic volume. Sectors are regularly combined or decombined to accommodate 

changes in traffic volume. CAASD described a much more complex example than the usual 

from Minneapolis ARTCC (ZMP). In the example ZMP experiences a number of unique 

scheduled and unscheduled events. The Oshkosh Fly-in is a scheduled event in which traffic 

escalates one time per year. Midwestern thunderstorms can disrupt normal traffic flow into 

Chicago and Detroit. CAASD authors noted that entire O’Hare inbound and outbound streams 

that would normally be managed by Cleveland ARTCC could be routed through ZMP airspace. 

In addition, hunting and fishing seasons bring heavy traffic. ZMP has very large sectors that 

may require multiple frequencies. In addition, they did not have enough workstations to 

accommodate all the split sectors they needed. As in other cases, the airspace adaptation 

designed the FPAs to allow resectorizing. However, due to the workstation limit, an FPA may 

be assigned to a specifically designated area workstation, instead of the adjacent workstation. 

These two items lead to unique issues in this resectorization strategy, in which the supervisor 

must ensure that flight progress strips and frequency assignments are routed to the appropriate 

workstation. CAASD noted that a controller might need to manually route the strips to the right 

printer. 

Finally, CAASD presented the example of Oceanic Track Changes, which result in dynamically 

changing airspace configurations. The winds aloft determine the most effective routes for 

oceanic traffic daily. When the winds shift, the optimal routes may also shift and result in 

changes in the oceanic tracks to take advantage of the winds. Most traffic in Oakland ARTCC’s 

(ZOA) oceanic airspace travels east to west across the Pacific Ocean. ZOA reconfigured their 

oceanic sectors to match the east-west tracks, such that each track was controller by only one or 

two sectors. This minimized the number of handoffs and workload for each controller and pilots. 

As the winds aloft shift, they adjust the sector boundaries to accommodate shifting tracks. 

CAASD notes this may occur a couple of times per day. ZOA found a work around in the 

computer to allow the total number of FPAs required to be flexible. At the time of the report, the 

same oceanic geographic map was used all the time in the ZOA oceanic area, so no new maps 
9 

were required. The supervisor drew temporary airspace lines on the map so controllers could 

visualize the airspace as new tracks were developed. 

All of these examples of limited DR are in use in today’s airspace. In closing their report, the 

CAASD authors identified some topics addressing concerns noted by facilities when they 

provided the examples above. After listing a number of candidate situations, the authors 

suggested that when the “effects of an identified factor are consistent each time the situation 

occurs, with predictable impacts that standard airspace configurations do not address, the LDR 

techniques presented in the six cases may be helpful (CAASD, MITRE, 2000, p. 4-2).” The 

CAASD authors pointed to existing airspace management guidelines, which include periodic 

refresher training, periodic evaluations to see if additional situations qualify, and periodic 

evaluation of the strategy. They noted that LDR is unlike a one-time airspace change, because 

LDR requires procedures for applying the strategy, which should cover the criteria for activation 

and the roles and responsibilities of the participating controllers. 

Most researchers who have worked with resectorization concepts and have included controllers 

in their process see both possibilities and limitations with DR. From the beginning of formal 

ATC, controllers themselves have been both the strongest asset (we have not been able to replace 

them with machines despite predictions to the contrary over twenty years ago), and they also 

represent the limitations that human beings bring to system operations. 

1.1.4 Human Factors Issues

The NAS is a highly structured environment. Structure provides benefits including predictability 

for the decision-maker, in this case, the air traffic controller. When something is unusual, it is 

easily spotted as out of the ordinary because of the brain’s capacity for pattern recognition and 

pattern anomalies. Expertise in ATC pattern recognition does not develop quickly. In the 

current system, it takes En Route controllers an average of about 3 years to certify as Certified 

Professional Controllers (CPCs, formerly Full Performance Level). In general, Enroute 

controllers, for example, must learn and checkout on at least 6 different sectors to become CPCs. 

As noted in the CAASD examples, there are situations when the structure is lost and typical 

patterns cannot be achieved as with weather systems and equipment outages. DR offers a tool in 

these situations to mitigate the loss of some of the decision makers' usual options. In our current 

system, DR options are highly structured, adding the benefits of that structure back to the 

controller’s strategy options. This section discusses the human factors issues associated with the 

controller and DR. 

1.1.4.1 Mental Models of the Airspace

A mental model can be defined as the mental structure within which an individual understands 

how a system works with respect to its internal components and processes (Kieras & Bovair, 

1984). In many respects, the mental model drives both prediction of system’s future states and 

problem solving. (Gott, Lajoie, & Lesgold, 1991). Controllers must rely on their mental model 

of the airspace and the air traffic patterns through the airspace to successfully and safely control 

traffic (Redding, Ryder, Seamster, Purcell, and Cannon, 1991). Controllers refer to their mental 

model as the "Picture". Recent efforts to remove structure from the system have been proposed 
10 

in Free Flight efforts and have raised safety concerns (Nunes, 2003). Controllers fear loss of 

their picture and sufficient situational awareness to maintain separation. 

Researchers have reported several studies of controllers’ conceptual structures. Mogford, 

Murphy, Roske-Hofstrand, Yastrop, and Guttman (1994) used two scaling techniques to 

investigate the controller’s internal representation of the relationships among ATC system 

elements. The researchers developed a list of 17 concepts from an ATC task analysis 

(Ammerman, Bergen, Davies, Hostetler, Inman, & Jones, 1987) and asked 11 controllers to 

provide ratings of relatedness between all possible pairs. They divided the 11 controllers into 

“high timers” (25-35 years experience) and “low timers” (5-10 years experience). Five 

individuals with no ATC experience also provided ratings. Researchers used multidimensional 

scaling (MDS) to infer the underlying dimensions among the concepts and Pathfinder network 

analysis to explore the importance of local relationships between concepts. The MDS derived 

two dimensions, planning and context. The pathfinder network analysis generates network 

diagrams among the concepts. Analyses demonstrate associations, centrality of concepts and 

cycles in which a concept is a beginning and end. The researchers found for example that high 

time controllers organized other concepts around weather. They found clear differences between 

the controller groups and the control group. In addition, they found subtle differences between 

the two controller groups. Experience leads controllers to more effectively structure their 

environment to temper the task load and make events more predictable. Sector structure has an 

impact on organization and how controllers think. Other studies as well have also focused on 

controller cognitive processes. 

Fields, Wright, Marti and Palmonari (1998) examined ATC using a distributed cognition 

analysis. This analysis regards cognition as a property of a system of individuals, such that the 

unit of analysis, in this case, is the group of controllers working on the approach sector, the pilots 

under their control and the artifacts that are part of their work. They describe ATC as 

“undertaking a computation to maintain separation between aircraft in a region of airspace, and 

at the same time attempting to improve the routing of aircraft through airspace” (p. 2). To do 

this, the authors suggest that the system has three parts. First, a controller is “looking for near 

intersections of four-dimensional trajectories (3 spatial dimensions and time), or rather looking 

for times where two 4D trajectories are not separated in 3D space” (p. 2). Second, a controller 

takes some action when a future conflict is detected within a number of possibilities (changing 

altitude, speed, or route or one or both aircraft). The controller continues to update the 

projection of the situation into the future to incorporate changes and new influences. The 

researchers point out that although solving the conflict detection problem is complex, they 

discerned a number of constrained special cases, which simplify the 4D problem. Recent efforts 

to remove structure from the system have been proposed in Free Flight have raised safety 

concerns (Nunes, 2003). 

Researchers at MIT (Davison, Histon, Ragnarsdottir, Major, & Hansman, (2003); Histon, 

Hansman, Aigoin, Delahaye, & Puechmorel (2002)) examined the concept of structure in 

airspace and procedures to support the reduction of cognitive complexity for controllers. 

Structure constrains the dynamic changes in the environment and, thus, reduces the cognitive 

load for controllers. When taskload increases to a point that controllers feel loaded they will 

impose more structure of their own by flying aircraft strictly by their flight plans and declining 

requests for change wherever possible. 
11 

Histon, et al., noted that much of the structure in air traffic today is codified in standard 

procedures, regulations, airways, and airspace boundaries. Thus, application of the structures 

helps limit and simplify cognitive complexity. For example, airways limit the number of entry 

and exit points in a sector. These researchers examined the use of structure to reduce cognitive 

complexity through subjective measures, system state measures, examination of the codified 

structure, communications and an empirical probe. Through interview with controllers at a 

number of facilities, the researchers identified key factors influencing complexity within three 

categories of airspace, traffic and operational constraints. From the interviews, they identified 

hard and easy sectors. Using Enhanced Traffic Management System (ETMS) data from the 

sectors, they demonstrated that the “hard” sector had more crossing and vertical transitions than 

the “easy” sector. They also observed, however, that a higher percentage of traffic was on 

standard routes (43%) in the hard sector, than in the easy sector (20%). The researchers found 

this to be consistent with the reduction of complexity through use of standard routes. They noted 

controllers develop an abstraction based upon an aircraft’s membership in a standard traffic flow, 

which is the foundation for managing high-density traffic. This allows the controller to manage 

the stream, rather than each individual aircraft, reducing cognitive complexity. The authors 

presented the approach pattern to Chicago O’Hare and demonstrated that there are three or four 

major merge points in the flow patterns. These points reduce the multidimensional aspects of 

merging numerous aircraft to a single dimension of separating by time over the merge point. 

Within the codified aspects of the NAS, the authors note that much of ATC is explicitly designed 

and documented in charts, as well as procedures and even dynamic elements, such as traffic 

management restrictions. 

In order to validate the structural abstractions, the researchers conducted an empirical probe in 

which the structural basis was manipulated as an independent variable. Using a simplified ATC 

task, participants were asked to use only speed control to achieve miles-in-trail separation when 

leaving the sector. The researchers manipulated the number of incoming streams and merge 

points. They either had one collocated merge point or multiple. While college students were the 

participants, results demonstrated that collocated merge points resulted in fewer separation 

violations, fewer speed changes, and lower difficulty ratings. 

It would seem that from a controllers perspective simpler is better. Also they see structure as a 

powerful tool to help them maintain control. Movement away from structure and towards more 

flexible resectorization will undoubtedly face resistance unless developers can show controllers 

how it improves their situation rather than making it more complex. 

1.1.4.2 Situational Awareness

The above section addressed the mental model in which we focused on structure of the airspace 

to support the model. This section addresses situation awareness to focus on the involvement of 

time and prediction. Endsley (1988) defines situation awareness (SA) as the “perception of the 

elements in the environment within a volume of time and space, the comprehension of their 

meaning, and the projection of their status in the near future.” She indicated that the basis of a 

controller’s SA is derived from the elements in the environment, such as displays, readouts, and 

communication channels (Endsley & Rodgers, 1996; Endsley & Smolensky, 1998). She 

additionally noted that SA is moderated by his or her capabilities, training, experience, 

preconceptions, objectives, and taskload. 
12 

The MIT (Wilhelmmsen, et al., 1999b) report provides some requirements for supporting SA in 

any DR system. They cite Endsley’s 3 levels of SA and the importance of providing support for 

each level in any system that uses DR. The first level of SA involves perception of the elements 

in the environment. Therefore, to ensure adequate SA in any DR system, the system must be 

able to perceive the status, attributes and dynamics of relevant elements in the environment. 

They emphasize that the controller must be able to clearly see the airspace on the screen for 

which he or she is responsible. The second level of SA is comprehension of the current 

situation. This involves understanding the meaning and significance of the elements in the 

environment. The MIT authors emphasize that it is necessary to facilitate the relationship of 

traffic to the new boundaries through enhancing attention and comprehension. Level 3 SA is the 

projection of future status and behavior of the elements in the environment. With DR, aids must 

ensure that decisions are made on the basis of the sector currently in operation as well as any 

history and prospective issues that will need to be handled regardless of sector geometry (i.e. 

clearing an aircraft to a new altitude at a time or location the controller planned for to avoid a 

potential conflict and not being distracted by the resectorization process). Maintaining SA 

requires effort on the part of the controller and this can add to his or her workload. DR may 

increase or decrease load depending on its implementation. 

Visual display of sector maps may become an issue for both the reestablishment of correct 

mental models and Situational awareness. As sector geometry changes in resectorization a 

change in the map provided to the controller could reduce mental workload centered on trying to 

code and store new geometry. Unfortunately according to Cochran et al. (1999a) both Host and 

the Direct Access Radar Channel DARC limit the number of available on line maps. There are 

also map size limitations both as well as in the Design System Replacement DSR system. This 

could have a direct impact on controller SA when trying to work traffic in transition and hold the 

map boundaries in working memory. This issue may or may not have been resolved through a 

NAS Change Request NCR in the intervening time since the MIT reports were written. 

1.1.4.3 Workload

A key concept of DR is to offload controller’s workload by redistributing traffic in a dynamic 

fashion. With today’s traffic management tools, is more possible than ever to dynamically 

distribute traffic. Again, the MIT report (Wilhelmsen, et al., 1999b) defined guidelines for a DR 

system to mitigate and more critically avoid creating a workload overload situation for a 

controller. The authors note that a sector demand metric must map reliably and consistently to 

controller workload under all conditions and must be relatively insensitive to variations in 

unpredictable aspects of the traffic. 

Hadley et al., (2000) raised a critical question which interrelates system dynamics and the human 

decision making process of when to resectorize. They expressed this question as follows: 

"However, several studies are needed to explore different approaches to dynamic 

resectorization. In addition, there are a number of operational and technical questions 

that need to be addressed. In particular, when should you resectorize? The goal of 

investigating this question is to determine the optimum point for resectorization to occur. 

Too early, is an inefficient use of resources, while changing airspace after the 

controller(s) is/are already busy may have negative consequences in terms of his/her 
13 

ability to safely and efficiently control traffic. A predictive capability, such as a measure 

of airspace complexity (i.e., dynamic density), would not only have to account for the 

sector complexity at some predetermined look-ahead time but would also be required to 

incorporate the transition time of the system to accommodate the resectorization process. 

For example, if a dynamic density index indicated that the sector complexity was going 

to exceed a given threshold in xx minutes and the transition period for the resectorization 

process is 30 minutes, then the controller working this sector should receive notification 

30 +xx minutes in advance. Is this amount of time sufficient for the controller to work in 

and become comfortable with this new airspace configuration before the predicted rush? 

This amount of time may be more than adequate, or controllers may require considerably 

more time to become accustomed. It is this period of time that needs to be addressed in 

simulation. (p. 25)." 

Human workload is an important issue in any person Machine System. Psychologists have been 

studying the construct for over 50 years. We know that controllers are motivated professionals 

and that they will continue to work harder as taskload increases. However, even they can be 

overwhelmed and at that point performance declines. DR may help manage taskload under some 

specific conditions and assist planners in optimizing workload. We know that workload can be 

too high and history demonstrates it can also be too low since most systems errors occur at low 

to moderate levels of traffic. When DR is used it will have to consider both current levels of 

demand and how that demand is redistributed between the new sectors as defined by the system. 

1.1.4.4 Communications

Communications take many forms in an active ATC facility. Requirements for communication 

and coordination are inherent in the job of the controller. According to Williamson et al. (1999): 

"The sector controller must maintain a current awareness of all activities, events, and 

conditions in his/her airspace, such as flight restrictions, reroutes, military operations, etc. 

He or she must also coordinate aircraft movements with adjacent sectors and facilities as 

required. This coordination may be done by landline, by interphone, or by direct verbal 

communication if the control position of the adjacent sector is close enough. 

Coordination activities related to traffic crossing, or flying close to, sector or facility 

boundaries can make up a large part of a controller's workload. They include handoffs, 

point-outs, interfacility coordination, and ground-to-air communication. The need to 

limit this boundary-related coordination and communication workload is an important 

consideration in the sector design process. While it is possible to take this into account 

when there is consistent structure in the traffic flow, it becomes increasingly difficult to 

do so when the traffic is more variable (p. 13).” 

Any change to sector structure whether preplanned or dynamic influences who the controller 

needs to talk with and when. If the changes and points of contact are not completely and instantly 

apparent there is the possibility that coordination may breakdown leading to systems errors. The 

more dynamic the system the more important it will be to provide the controller with the critical 

information he/she needs on or preferably before any sector transition occurs. 
14 

1.1.4.5 Some Proposed Workarounds

Cochran et al. (1999b) interviewed controllers in operating facilities and proposed some 

technology work arounds that may facilitate the use of resectorization at least on a limited basis. 

Controllers are generally very bright and often creative people; this is especially true when they 

are told to use systems and procedures that may not be all that they could have wanted if they 

had been involved in the original design. 

One controller proposed using a dummy data tag on a radar display as a memory jogger that an 

airspace change would occur at a point in time. This would involve starting a track on an aircraft 

with no velocity and entering text in the tag as a reminder that for example an piece of Special 

use airspace would be activated at time “x.” This would only be useful for anticipated actions 

that might be foreseen under LDR for example. 

Some controllers today use grease pencils directly on the PVD surface to indicate altered 

boundaries and other relevant information. Apparently they cannot do this on DSR monitors but 

some use a dry erase marker, which is discouraged but undoubtedly used by some controllers. 

Given additional requirements with no underlying technology controllers will often find a way to 

make it work and stay within safety parameters. However implementing procedural change with 

out supporting technology is not a systems engineering approach. It can lead to unique solutions 

that are not transportable to other facilities and operations. 

1.1.4.6 The Search for Resectorization Triggers

Over the years both within the human factors community and beyond researchers and 

mathematicians have been looking for a method to operationalize decisions in order to manage 

the load and the subsequent workload of controllers. First line ATC supervisors routinely watch 

the traffic flow in order to determine if they need to add people to sectors and or combine or 

decombine those sectors. This is a subjective process honed by years of experience working as 

an operational controller then as a supervisor. While there are checklists to compute complexity 

(which generally is interpreted uniquely by each controller) supervisors make decisions based on 

what feels right for the moment. Mogford, Guttman, Morrow, and Kopardekar (1995) after 

interviewing many controllers drew the conclusion that complexity, despite an official FAA 

definition, is and was a subjective construct used by control personnel in their own unique way. 

Dynamic density DD is another construct that materialized in the 1990s with various 

formulations by different researchers. Developers looked for a math model that could capture 

current operations and anticipate the future so that both controllers and traffic management units 

could better balance the operation for smooth functioning and eliminate bottlenecks or choke 

points. Laudeman, Shelden, Branstrom, and Brasil (1998) developed one model at NASA. Their 

definition was as follows: 

"The definition of metric of air traffic controller workload based on air traffic 

characteristics is essential to the development of both air traffic management automation 

and air traffic procedures. Dynamic density is a proposed concept for a metric that 

includes both traffic density and traffic complexity. It was hypothesized that a metric 

that includes terms that capture air traffic complexity will be a better measure of air 
15 

traffic controller workload than current measures based only on traffic density. A 

weighted linear dynamic density function was developed and validated operationally. 

The proposed dynamic density function includes a traffic density term and eight traffic 

complexity terms. A unit-weighted dynamic density function was able to account for an 

average of 22% of the variance in observed controller activity not accounted for by traffic 

density alone. A comparative analysis of unit weights, subjective weights, and regression 

weights for the terms in the dynamic density equation was conducted. The best predictor 

of controller activity was the dynamic density equation with regression-weighted 

complexity terms. (Abstract)." 

Kopardekar and Magyarits (2003) defined dynamic Density as “... the collective effect of all 

factors, or variables, that contribute to the sector level ATC complexity or difficulty at any given 

time (p. 1.)" They noted that DD was a construct with much in common with "Complexity" and 

or the difficulty of operating the sector. These researchers collected traffic samples from a 

number of centers a ran linear regressions between DD variables in various combinations against 

complexity ratings of the same sectors at the same time frames. The results indicated, "A unified 

DD metric composed of variables from several organizations performed the best. The results 

indicated that DD represents instantaneous sector complexity better than aircraft count, which is 

the currently used method. The results also indicated that the prediction of complexity using DD 

is somewhat better than the prediction using aircraft count most likely due to the inherent 

inaccuracy of predicting aircraft count (p. 1)." 

Smith, Scallen, Knecht, and Hancock (1998) used the DD concept to predict collision risk within 

a sector of airspace. While their model evaluated using a series of sensitivity analyses focused 

on the cockpit the authors believed it was applicable to Enroute ATC. They proposed that traffic 

management units in ARTCCs could use the technique for adjusting the criteria they use for 

computing the critical capacity of sectors. 

The FAA does have a strategic planning tool used by traffic management coordinators for 

planning purposes. This is called Monitor Alert or M/A and is a component of the Enhanced 

Traffic Management System (ETMS). M/A analyzes traffic demand for airports, sectors and 

reporting fixes. It compares current and projected demand against capacity and provides alerts 

when the load may exceed capacity (NASA, 2005). The criterion for an alert is known as the 

Monitor Alert Parameter (MAP). These are computed based on average sector flight time. The 

longer the flight time the less efficient and more loaded is the sector. This leads to a higher MAP 

more likely to trigger an alert. For the more serious alerts the coordinator may notify the 

area/sectors affected. Primarily traffic management coordinators to take strategic actions to 

avoid sector overload use this information. It does not appear that tactical supervisors routinely 

have the information or use it currently to make resectorization decisions (Phillips, personal 

communication, April 15, 2005). 

There was an interesting addendum to this issue of automated planning and alerting tools. A 

near miss situation occurred in airspace that was using the M/A tool a number of years ago. 

National Transportation Safety Board (NTSB) investigators noted that the MAP is supposed to 

be updated and adjusted periodically to reflect current conditions and it had not been so the 

controller working the Bradford sector may have been overloaded. The investigators went on to 

say: 
16 

"The FAA is increasingly relying on the use of automated systems, such as ETMS, for 

traffic monitoring and demand assessment. Much of the data used by ETMS to perform 

trajectory estimation and other predictive tasks are sent to the system as an indirect result 

of NAS computer entries made by controllers. However, air traffic controllers questioned 

about ETMS and monitor alerts appeared to have very little knowledge about ETMS 

processing and what information it uses to produce its predictions. The Safety Board is 

concerned that if controllers do not understand the effects that their actions have on the 

data made available to ETMS, they may inadvertently mislead the system and reduce its 

effectiveness, as occurred in this case. Therefore, the Safety Board believes that the FAA 

should provide air traffic controllers with annual refresher training designed to ensure 

that they understand the relationship between NAS and ETMS, including an overview of 

ETMS predictive functions, the data flow and message types exchanged between NAS 

and ETMS, and the various factors that may affect the accuracy of ETMS predictions. 

The excessive traffic level in the Bradford sector was discovered only as a consequence 

of the subsequent operational error investigations and would possibly not have been 

discovered or received management attention if the error had not occurred. The FAA 

appears to have no formal process for the identification and investigation of situations in 

which a sector is subjected to excessive traffic demand when no otherwise reportable 

event takes place. The Safety Board is aware that the FAA has implemented a process 

for reporting and tracking ATC equipment problems through Unsatisfactory Condition 

Reports (UCR); however, no similar system exists for procedural problems. Therefore, 

the Safety Board believes that the FAA should establish a formal method for ATC 

personnel to report instances in which sectors become overloaded (similar to the UCR 

process), so that the circumstances causing or permitting overloading can be identified 

and addressed (NTSB 2000, p. 5.)."

It appears that automated tools without both initial and recurrent training may not do what is 

expected of them. Further, facilities using the tools require clear procedures for implementation 

and updating of the required data fields as the situations change. 

1.2 Discussion 

NAS is a complex person machine system, which will undoubtedly evolve as the demand for 

airspace and services continues to grow. Predictions are in accordance with projections from the 

FAA and other affiliated organizations such as RTCA (2002). 

The NAS is built upon the performance of humans in both the cockpit and controllers. In the 

current system, it takes En Route controllers an average of about three years to certify as CPCs. 

Thus, both the agency and the individual invested quite a lot in achieving the workforce we have 

today. They do what they do quite well given the technology they have and the training that is 

used to bring them up to speed. There are many organizations and individuals who have a stake 

in airspace and how it is used. 

RTCA (2002), a private not for profit interest group, focused primarily on the airspace users. 

The authors of their documented concept of operations for the future indicated that while human 

operators in a ground control function perform best at higher order decision making they have 
17 

finite limits in terms of the number or aircraft they can monitor and deal with at any point in 

time. RTCA anticipates that automation will increase the efficiency to the point that controllers 

would only intervene by exception while maintaining responsibility for separation and higher 

order decisions. It is somewhat ironic that these same words or words of the same intent 

appeared in documentation on the Automated Enroute ATC Automation system (AERA) over 

twenty years ago when predictions for the capabilities of computers suggested that controllers 

would be obsolete in a few years. Yet controllers still make tactical decisions in today's system 

and when they reach a load point where things are too busy or too slow there are manual 

techniques for rebalancing the load. 

In the current system resectorization exists and is used effectively in response to changing traffic 

flows and weather. The system provides some guidelines and procedures for how this is done 

and controllers know what to expect based on past experience in airspace with which they are 

familiar. In today's system resectorization decisions are made by first level supervisors based on 

their experience with the airspace and the people working in it. Guidelines are often implied 

rather than written and despite the idiosyncratic aspects of this, it seems to work rather well. In 

the future it may be possible and even feasible to expand low technology resectorization under 

the limitations that structure is maintained and controllers expectations are not violated. Written 

procedures and standardized training may facilitate this. However, further use of limited or 

unlimited DR with more dynamics may require additional systems innovations and new 

technology. Issues concerning the appropriate triggers for resectorization remain to this day. 

While there are math models that attempt to identify current system demand and anticipate future 

demand, they generally emphasize system variables and essentially discount the human operator. 

Math models applied systematically are likely to be better than completely arbitrary or from the 

gut decision processes, they fail to capture both the strengths and weaknesses of the operators 

currently working. It seems likely as resectorization plans and procedures develop along with 

new technology, that both better models and decision criteria will be needed. The final decision 

on when and where to resectorize will likely remain with the supervisor on duty who hopefully 

has the best understanding of the people working the traffic and what they can and cannot do 

effectively. This involves a balancing of the technology and the human factor on an individual 

basis that likely will provide the best system performance so that resectorization decisions are 

not accomplished too early or too late. 

Some believe that the primary answer to all the airspace issues will be that: 

"Automation technology must be fully exploited in order to increase the traffic handling 

capacity of the airspace while at the same time giving the controllers and other ATC 

specialists the tools they need to ensure the safe, orderly, and expeditious flow of traffic. 

DR is one element of this new automation technology. Properly implemented and used, 

DR will allow the threshold between free flight and organized traffic to shift in the direction 

of free flight. Such a shift is likely to produce significant economic benefits. (Williamson et 

al, 1999, p. 16)". 

In today’s NAS environment, DR provides a valuable tool for air traffic managers to maintain 

traffic flows while accommodating equipment outages, weather systems, and such. However, in 

the current system, the application of DR is limited by a number of factors, including limited fix 

posting areas in the HOST computer and limited numbers of frequencies. Within this limited 
18 

DR, the application is highly structured. Airspace is completely defined. The HOST adaptation 

databases are modified. New maps are presented to the controller. Letters of agreement and 

memoranda of understanding incorporate procedures for implementation. All controllers are 

trained. All parties coordinate the time of the switch. 

The operational concept for DR, however, is much more expansive than today’s limited 

implementation to include unlimited DR. We can certainly imagine a day when the computer’s 

limits could accommodate real-time adjustments of sector boundaries. Today’s controllers build 

expertise of the specific airspace they work, which includes knowledge of obstacles, radar 

coverage, traffic patterns and adjacent frequencies. Dynamically changing their control risks 

loss of situational awareness. 

As we move forward toward more flexibility in the NAS we must support the controllers’ 

situational awareness. Athenes, Averty, Puechorel, Delahaye, and Collet (2002) suggested that 

controlling air traffic might not be so much problem solving task, but more a perceiving and 

decision-making task. Therefore, an unlimited DR must clearly indicate what airspace a 

controller is responsible for at all times. That means a real time adjustment to sector boundaries 

must be displayed in real time to the controller. The controller must know what new frequencies 

to change aircraft to as they transition out of the new sector. Controllers in surrounding areas 

must know they have lost airspace to a new DR and know precisely and graphically what they 

control. This is problematic, because today’s sectors are not neatly shaped. Some have shelves 

and extensions that must be clearly depicted. In addition, the 2 dimensional displays do not 

indicate altitude strata under a controller’s responsibility. 

Adding structure and constraints to the resectorization can assist the controller’s situation 

awareness. For example, if a traffic flow must avoid a weather system dynamically, limit the 

controller’s responsibility to just that flow. Block the altitudes and inhibit crossing traffic. 

Ensure that surrounding controllers don’t clear anyone into the new airspace. 

To avoid the situation in which a controller may not be aware of obstacles or other airspace 

constraints, ensure that the DR occurs only in areas in which the controller has knowledge of the 

airspace. Training and practice in DR is critical in order for controllers to build expertise about 

potential problems. 

As the NAS develops, we need to off load secondary tasks from the controller. For example, one 

of the problem today’s controllers express about unlimited DR is the frequency change. Moving 

boundaries would leave the controller not knowing exactly which frequency to switch an exiting 

aircraft to. We could envision a day where frequency switches are automatic between ground 

and the flight deck so that this task could be offloaded from the controller and leave the 

controller to separate aircraft. Likewise, offloading verbal communication to the flight deck 

through a data link technology would also free up controller cognitive resources to manage 

separation tasks. In fact, to accommodate an unlimited DR, even more supportive methods of 

communication, such as the ability for a controller to just draw a new route and have it up linked 

to an aircraft, would offload communication and typing requirements for the controller, as well 

as ensuring precise accuracy in the new route. 
19 

The key to avoiding both controller underload and overload is a combination of good design and 

training. Training only supplements design and should not be used as a patch for a system that was 

not adequately thought out. The current use of resectorization in facilities is structured and scripted. 

It appears that controllers are trained to transition to combined and decombined sectors and have 

done this frequently. As the MIT reports point out advanced use of DR and unlimited DR will 

require new technology in addition to training. The technology will need to provide the controller 

with the information and SA he/she needs to safely move traffic in an expeditious manner. 

Most researchers and airspace developers agree that change will be a necessary and there will be 

continuing requirements for airspace management. They are unanimous that above everything safety 

has to come first, and that planners should consider all relevant variables when proposing changes. 

Change for its own sake is not desirable. Both structured change within Limited DR and Unlimited 

DR pose challenges for the controllers in terms of workload and situational awareness. As long as 

controllers are in the loop and responsible for the outcomes they will need both current and 

anticipated information so they can hang onto their accurate pictures in their working memories. 

Working memory is a limited capability. Experts have greater capacity than novices because in part 

they have learned how to organize the information, how to filter the extraneous, and methods or 

schemas that work most of the time within their realm of experience. LDR, which is trained for and 

expected, can build new schemas that once again will likely work most of time. Exceptions when 

anticipated can also be worked safely. It is the unanticipated especially in an overload or 

underloaded condition that could be very problematic. When underloaded the controller may simply 

operate on rote and miss key indicators. When overloaded, which can happen even in today's 

system, controllers can reach a point where they have no cognitive resources left to give. 
20 
21 

References 

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Acronyms 

AATT Advanced Air Transportation Technologies 

AOC Airline Operations Center 

ARTCC Air Route Traffic Control Center 

ATC Air Traffic Control 

ATCS Air Traffic Control Specialist 

ATWIT Air Traffic Workload Input Technique 

CAASD Center for Advanced Aviation System Development 

CHI Computer Human Interface 

CPC Certified Professional Controller 

CS Resector Command 

D-side Data (or Radar Associate) Controller Position 

DESIREE Distributed Environment for Simulation, Rapid Engineering, and Experimentation 

DOD Department of Defense 

DR Dynamic Resectorization 

DSR Display System Replacement 

ETMS Enhanced Traffic Management System 

FAA Federal Aviation Administration 

FL Flight Level 

FPA Fix Posting Areas 

HITL Human-In-The-Loop 

IPT Integrated Product Team 

LDR Limited Dynamic Resectorization 

MAP Monitor Alert Parameter 

MDS Multidimensional Scaling 

MIT Massachusetts Institute of Technology 

NAS National Airspace System 

NASA National Aeronautics and Space Administration 

NATCA National Air Traffic Controller Association 

NTSB National Transportation Safety Board 

R-side Radar Controller Position 
25 

RDHFL Research Development and Human Factors Laboratory 

RTCA Radio Technical Corporation of America 

SA Situational Awareness 

SME Subject Matter Expert 

TGF Target Generation Facility 

TLX Taskload Index 

TMA Traffic Management Advisor 

TOC Transfer of Communications 

UCR Unsatisfactory Condition Reports 

WJHTC William J. Hughes Technical Center 

ZOA Oakland ARTCC