ICAO: Human Factors in CNS/ATM Systems


ICAO addressed the importance of human factors considerations in the design of automated and advanced communication, navigation, and surveillance (CNS) and air traffic management (ATM) systems in the 1994 circular Human Factors Digest no 11.1 The circular presents the human factors implications of automated and advanced technologies in CNS/ATM systems, and introduces the concept of human-centered automation. The following discussion summarises the key messages from the circular.

Chapter 1. The ICAO CNS/ATM concept

1.5.2 The main features of the ICAO CNS/ATM system concept are set out below.3


  • Extensive use of digital communications systems for greater efficiency.
  • Optimum use of automation in aircraft and on the ground.
  • Data and voice communication by satellite relay, except for terminal and high-density areas with terrestrial coverage.
  • Continued use of VHF in many continental and terminal areas.
  • Use of an SSSR Mode S air-ground data link for high-density airspace.


  • Progressive introduction of area navigation (RNAV), and the development of the concept of requried navigation performance (RNP).
  • Use of global navigation satellite systems (GNSS) for aircraft navigation and non-precision type approaches.
  • Continued use of radio navigation systems, to eventually be entirely replaced by satellite systems.


  • Continued use of secondary surveillance radar (SSR), enhanced with Mode S.
  • Use of automatic dependent surveillance (ADS) mainly in non-radar coverage areas. This involves aircraft automatically transmitting position and navigation data from on-board navigation systems by data link.


  • Air traffic management (ATM) consists of an air part and a ground part, with the ground part consisting of air traffic services (ATS), air traffic flow management (ATFM), and airspace management (ASM).
  • ATM’s objective is to enable on-time and minimally constrained aircraft operations without compromising agreed levels of safety.

1.6. This systems concept is seen as permitting enhanced safety by achieving, for example: less ATS communication interruptions, improved conflict detection and resolution, and better weather information. It is also seen as permitting more flexible and efficient use of airspace.

Chapter 2. Automation in future aviation systems

2.1. A major issue in the CNS/ATM system is the impact of automation and advanced technology on the human operator. To be effective, automation must meet the needs and constraints of designers, purchasers, and users of the system.

Role of the human operator

2.2. Technology is so advanced that nearly all continuous air traffic control and surveillance and air navigational tasks could be automated.

2.3. However, the view that automation may enable the entire replacement of humans in the operation of such systems is unrealistic. Rather, automation shifts the human to a different role within the system.

2.4. The safe operation of aviation systems requires real-time responses. Human responses involve the use of human senses, initiative, and common sense. Automated responses rely on programming to ensure that the right action is taken at the right time. That programming cannot take into account all of the uncontrollable variables of the aviation operating environment. Automation can also be unpredictable in its operation. These differences between human and machine can have safety implications for the operation of the aviation system.

2.5. While humans are not perfect sensors, decision-makers, and controllers, they do have the ability to reason, and to apply abstract thought and conceptual analysis to problems and uncertain situations. Human problem-solving is also more flexible and adaptable than an automated system, for the reasons mentioned above.

2.6. Automation should be seen as a tool enabling the human to perform a task that would otherwise be difficult or impossible, or to direct a machine to independently perform a task that would require increased human attention or effort.

Delta Airlines’ “Statement of Automation Philosophy”:

The word “automation”, where it appears in this statement, shall mean the replacement of a human function, either manual or cognitive, with a machine function. … The purpose of automation is to aid the pilot in doing his or her job. … The pilot is the most complex, capable and flexible component of the air transport system, and as such is best suited to determine the optional use of resources in any given situation. … Automation should be used at the level most appropriate to enhance the priorities of Safety, Passenger Comfort, Public Relations, Schedule and Economy …


2.10. Automation means “a system or method in which many of the processes of production are automatically performed or controlled by self operating machines, electronic devides, etc.”.4

2.13. Technological advances, particularly those involving automation, can change the procedures and practices of the global aviation system, the working environment, and the role of human operators within the system. Accordingly, where changes are contemplated a system safety analysis is necessary to identify safety deficiencies and to manage potential human or technological failures.

2.14. To achieve the benefits of automation, automated systems must be matched with human capabilities, and the relative strengths of each component (machine and human) must be recognised and allowed for. In aviation automated systems, the human with ultimate responsibility for safety must remain the key element, with automation assisting that human.

2.15. One principle to be recognised in the development of ATM procedures using automation technologies is that automation must be designed to assist the human operator, and should be human-centered.

Issues and concerns about automation

2.20. There are a number of issues or concerns about the impact of a technology-centered approach to automation, which include, for example:5

  • Loss of systems awareness where the human operator lacks knowledge of the capabilities and limitations of the automated system.
  • Poor interface design.
  • Poor attitudes towards and lack of acceptance of automated systems.
  • Loss of control resulting in lowered motivation and job satisfaction of the operator.
  • Over-reliance on automation.
  • Boredom, complacency, and inattention.
  • Intimidation of the human operator by the extent, complexity, and inherent authority of automated system components.
  • Distrust.
  • Confusion and misapplication in the operation of automated functions by the human operator.
  • Increased workload and impaired team function.

Chapter 3. Human-centered technology

3.2. Nearly all current automated human-machine systems are designed to assist and reserve certain functions for the human operator.

3.3. However, the trend towards automation has the potential to isolate human operators and decrease their awareness of the state and situation of the system, caused by lack of consideration of capabilities and limitations. This may be countered by better human factors design decisions at the beginning of the systems design process.

3.4. This influence of human factors on design is the goal of the human-centered automation concept.

3.5. The human-centered technology approach to automation helps to prevent disasters and accidents, which are frequently
caused by unexpected or unusually technological interactions resulting in unforeseen failures.

3.11. The human-centered technology approach also has a cost benefit. Systems that integrate human capabilities, limitations, and expectations into their design are easier to learn and operate for the human, reducing training and operation costs.

Chapter 4. Principles of human-centered automation

4.3. It is generally accepted that human operators are required to take full responsibility for the safe operation of the system. If that is the case, then automated tools designed to assist them in fulfilling that responsibility should be designed with the human operator in mind. ICAO has articulated the following principles.

Principles of Human-Centered Automation:

The human bears the ultimate responsibility for the safety of the aviation system.


  • The human must be in command.
  • To command effectively, the human must be involved.
  • To be involved, the human must be informed.
  • Functions must be automated only if there is a good reason for doing so.
  • The human must be able to monitor the automated system.
  • Automated systems must be able to monitor the human operator.
  • Each element of the system must have knowledge of the others’ intent.
  • Automation must be designed to be simple to learn and operate.

Chapter 5. Qualities of human-centered automation

5.2. Human factors researchers and specialists, accident investigators, and others, agree that making automation human-centered can solve most problems associated with human error, and that automation can be designed to make the system more resistant to and tolerant of human errors in designing those systems. If the qualities of human-centered automation are defined and recognised, it is hoped that the system designed will be made inherently more useful to the human operator.

5.4. In summary, those qualities are that human-centered automation must be:

  • accountable to the human operator. It must inform the human, justify its decisions, and anticipate human requests.
  • subordinate to the human operator’s command, and must be capable of being countermanded by the human operator.
  • predictable by the human operator, so that its behavior can be anticipated and understood.
  • adaptable by the human operator to meet the requirements of changing circumstances.
  • comprehensible by the human operator.
  • flexible, for application with different environmental, operational and human variables.
  • dependable, such that the system always does what it is ordered to do, never does what it is not ordered to do, and never worsens a situation.
  • informative, to permit the fullest possible involvement of the human operator.
  • error-resistant, such that it prevents errors being made by the human operator.
  • error-tolerant, such that it detects and mitigates the effect of errors made by the system or the human operator.
1. ICAO - INTERNATIONAL CIVIL AVIATION ORGANIZATION (1994). Human factors digest no 11. Human Factors in CNS/ATM systems. Circular 249-AN/149. ICAO (Montreal, Canada), 1994.
2. ICAO - INTERNATIONAL CIVIL AVIATION ORGANIZATION (1992). Human factors digest no 5. Operational Implications of Automation in Advanced Technology Flight Decks. Circular 238-AN/142. ICAO (Montreal, Canada), 1992.


1 ICAO - INTERNATIONAL CIVIL AVIATION ORGANIZATION (1994). Human factors digest no 11. Human Factors in CNS/ATM systems. Circular 249-AN/149. ICAO (Montreal, Canada), 1994. A useful summary of ICAO’s human factors programme appears at paragraphs 1.7 to 1.19 of the circular.

2 Paragraph numbers in this document correspond to the paragraph numbers in the ICAO circular. Because this document is intended as a summary, certain paragraphs have been omitted.

3 Refer paragraph 1.5 of the ICAO circular for the full summary.

4 ICAO - INTERNATIONAL CIVIL AVIATION ORGANIZATION (1992). Human factors digest no 5. Operational Implications of Automation in Advanced Technology Flight Decks. Circular 238-AN/142. ICAO (Montreal, Canada), 1992.

5 See ICAO Human Factors Digest No. 5, cited above.

Want to know more?

ICAO recommends a number of publications in Appendix 2 to the circular, including:

Bainbridge, L. "Ironies of Automation". In Analysis, Design, adn Evaluation of Man-machine Systems, Proceedings of the IFAC/IFIP/FFORS/IEA Conference. G. Johannsen and J.E. Rijnsdorp (eds.). Pregamon Press, New York, 1982, pp. 129-135.:

Contributors to this page

Authors / Editors

Nick ChristiansenNick Christiansen

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