ICAO: Human Factors in Air Traffic Control

ICAO addressed the topic of human factors in Air Traffic Control (ATC) in the 1993 circular Human Factors Digest No. 8. Although a relatively old publication, the main ideas that are addressed in the circular remain relevant in the 21st century and described below is a synopsis of the contents in the circular.


11. This digest focuses on human factor issues pertaining to ATC with the aim of articulating mainly to the ATC fraternity and to the larger aviation community the way forward for human factors vis-a-vis automation in ATC; Human Factors within Systems are introduced, whilst demonstrating how ATC performance and safety could be compromised due to inherent limitations in humans .

Chapter 1: The Evolution of ATC

Human Factors within Systems

1.1-1.2 An ATC system is a model of a human-machine system with the aim of achieving a safe and efficient flow of air traffic; A necessary pre-requisite for this being the continued supply of professional controllers (humans) well versed in the interaction and usage of technology (machines) available.
1.3 This interaction/matching of the human-machine system is an ongoing and changing process, hence it is imperative that humans are matched with systems successfully with the correct application of Human Factors data available so that the full benefits of the ATC system can be harnessed.
1.4 These Human Factors knowledge must thus be applied both to the effects of the human on the system and to effects of the system on the human with the aim of improving safety and prevention of accidents.
1.5 Controllers also must have a firm grasp on how the system operates whilst utilising his own professional attributes for the above to be achieved.
1.6 The SHELL model will be applied extensively to the ATC system in the digest and illustrates the main interactions of humans (Liveware) with other aspects of the system such as machines (Hardware), materials/procedures (Software), Environment and with other humans (Liveware).
1.7 However it is important to note that Human Factor topics are always interlinked and must be dealt through its various interactions with each other instead of as independent entities.

Evolution of ATC

1.8 Air Traffic came into prominence with the forming of ICAO in 1947 and the subsequent establishment of Rules of the Air and Air Traffic Services at a period of burgeoning demand for air travel and advances in technology both in aircraft and ATC equipment.
1.9 Airspaces round the world were demarcated into flight information regions (FIRs) under different international states; FIRs were further divided into classes of controlled and uncontrolled airspace (Class A - E) stipulating which type of ATC services were provided.
1.10 3 main categories of air traffic control (Area, Approach and Aerodrome) were identified specifying ATC responsibilities of aircraft from taxy to take off, enroute in the area, arrival phase and landing.
1.11 For the above to work properly, a flight plan must be submitted by the pilot with details containing identity, aircraft type and destination. Using these details which are displayed on the radar screen, controllers can de-conflict the aircraft (either vertically of horizontally) through the means of mandatory radar control via radio communications.

Future of ATC

1.13 There is now an exponential increase in air traffic round the world with ATC systems operating at close to near capacity for longer periods of time.
1.14 With the distinct possibility of these capacities being exceeded, new avenues must be explored to deal with these increased demands; this include increased automation, availability of better up to date information to the controller and changing existing mental models of reacting to new traffic scenarios to better advanced forward planning of efficient traffic flows.
1.16 The lack of an increase of controllers could also exacerbate the increasing demand to the limited airspace and restrictions present which might require more controllers to intervene should a potential confliction emerge.
1.17 In spite of all these possible changes and developments through the improvements in radar, collision avoidance and navigation systems, these must be balanced with the capabilities of humans with safety in ATC never compromised.

Transfer of Information

1.18 Efficiency of flight in ATC is dependent on several factors such as characteristics of aircraft, equipment, how the aircraft is controlled, professionalism of ATC/ pilot, information and number of aircraft and environmental factors.
1.19 With experience, controllers are able to identify and react to poor quantitative (performance based) and qualitative (accuracy of data) information about aircraft.
1.20 In most instances, it is the the qualitative automation that determines how close aircraft can be from one another, hence increasing efficiency in the ATC system.

Chapter 2: The Controller's Workspace

Application of Ergonomic Data

2.1 Controlling in ATC is dependent on the interaction of the human-machine interface and it is paramount that the workspace be set up with all of constraints resolved to facilitate maximum task performance through the application of ergonomics.
2.2 The broadest application of ergonomics to ATC workspaces is environmental and examples include:

  • 2.3-2.4 Buildings must be designed with basic amenities close by.
  • 2.5-2.7 Rooms must be spacious enough to accommodate all staff members comfortably.
  • 2.8-2.10 Controller's workstations must be equipped with all the necessary tools for controlling and if there is a need for sharing, information must be displayed clearly and within reach for both controllers. If various control positions collapse due to lull traffic, supervisors at their workstation must still be in the position to monitor all the controllers.
  • 2.11-2.12 In the aerodrome environment, there must be unimpeded visual access of ground vehicles and aircraft at critical phases of flight. There must also be a simple and seamless flow of information so that aircraft details are always up to date and available for handover to the next control agency.
  • 2.13 The controller's console must meet the ergonomic requirements for all body types of controllers with easy access to critical and frequently used controls placed within eye distance.
  • 2.14-2.15 Anthropometry in the environment whereby the controller is working at his most comfortable position with ample space through the adjusting of his workspace must also be factored in.
  • 2.15-2.17 The layout of equipment must fulfill its roles and responsibilities and not placed for the sake of being there and should be configured to minimize and prevent distractions.
  • 2.20-2.24 Lighting in the form of ambient light and glare must be taken into account be it in a radar or aerodrome environment so that all the displays and controls remain visible for controlling without any distraction.
  • 2.25-2.27 The thermal environment must be maintained at a comfortable level through sound management of temperature and air flow rate for controlling.
  • 2.28 Noise levels within workstations should be kept at a minimum less it becomes a distraction, causing important information and messages to be missed out by controllers.
  • 2.29-2.34 Visual displays containing information for controllers must factor in human capabilities in how they see and process the information for it to be of good use. Hence, visual displays must be clear and visible with information portrayed in the correct colour contrast, colour differentiated to avoid ambiguity if need be, with symbols and alphanumerics having the appropriate spacing for easy deciphering.
  • 2.35 Input devices (keyboard, mouse) in which the controller executes all control actions must also be easy to use with the necessary response sensitivity and feedback.
  • 2.37-2.41 An essential ingredient of ATC is communications and controllers must be aware its uses and whether the system is functioning properly in his transmissions out to the pilot. Standard radio telephony should be articulated slowly and clearly at all times to prevent any ambiguity and controllers must always verify again if there is any doubt in the content of transmission.

Controller Proficiency

2.42-2.44 ATC involves a large amount of multi-tasking in which controllers must utilise all the different information available and draw on his past experiences and memory to make the right decisions under at all times. Hence Human Factors addresses the thinking processes of controllers and the effects of equipment changes on them with the aim of not drastically changing his thinking processes, although in minor instances, retraining has to be done so that he can unlearn old habits and learn new techniques.

Classes of Information

2.45-2.51 Controllers must have a thorough understanding of all the forms of information available to them through the myriad of sources; these include those from speech transmissions and displayed information on the radar screen. Future ATC systems will attempt to help controllers through more concise and processed information, alerting controllers to anomalies and conflictions and help in predicated future problems by offering solutions of projected confliction in flight paths. Notwithstanding the development of these sophisticated systems, controllers still must be conversant of all available information in the event of system contingencies, hence all information needed by the controller must be reliable and up to date.

Chapter 3: Automation in ATC

Full or Partial Automation

3.2 Human Factors implications related to the liveware-software interface concerning partial automation intended to assist controllers will be delved into due to the problems associated with it.

Reasons for Automation

3.3-3.6 With technological advances and an increasing amount of available information, automation is necessary to aid the controller so that the essential processes in information management and safety in ATC are not compromised.

Goals of Automation

3.7 Automation could be of great benefit and an asset to controlling as it could bring about improvements in efficiency, safety, error prevention and reliability. Hence it is must correctly matched with human capabilities through the maximizing of attributes and potential of both human and machine.
3.8 In spite of this, humans will still be the central element of the human-machine system with the machine helping the human and not vice versa. Hence it is imperative that the human-machine system be studied carefully from an infant stage so that it is not made redundant and jeopardize the ATC system.


3.9 Some of the constraints that need to be dealt with that are brought about by human functions within an automated ATC system include:

  • Humans must not become over-reliant on automation so much so that they aren't able to handle situations arising from system failure. Hence, human expertise levels must be kept up to standards through regular currency.
  • Controllers must remain plugged in with the whole ATC control process and not allow his awareness of traffic to be diminished by taking automation for granted.
  • There must be a constant balance of workload between a minimum and maximum threshold so that safety is not compromised.
  • The differing nature of workload involves non-interchangeable skills which may still need humans to verify certain automated functions.
  • With automation, there is a distinct possibility that job satisfaction for humans may be eroded.
  • As part of a controller's proficiency, they must be cognizant of the benefits and limitations of the systems so that they can trust and use it appropriately.
  • Controllers must be clear of the division of task responsibilities between himself, other controllers and the automated system to prevent confusion.
  • The human-machine co-ordination process must be clearly defined so that the transmission of information between different parties are always clearly acted upon without any interference or ambiguity.

3.10 With all these constraints in mind, it is important that humans participate activiely in the whole system development from developmental till implementation stage so that the information presented and integrated in the human-machine interface are timely and relevant.

The Application of Automation

3.11 Over time, the application of automation has taken a more "active" role in the work processes of controllers; moving from collating of various information to integrating this wealth of information "intelligently" and providing assistance to controlling problems.
3.12-3.15 However, problems still exist in this automated integrating process due to the overwhelming number of areas where information needs to be tapped as seen from the mismatch in information from the paper flight strip containing aircraft details and what is displayed on the radar console, thus generating problems of duplication.

Further implications of Automation

3.16-3.18 With automation in an advisory capacity, critical decisions can only be made with agreement from the controller. Hence this advisory role would be more suited in the planning aspect through the manipulating of constraints defined by the controller; this would enable true human-machine cooperation to be achieved with planning handled by the machine (sequencing of aircraft) and executive decisions (traffic deconfliction) still in the hands of the controller, thus retaining human expertise.
3.19 Another role machines could partake would be identifying and solving certain simple problems so as to relieve some controller's workload.
3.20 Nonetheless, automation of data could lead to Human Factors problems as controllers might not have a good feel regarding the reliability of information. Also, certain routine tasks which aid the basic fundamentals of controlling could be eroded. It is thus imperative that all these problems be looked at and catered for in advance in the developmental stage of the system.

Team Functions

3.22-3.23 Traditional team ethos in assistance of controlling by colleagues or supervisors might be rendered irrelevant what with automated assistance enabling controllers to be self sufficient in their constant interaction with the machine.
3.24 This would make assessment of competency level in controllers difficult due to reliance of machines with the leaning towards simulators as a means to gauge the proficiency of controllers.


3.25-3.27 The imposing of standardization across communications and controlling still leaves room for debate due to a de-sync between the degree of standardization that should be accepted as it brings about its own set of problems by discouraging human flexibility and its incompatibility of ingrained verbal practices.

Human-Machine Interface and Human Error

3.28-3.29 Traditional and existing means of the human-machine interface will be changed with automation as new methods in the conveying of information are utilised. This brings about a whole set of new problems as human error becomes less predictable and hence new kinds of human error need to be identified.

Chapter 4: The Selection and Training of Air Traffic Controllers

Selection of Applicants

4.1 Due to the demanding nature of air traffic, only the selection of the right candidate capable of becoming a proficient controller will lead to the continued safety and efficiency in the job.
4.2-4.3 This is done by attracting a large pool of applicants through publicity, through extolling how desirable ATC as a profession can be. Thereafter, this large pool can be downsized through impartial yet stringent selection procedures based on Human Factors principles.


4.4 Prior to the administrating of standardized tests, particular desirable attributes in controllers are first identified so that they are assessed accordingly in candidates.
4.5 However, no one test has gained universal acceptance which would enable it to be totally dependent in controller selection and these tests are generally used as a means to find out more about a particular attribute of the candidate.
4.6 Tests are also slowly becoming more automated but care must be taken to ensure candidates receive the proper practice and familiarization with the automated testing procedures less it would impact their performance.
4.7 These selection process must continuously evolve as and when changes occur in ATC and when new examinable human dimensions are discovered so that the process is not made irrelevant.
4.8-4.9 Items that could potentially impede controller's performance also need to be looked at and this includes medical histories, addictions, physical handicaps, emotional problems and mental limitations.
4.10 To make the selection process holistic, an impartial interview should be conducted to gauge two essential ingredients of ATC success; expression of speech and how good a team player the candidate is.


4.19 A proficient controller needs to be cognizant in the fundamentals of how ATC is conducted in line with acceptable standards whilst utilising the required information, methods of communication and equipment. They must also know how to work in a team, detect system failures and how to gauge aircraft performances. All these can only be achieved through proper training.
4.27 Pertinent issues which should be addressed by specific Human Factors training include: learning and comprehending procedures related with safe conduct of ATC, how to liaise with controllers and pilots, identifying and preventing human error, knowledge of one's weakness and the motivation to eradicate them through remedial training, garnering of professional attitudes and aptitudes whilst seeking high standards.
4.11-4.12 The training process must thus allow trainees to make the best use of human capabilities whilst negotiating its limitations in relation to the learning, understanding and remembering processing of information. Only then would it lead to controllers being able to discharge their duties in accordance to highly acclaimed international standards.
4.13 For training to be beneficial, it should be conducted in a progressive manner so that trainees are able to master the fundamental principles so that the knowledge they acquire will hold them in good stead when dealing with more complex problems later. How far the trainee has progressed can be tested through assessments along the way.
4.14 It is also important to ensure that any ambiguities arising from new forms of material must be properly taught and learned else it would lead to new human errors.
4.15 Although more self-training packages for students to practice their skills on the computer are in the pipeline, the current flavour in teaching methods is through active participation via practical sessions and simulation. Even on the job training entails honing whatever they have learnt with qualified controllers in real life scenarios at ATC centers.
4.16 On the job training is vital in the training process as a gauge to check how students deal with the pressures of controlling. Hence it is important that the instructor possesses the necessary experience and acumen in coping with the twin responsibility of controlling real life traffic and imparting ATC skills to the student.
4.17-4.19 With the advent of more automated and sophisticated systems, it is essential that controllers retain the fundamental ATC practices and procedures through refresher and competency checks so that they are not over-reliant on automation and can still deal with contingencies in the event of system failure.
4.20 Due to the fact that currently ATC cannot function autonomously, essential objectives of training should include and identify everything that the controller needs to know, do and say at the appropriate time in the appropriate manner.
4.21-4.24 Training should hence follow recommended Human Factors procedures and practices that would cater to their individual needs and provide an insight into their limitations and capability whilst being able to select the appropriate tools to enhance their work performance. It is also important in instilling good habits, knowledge and skill so that the controller can plan, deconflict, multi-task and deal with contingencies; these skills must be maintained actively and reinforced from time to time. Training must encourage constant scanning and alert techniques so that the controller does not succumb to tunnel vision and become overly preoccupied with certain tasks and neglect other important tasks and must prepare the controller for both types of light and heavy traffic so that he is capable of dealing with each of them confidently and safely.
4.25 A sign of good training is whereby controllers are able to deal with all kinds of scenarios through the breeding of self confidence based on past and achieved performance.
4.26-4.27 When new changes arise in ATC systems, it is imperative that there is a careful redefinition of all the consequent changes in the controller's knowledge and procedures. Retraining should be done before the controller encounters these changes in real life scenarios.
4.28 However, initial training of new controllers and retraining of qualified controllers may not be the same as retraining involves the unlearning and discarding of familiar knowledge and inappropriate practices ingrained.

Chapter 5: The Human Element - Specific Attributes

Recognition of their significance

5.1 For Human Factors in ATC to be holistically addressed, besides those that are addressed through the SHEL model, two other categories associated with specific human attributes have to be considered and will be elaborated further.
5.3 The first category concerns the effects of ATC on those who work within it; this can be managed by modifying the ATC system. The second category refers to innate human attributes which are independent of ATC environments and to which ATC must therefore be adapted to. In both cases, solutions of particular problems may differ due to the myriad of different causes which potentially might lead to either changes to the system or to the human.

The first category includes attributes such as:

  • 5.5-5.8 Stress - Workload of controllers are at the very high end of the stress spectrum due to the demanding nature of the job. Factors that have contributed to stress include the hostile environment involving fear of blame, poor relationships with management, shift work which affect the cicardian cycle and modern lifestyle. Although prevention is better than cure, what is needed is correct diagnosis of how stress came about. Some ways to alleviate the problem include redesigning tasks and reallocating responsibilities by transferring the controller to a less demanding job or to adjust the number of hours of work through proper shift rostering.
  • 5.9-5.12 Boredom - Although still not very well comprehended, it is caused by little activity and routinization. Hence there is a need to alleviate this by allowing controllers leeway to plan their workload, providing ample skilled work to do and have groups of people around work to interact while working.
  • 5.13 Confidence and Complacency - Confidence is essential in controlling but is detrimental if it leads to complacency. This can be reduced by constantly dishing out high work levels and providing stimulating problems to solve.
  • 5.14-5.17 Error Prevention - To err is human, and from this basis although efforts have been made to prevent human error, it may not be sensible to predicate the safety of the ATC system on the assumption that every human error can be prevented; it is more important to make the system error tolerant through system design. Although various classifications of human error in ATC have been compiled through reported ATC incidents containing details of human errors, an alternative approach worth considering is through procedures which are formulated to remove error and prevent further consequences by classifying error classification based on error tasks, execution and knowledge.
  • 5.18-5.21 Fatigue - Fatigue is an important aspect as it would lead to impairment of judgment and hence safety to be compromised in ATC. Hence, adequate rest breaks must be provided along with provision of meal breaks and to be mindful of stretching long, continuous working hours vis-a-vis traffic intensity.

The second category includes:

  • 5.22-5.23 Needs at work - Humans, unlike machines have job expectations and it is important to balance their satisfaction, with their aspirations managed for fear of being made redundant as automation is slowly introduced.
  • 5.24-5.26 Attitudes - Favourable attitudes towards the ATC environment and as a profession both individually and from outsiders are important for controllers so management must continuously strive to achieve this.
  • 5.27-5.28 Functions of Teams - With the introduction of automation, there is a need to look into all facets of team functions to restore the optimum match between human and machine.
  • 5.29-5.30 Individual differences - The current selection and training processes involves selecting and training candidates of similar characteristics so management know what to expect in their performance while on duty. With automation, this trend may move to honing individual differences and strengths so as to tap on their varied potential and background.

A General Human Factors View

5.31 The genetic makeup of human's mental faculties in their thought process of understanding and decision making must be factored into the ATC system in order for task competency in training and jobscope to be maximized.
5.32 For optimum performance in ATC, there is a need for reviews at appropriate times so that there is not any degradation in working hours, shift cycles and ergonomic factors at work.
5.33 The future is pointing towards greater air traffic which would entail current ATC systems which are working at close to or at maximum capacity to be further enhanced and developed.
5.34 With this prospect of further development and technological changes, it is necessary to identify and resolve quickly the associated Human Factor consequences.

1. ICAO - INTERNATIONAL CIVIL AVIATION ORGANIZATION (1993). Human factors digest no 8. Human factors in Air Traffic Control. Circular 241-AN/145. ICAO (Montreal, Canada), 1993.

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ICAO's Human factors digest no 8

ICAO - INTERNATIONAL CIVIL AVIATION ORGANIZATION (1993). Human factors digest no 8. Human factors in Air Traffic Control. Circular 241-AN/145. ICAO (Montreal, Canada), 1993.

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