Air traffic controller strategies in operational

Paper for the 30th EAAP Conference 2012, Villasimius, Sardinia, Italy.

Air traffic controller strategies in operational disturbances - An exploratory study in radar control Marian J. Schuver-van Blanken* & Jeroen J.G. van Merriënboer** * Human Factor Department, ATC the Netherlands **Department of Educational Development and Research, Maastricht University Summary Within a wide range of operational disturbances, air traffic controllers are able to ensure safety whilst keeping the additional goals of efficiency and environment in optimal balance. Key elements to success in these situations are the controller strategies underlying air traffic control (ATC) task performance. Controllers continuously use strategies to adapt their task performance to the characteristics and dynamics of the situation and the operational constraints. The scope of available research has predominantly focused on strategies used by controllers in response to an increase or decrease in traffic volume or density, in experimental settings on part-tasks or on standard (canonical) ATC situations. However, research also suggests that controllers use strategies in response to the characteristics of the situation, such as the weather situation or other complexity factors. Additional research is needed to couple and expand the controller strategies found in literature to the operational ATC environment. This paper provides the initial results of an exploratory study started at ATC the Netherlands (LVNL) focusing on the air traffic controller strategies applied in response to disturbed operational ATC situations in radar control at LVNL. Based on a replay of an operational disturbance that radar controllers handled in practice, controllers are retrospectively interviewed. On the basis of these data, strategies controllers use in operational disturbances are identified based on a list of strategies from literature, supplemented with new strategies found in practice. These results will provide input for an in-depth study to identify the strategies that are critical for ATC performance, both within the operation as well as in training. In addition, it provides a basis to incorporate strategy learning in ATC training as well as to fit the design of decision support tools to air traffic controller strategies. Introduction Air traffic control is characterised by the crucial role of the human in establishing and maintaining a safe and orderly flow of air traffic. In the complex dynamic environment of air traffic control (ATC), a high level of air traffic controller (ATCo) performance is required due to strict safety requirements, in combination with increasing efficiency and traffic intensity and growing environmental regulations, especially in serving busy and complex airports such as Schiphol Airport (Oprins, Burggraaf & van Weerdenburg, 2006). Air traffic controllers are able to successfully intervene in a wide range of situations often with conflicting goals, high dynamics and under high time pressure, in which their performance is one of the most significant elements of ATC to ensure safety and efficiency. Moreover, also with growing demands and increasing technology in air traffic control in the future, the air traffic controller proficiency remains crucial, as human operators have proven to be the most flexible and creative element to manage threats, errors and unpredictable events in a range of situations (SESAR, 2007). Major improvements in selection and in competence-based assessment in air traffic control training have yielded insights in the bottlenecks in the acquisition of air traffic controller competences (Oprins, 2008). One of the key elements to success in training appears to be the acquisition of strategies underlying air traffic controller performance (Nunes & Mogford, 2003; Oprins, 2008), as air traffic controller strategies are crucial in ATC task performance. Despite the fact that the importance of controller strategies in ATC performance and training is emphasised in literature, studies that deal with these issues are limited (Fothergill & Neal, 2008; Nunes & Mogford, 2003; Oprins, 2008). There is a need to identify the critical strategies for air traffic control performance. Air traffic controller strategies – overview from literature and research Strategies reduce the likelihood of overall task performance being compromised (Histon & Hansman, 2008; Loft, Sanderson, Neal & Mooij, 2007; Malakis, Kontogiannis & Kirwan, 2010; Mogford, Guttman, Morrow & Kopardekar, 1995; Nunes & Mogford, 2003). Controllers continuously adapt their strategies according to the characteristics of the situation (D’Arcy & Della Rocco, 2001), and to ensure safety whilst keeping the goals of efficiency, environment and their own mental workload in optimal balance (Oprins, 2008). A strategy is defined as a working method or specific class of air traffic control activities that achieves one or more objectives (e.g. safety, orderliness, expeditiousness) within a certain investment of time and effort (Loft et al., 2007). Based on available research, strategies involved in air traffic control can be categorized into the cognitive processes involved in air traffic control as distinguished in the ACoPOS model (the Air traffic controller Cognitive Process and Operational Situation model; Schuver-van Blanken, Huisman & Roerdink, 2010):

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Paper for the 30th EAAP Conference 2012, Villasimius, Sardinia, Italy. ATCo Cognitive process Situation AssessmentPerception Situation AssessmentInterpretation

Related types of strategy available from literature st nd rd 1,2,3,4  Selectively extract a/c data for conflict detection (altitude 1 , heading 2 , speed 3 )

Situation AssessmentAnticipation

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Attention & Workload management

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Solving situations

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Planning

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Taking decisions

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References: 1. Amaldi & Leroux (1995) 2. Bisseret (1971) 3. Nunes & Mogford (2003) 4. Rantanen & Nunes (2009)

Identifying patterns in a situation:  Grouping aircraft into categories or groups (e.g. standard/non-standard, 6,12 important/non-important, same characteristics) 1,6,7,8,10,11  Grouping aircraft into standard or non-standard streams or flows of traffic  Identifying critical points or hotspots (where conflicts typically occur or at intersection 1,6,8 of routes) Thinking ahead of possible threats (e.g. meteorological conditions, congested airspace, 5 aircraft malfunctions) to manage uncertainty 5 Play out mentally the progression of events 2,10,12 Increasing attention for aircraft depending on control action or situation 5,9,10,11 Switch attention between tasks on the basis of available time and importance Saving attentional resources to keep workload at an acceptable level (e.g. avoiding 9,14 monitoring) Conflict resolution 8  Refer to previously used solutions 12  Speed separation (within streams) vs. altitude control for aircraft separation Problem solving techniques 14  Narrowing or dividing a problem into smaller parts 14  Partially solving a problem and fine tune later 14  Formulating simple solutions (e.g. few actions, less coordination) Problem prevention 9,10,13  Increasing safety buffers to manage uncertainty or become more cautious  Preventing potential problems/mitigate consequences before they actually result in a 5 problem situation 5,9 Formulating a back up plan/alternative plan in case initial plan does not work Reverting to standard routings or a routinized/standard working method to ensure safety 9,10,11 instead of achieving efficiency Being selective in when to intervene, depending on probability or risk of situation (wait 1,9 and see vs acting immediately) 5. Malakis, Kontogiannis & Kirwan (2010) 6. Histon & Hansman (2008) 7. Redding, Ryder, Seamster, Purcell & Cannon (1991) 8. Seamster, Redding, Cannon & Ryder & Purcell (1993) 9. D’Arcy & Della Rocco (2001)

10. Sperandio (1971) 11. Sperandio (1978) 12. Gronlund, Ohrt, Dougherty, Perry & Manning (1998) 13. Loft, Humphreys & Neal (2003) 14. Flynn & Kauppinen (2002)

Table 1, List of air traffic controller strategies derived from literature categorized into ATCo cognitive processes

The scope of available research in controller strategies has predominantly focused on strategies in situations of high or low task demands mainly in terms of an increase in traffic volume or density. In addition, strategies found in research predominantly originate from experimental settings focusing on part task situations, research in standard (canonical) situations or from inventories on the basis of interviews or questionnaires. One study focused on strategies in exceptional situations (emergency and abnormal situations) (Malakis et al., 2010). Further research is needed to provide a classification of the various types of strategies and situations (Nunes & Mogford, 2003) within the operational ATC environment (D’Arcy & Della Rocco, 2001, Rantanen & Nunes, 2009, Fothergill & Neal, 2008). Coupling the strategies revealed from research to air traffic controller cognitive processes and to prototypical operational situations can yield the necessary step to be able to identify critical strategies involved in air traffic control performance. To ensure safety whilst efficiency and environment are not sacrificed, controllers need to employ a combination of strategies, adjusted to the characteristics of the situation as well as operational constraints. This way, the air traffic controller is able to successfully intervene in a range of situations requiring standard, disturbed or exceptional operation. The air traffic controller’s proficiency lies especially in effectively managing the operational disturbances and unpredictable events and fluctuations (e.g. Redding, Seamster, Purcell & Cannon, 1992; Schuver-van Blanken et al., 2010). Although research has provided strategies for standard operation as well as exceptional operation, research has not yet revealed which set of strategies are critical for especially handling traffic in disturbed operational situations, in which safety is ensured while efficiency and environmental constraints as well as own mental workload are continuously balanced.

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Characteristics of disturbed operation Standard operation comprises routine or standard traffic handling, whilst exceptional operation comprises extreme (non-routine) situations in which safety is maintained but efficiency of traffic handling is affected. However, daily air traffic control frequently is characterized by disturbed operation as a result of disturbances, unpredictable events or complex situations. Disturbed operation (in between standard and exceptional operation) requires adapted traffic handling, in which safety is ensured whilst efficiency is not sacrificed. Disturbed operational situations are defined as situations in which traffic handling has to be adjusted from standard traffic handling to a different traffic handling method. In this, traffic handling has to be (temporarily) adjusted or traffic streams have to be (temporarily) rebuild, while safety is ensured and optimal efficiency is aimed for. These situations frequently occur in ATC traffic handling, and are not only occurring in extreme or exceptional situations. The following prototypical factors characterise standard, disturbed and exceptional operation (Figure 1):

Figure 1, Prototypical factors characterizing standard, disturbed and exceptional operation

Disturbances in operational traffic handling can occur as a result of one single factor or a combination of factors that disturb standard operation in order to maintain safety and efficiency. In disturbed operation, traffic handling and working methods have to be adapted for a shorter or longer period to mitigate the disturbance. Examples of prototypical factors that may cause (temporarily) disturbed operation are:  Variability in aircraft performance: e.g. slow rate of climb, turn later than expected, aircraft performance characteristics;  Switch in operating mode: e.g. a planned switch in runway combination or switch from inbound to outbound peak in which traffic flows have to be rebuild or adapted to accommodate the switch from the old to the new operating mode;  Crossing/regional or non-standard traffic: e.g. aircraft from regional airports or crossing traffic in conflict with inbound/outbound traffic flows, missed approach or go-around, non-standard approach excluded from standard inbound flows;  Weather situation: e.g. heavy rain, wind, thunderstorms in the sector, reduced visibility;  High density or bunching of aircraft; e.g. high traffic volume or bunches in traffic flows that have to be merged, delays of aircraft resulting in bunches of aircraft in the inbound traffic flow.  Unexpected change in operating mode: e.g. sudden closure of a runway, an unexpected/unplanned change of runway combination in the inbound peak, sudden unavailability of airspace;  Technical problems: e.g. aircraft returning to airport due technical problems; loss of RT contact. Research questions To get further insights in the air traffic controller strategies that are applied in disturbed operational situations, we designed an exploratory study to identify the existence of cognitive strategies used by radar controllers in response to disturbed operational situations. More specifically, the exploratory study will answer the following questions: 1. Based on a list of categorized strategies compiled from literature: Which strategies do air traffic controllers use in disturbed operational situations? 2. Do air traffic controllers use other strategies in disturbed operational situations supplementary to the list of strategies from literature? And if so, which strategies do they use?

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Method Participants The focus of the study is on radar control, both within area and approach control at ATC the Netherlands. The aim is to interview ten expert air traffic controllers. Thus far, three expert air traffic controllers have been interviewed (mean experience 20 years as air traffic controller) on the disturbed operational situation they handled. Materials  Disturbed operational situations: Our exploratory study focuses on disturbed situations that air traffic controllers have handled by themselves during their operational duty. The categories of prototypical factors determining disturbed situations (see Figure 1) provides the guideline for determining the suitability of the selected disturbed situations.  Retrospective interview: To reveal the strategies used in the disturbed operational situation, a semistructured retrospective interview was held with the air traffic controller based on the critical decision method (CDM) (Klein, Calderwood & MacGregor, 1989). The CDM is a knowledge elicitation method in complex and dynamic environments, comprising a retrospective semi-structured interview technique to provide information on the processes underlying decisions (Hoffman, Cranda, Shadbolt, 1998), using probing questions to elicit thinking processes. A replay of the traffic handling in the disturbed situation based on a radar data recording system at ATC the Netherlands was used as a basis of the retrospective interview. Procedure Due to the fact that disturbed operational situations cannot be predicted, air traffic controllers are instructed on the kind of disturbances the study is focusing on, using the prototypical factors and examples of disturbed situations and are subsequently asked to log a disturbed situation they handled themselves during their work. After having logged the time, sort of situation and the sector by the air traffic controller, the suitability of the selected situation is evaluated by the researchers, by checking the presence of prototypical factors characterizing disturbed operation, using the categories of prototypical situations (Figure 1), together with questions focusing on the effects the air traffic controller has experienced (such as accelerating task execution, (temporarily) adapting traffic handling when routine traffic handling is not sufficient, unexpected changes, situations which turn out differently than planned or expected). Next, the replay of the traffic handling was made, followed by the semi-structured retrospective interview. Critical points or events in the traffic handling were identified together with the air traffic controller on the basis of the replay, and probing questions were asked to elicit the controller to express his/her thinking in that situation (for example: What was important information or were features you were noticing?; On the basis of which factors or principles did you choose your solution or course of action?). The retrospective interview was recorded and subsequently transcribed. Data analysis The transcript of the interview is divided into meaningful segments of sentences. Using a coding scheme in Excel, the meaningful segments are categorized into the ATC cognitive process involved. For each segment the strategy type is identified, based on the list of strategies compiled from literature as displayed in Table 1. Each category of strategies is extended by a category of ‘other strategy’ in case new strategies were identified supplementary to the strategies available from literature. Notes are incorporated for each segment providing more detailed information on the strategy in relation to the specific air traffic control situation and the underlying goal. In the case of ‘other strategy’, notes contain a characterization of the type of new strategy involved, in addition to the situation and goal. Initial results Although the exploratory study is still ongoing, we provide initial results of the research thus far. Three situations have been analysed with the concerning controller. Two situations that were selected by two other controllers were not suitable for the research, one because it was categorized as standard operation, the other because it concerned a distraction outside the radar view which influenced traffic handling. The analysed disturbed operational situations are different in their characteristics:  Situation 1 (consisting of 6 events): Handling air traffic in an inbound peak operation in a dense approach airspace with non-standard aircraft and regional aircraft in combination with merging inbound traffic flows with bunches of aircraft and complex conflict situations.  Situation 2 (consisting of 5 events): Handling air traffic in a situation of traffic delays in an inbound peak in a dense area control sector, with blocking radio/telephony, crossing traffic, regional traffic and complex conflict situations. 4


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Situation 3 (consisting of 1 event): Handling air traffic in a situation of weather disturbances in a dense area control sector, caused by heavy wind and thunderstorm. In total, 376 meaningful segments were analysed across all 12 events. All categories of strategies were found in the analysed situations, distributed as displayed in figure 2 among the ATC cognitive processes (including both strategies available from research as well as new types of strategies found).

Figure 2, Distribution of strategies in cognitive processes

Overall, all strategies from literature were found to be applied in the analysed cases with operational disturbances. In addition, we found indications for new types of strategies supplementary to the strategies available from literature, especially for perception, solving, planning and decision making. Table 2 provides the initial results indicating the presence of strategies in the analysed segments related to that cognitive process. The category of ‘other strategy’ indicates existence of new types of strategies.

Table 2, Strategies identified within each cognitive process and their percentage, including other (new) strategies identified supplementary to strategies available from literature

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Main strategies, including other (new) types of strategies supplementary to strategies available from literature, can be characterized as:  Perception strategies In addition to extracting flight level and speed of especially inbound aircraft to detect conflicts and to determine an inbound sequence, air traffic controllers indicate to first have a “look around the corner” for planning information (e.g. information of expected aircraft that are not yet on the radar or still in the previous sector, the departing moment of outbound traffic or aircraft delay information). In addition, additional information is extracted depending on the disturbance (e.g. type of aircraft & performance, runway configuration in use or weather situation).  Interpretation strategies Air traffic controllers indicate to identify patterns in the situation. Aircraft are grouped into categories, particularly on the basis of their destination (e.g. from/to adjacent airport, in/outbound) or potential conflict (e.g. aircraft on different routes that need to be merged). In addition, standard and nonstandard flows are identified (e.g. standard inbound traffic flows in relation to the runway configuration, departure routes and routes of crossing/regional traffic). Critical points or hotspots are identified in an early stage (e.g. typical conflicts or conflict points between routes in a specific runway configuration, merging points between inbound flows or points where routes are geographical separated). No additional types of interpretation strategies were identified.  Anticipation strategies Possible threats are continuously identified to determine possible further disturbances and proactively mitigate threats (e.g. considering a different climb performance, possible departing traffic at a specific moment, possible changes in weather; or a further increase in amount of traffic). In addition, air traffic controllers indicate to identify effects of the developing situation on the traffic handling method on efficiency (e.g. switch to basic circuit while not missing a landing slot, vectoring out the delay vs. holding); the effects of the solution on the resulting situation (e.g. in generating new conflict situations, progression/duration of conflict situation, limits of maneuvering/solution space); and the effects of the developing situation on expected attention/workload within a few minutes.  Attention and workload management strategies Air traffic controllers indicate to manage monitoring and save attentional resources by means of: solving a situation instead of monitoring a situation; monitoring one situation at a time (not monitoring on two places) and taking action if monitoring takes too much time and attention. A situation is monitored to verify whether a solution takes effect or attention is increased at a critical moment (e.g. moment of turn in, or when to continue further climb/descend). To focus attention on traffic in a non-standard situation, other (standard) traffic is kept on standard routing as much as possible (requiring minimum but sufficient attention and saving attentional resources needed for the disturbance). A supplementary attention and workload management strategy is “always keeping spare time” to be able to focus on unexpected complex situations or disturbances. Another attention and workload management strategy refers to “considering the (observed) taskload of the next air traffic controller” in the team, where the air traffic controller might also decide to increase his/her own workload and redistribute tasks to maintain safety and efficiency in the sector and the adjacent sector.  Solving strategies Conflicts are often solved by level separation first, especially to solve conflicts between inbound and outbound traffic or conflicting routes. In this, safe or available flight levels are searched for and the maximum possible level (e.g. highest possible level for outbound) is aimed for to keep efficiency. With respect to problem solving strategies, level separation is followed by vectoring or further climb/descend for efficiency (i.e. partially solving a problem and fine tune later) or for solving additional parts of the conflict (i.e. dividing a problem in smaller parts). This way time is created or gained to get a better or detailed insight of the evolving situation and to finetune later. In problem prevention especially the early or immediate solving of a complex or time critical situation is emphasized, as well as solving situations with a minimum of domino-effects (e.g. consequences are minimized, additional conflicts or attention increase are avoided). Further level buffers are incorporated to maintain safety, and to focus attention on other situations that are more complex or time critical. Other (supplementary) solving strategies that have been found can be characterized as the “creating space” for solution or maneuvering, by applying non-standard routing or moving/transferring crossing points. Creating space is not only applied to prevent potential problem situations or to create separation in case level separation is not sufficient, but is considered crucial to keep efficiency for an aircraft (e.g. a continuous climb); to create time to solve a situation; to avoid domino-effects for other aircraft or to prevent increase of attentional resources (e.g. avoid the need for monitoring a situation). 6


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Planning strategies Escape possibilities are identified in case the initial plan might not work or might not develop as expected, which are partly also ensured by building in safety buffers. Air traffic controllers keep to standard routings or a routinized working method as much as possible to ensure safety and to divide attention. However, the switch from standard routing to a routinized working method (e.g. switch from standard inbound route to basic circuit), is not only used to ensure safety, but often also to create maneuvering space and maintain efficiency of traffic handling. Further, the working method is adapted to sector layout, runway configuration, available solution space and traffic handling in next sector to facilitate optimal efficiency. A supplementary planning strategy can be characterized as: “creating a temporary standard or pattern” in non-standard routings (e.g aircraft follow same non-standard route) to manage workload and maintain efficiency. In addition “avoiding becoming reactive” in traffic handling is emphasized, or at least keep this period as short as possible (e.g. by forming an initial plan before the aircraft enters the sector). Decision making strategies Air traffic controllers indicate to act immediately on the disturbed situation, to partially solve the problem or prevent additional problems, and to both maintain safety and efficiency. In some situations in which air traffic controllers indicate to have sufficient time they sometimes wait and see before they intervene. A supplementary decision making strategy can be characterized as: “reflection on action” in which a decision is adapted or withdrawn in case the solution takes too long to take effect or turns out differently than expected.

Initial conclusions & way forward The initial results reflect that strategies from literature are used in the analysed operational cases. In addition, initial results revealed the presence of new (supplementary) strategies for perception, attention and workload management, problem solving, planning and decision making. Analysis of additional cases is needed and still ongoing to determine strategies used in response to disturbed operational situations. Subsequently, all results will be analysed into more detail to answer the research questions for this exploratory study. The results of the exploratory study will provide input for an in-depth study to identify the strategies that are critical for ATC performance, both within the operation as well as in training. Questions to be answered in this in-depth study focus on the extent in which strategies differ among different types of disturbances or events; the extent in which the other types of strategies found are unique or critical for disturbed operational situations; the extent of difference between area and approach control; the extent of difference of strategies among different levels of experience and how strategies differ between low, moderate and high performers in training. Identifying the critical strategies in ATC performance will provide a basis to incorporate strategy learning in ATC training as well as to fit the design of decision support tools to air traffic controller strategies. References Amaldi, P. & Leroux, M. (1995). Selecting relevant information in a complex environment: The case of air traffic control. In: Norros (Eds.), 5th European Conference on Cognitive Science Approaches to Process Control (pp. 89-98). Finland: VTT Automation. Bisseret, A. (1971). An analysis of mental model processes involved in air traffic control. Ergonomics, 14, 565-570. D’Arcy, J.F. & Della Rocco, P.S. (2001). Air traffic control specialist decision making and strategic planning – A field survey. DOT/FAA/CT-TN01/05. US Department of Transportation FAA. Flynn, M. & Kauppinen S. (2002). Investigating air traffic controller resolution strategies. European Air Traffic Management Programme Rep. No. ASA.01.CORA.2.DEL04-B.RS. Brussels: Eurocontrol. Fothergill, S. and Neal, A. (2008). The effect of workload on conflict decision making strategies in air traffic control. In: Proceedings of the 52nd Annual Meeting of the Human Factors and Ergonomics Society (39-43), New York. Gronlund, S.D., Ohrt, D.D., Dougherty, M.R.P., Perry, J.L. & Manning, C.A. (1998). Role of memory in air traffic control. Journal of Experimental Psychology: Applied, Vol (4)3, sep 1998, 263-280. Histon, J.M. & Hansman, R.J. (2008). Mitigating complexity in air traffic control: The role of structurebased abstractions. ICAT-2008-05. Cambridge, MA: MIT. Hoffman, R.R., Cranda, B., Shadbolt, N. (1998). Use of the critical decision method to elicit expert knowledge: A case study in the methodology of cognitive task analysis. Human Factors(40)2, 254-276. Klein, G.A., Calderwood, R. & MacGregor, D. (1989). Critical decision method for eliciting knowledge. IEEE Transactions on Systems, Man, and Cybernetics, 19, 462-472.

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Koros, A., Della Rocco, P.S., Panjwani, G., Ingurgio, V. And D’Arcy, J. (2003). Complexity in air traffic control towers: A field study, part 1 - complexity factors. DOT/FAA/CT-TN03/14. Atlantic City: US Department of Transportation, FAA William J. Hughes Technical Center. Loft, S., Humphreys, M. & Neal, A. (2003). Prospective memory in air traffic control. In G,. Edkins & P. Pfister (Eds.), Innovation and consolidation in aviation (pp. 287-293). Aldershot, UK: Ashgate. Loft, S. Sanderson, P., Neal, A & Mooij, M. (2007). Modeling and predicting mental workload in en route air traffic control. Human Factors, 39 (3), 379-399. Malakis, S., Kontogiannis, T. & Kirwan, B. (2010). Managing emergencies and abnormal situations in air traffic control (part I): taskwork strategies. Applied Ergonomics, 41(4), 620-7 Mogford, R.H., Guttman, J.A., Morrow, S.L., & Kopardekar, P. (1995). The complexity construct in air traffic control: a review and synthesis of literature. DOT/FAA/CT-TN95/22. Atlantic City International Airport, NJ: DOT/FAA Technical Centre. Nunes, A., Mogford, R. H. (2003). Identifying controller strategies that support the ‘picture’. Proceedings of the 47th Annual Meeting of the Human Factors and Ergonomics Society. Santa Monica, CA. Oprins E. (2008). Design of a competence-based assessment system for air traffic control training. Doctoral dissertation, University of Maastricht. Oprins, E, Burggraaff, E. & van Weerdenburg, H. (2006). Design of a competence based assessment system for air traffic control training. International Journal of Aviation Psychology, 16(3), 297-320. Rantanen, E.M. & Nunes, A. (2009). Hierarchical conflict detection in air traffic control. The international journal of aviation psychology, 15(4), 339-362. Redding. R.E., Ryder, J.M., Seamster,, T.L., Purcell, J.A. & Cannon, J.R. (1991). Cognitive task analysis of en route air traffic control: Model extension and validation. ERIC document No. ED 340 848. McLean, VA: Human Technology. Schuver-van Blanken, M.J., Huisman, H. & Roerdink, M.I. (2010). The ATC cognitive process & operational situation model – a model for analysing cognitive complexity in ATC. Proceedings of the th 29 EAAP Conference, Budapest, Hungary. Seamster, T.L., Redding, R.E., Cannon, J.R., Ryder, J.M. & Purcell, J.A. (1993). Cognitive task analysis of expertise in air traffic control. International Journal of Aviation Psychology, 3, 257-283. SESAR (2007). Human Factors Impact. SESAR Consortium: SESAR Definition Phase - Task deliverable WP1.7.1./D2. Sperandio, J.C. (1971). Variation of operator’s strategies and regulating effects on workload. Ergonomics, 14, 571-577. Sperandio, J.C. (1978). The regulation of working methods as a function of workload among air traffic controllers. Ergonomics, 21, 195-202. Contact information Marian J. Schuver-van Blanken Human Factor department, ATC the Netherlands [email protected]; +31 20 4063434

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