Human Factors Analysis and Classification System

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The Human Factors Analysis and Classification System (HFACS) identifies the human causes of an accident and provides a tool to assist in the investigation process and target training and prevention efforts.[1] It was developed by Dr Scott Shappell and Dr Doug Wiegmann, Civil Aviation Medical Institute and University of Illinois at Urbana-Champaign, USA, respectively, in response to a trend that showed some form of human error was a primary causal factor in 80% of all flight accidents in the Navy and Marine Corps.[1]

HFACS is based in the "Swiss Cheese" model of human error[2] which looks at four levels of active errors and latent failures, including unsafe acts, preconditions for unsafe acts, unsafe supervision, and organizational influences.[1] It is a comprehensive human error framework, that folded Reason's ideas into the applied setting, defining 19 causal categories within four levels of human failure.[3]


The HFACS taxonomy describes four levels within Reason's model and are described below.[4][5]

Level 1: Unsafe Acts[edit]

The Unsafe Acts level is divided into two categories – errors and violations – and these two categories are then divided into subcategories. Errors are unintentional behaviors, while violations are a willful disregard of the rules and regulations.


  • Skill-Based Errors: Errors which occur in the operator’s execution of a routine, highly practiced task relating to procedure, training or proficiency and result in an unsafe a situation (e.g., fail to prioritize attention, checklist error, negative habit).
  • Decision Errors: Errors which occur when the behaviors or actions of the operators proceed as intended yet the chosen plan proves inadequate to achieve the desired end-state and results in an unsafe situation (e.g. exceeded ability, rule-based error, inappropriate procedure).
  • Perceptual Errors: Errors which occur when an operator's sensory input is degraded and a decision is made based upon faulty information.


  • Routine Violations: Violations which are a habitual action on the part of the operator and are tolerated by the governing authority.
  • Exceptional Violations: Violations which are an isolated departure from authority, neither typical of the individual nor condoned by management.

Level 2: Preconditions for Unsafe Acts[edit]

The Preconditions for Unsafe Acts level is divided into three categories – environmental factors, condition of operators, and personnel factors – and these three categories are then divided into subcategories. Environmental factors refer to the physical and technological factors that affect practices, conditions and actions of individual and result in human error or an unsafe situation. Condition of operators refer to the adverse mental state, adverse physiological state, and physical/mental limitations factors that affect practices, conditions or actions of individuals and result in human error or an unsafe situation. Personnel factors refer to the crew resource management and personal readiness factors that affect practices, conditions or actions of individuals, and result in human error or an unsafe situation.

Environmental Factors

  • Physical Environment: Refers to factors that include both the operational setting (e.g., weather, altitude, terrain) and the ambient environment (e.g., heat, vibration, lighting, toxins).
  • Technological Environment: Refers to factors that include a variety of design and automation issues including the design of equipment and controls, display/interface characteristics, checklist layouts, task factors and automation.

Condition of Operators

  • Adverse Mental State: Refers to factors that include those mental conditions that affect performance (e.g., stress, mental fatigue, motivation).
  • Adverse Physiological State: Refers to factors that include those medical or physiological conditions that affect performance (e.g. medical illness, physical fatigue, hypoxia).
  • Physical/Mental Limitation: Refers to when an operator lacks the physical or mental capabilities to cope with a situation, and this affects performance (e.g. visual limitations, insufficient reaction time).

Personnel Factors

  • Crew Resource Management: Refers to factors that include communication, coordination, planning, and teamwork issues.
  • Personal Readiness: Refers to off-duty activities required to perform optimally on the job such as adhering to crew rest requirements, alcohol restrictions, and other off-duty mandates.

Level 3: Unsafe Supervision[edit]

The Unsafe Supervision level is divided into four categories.

  • Inadequate Supervision: The role of any supervisor is to provide their staff with the opportunity to succeed, and they must provide guidance, training, leadership, oversight, or incentives to ensure the task is performed safely and efficiently.
  • Plan Inappropriate Operation: Refers to those operations that can be acceptable and different during emergencies, but unacceptable during normal operation (e.g., risk management, crew pairing, operational tempo).
  • Fail to Correct Known Problem: Refers to those instances when deficiencies are known to the supervisor, yet are allowed to continue unabated (e.g. report unsafe tendencies, initiate corrective action, correct a safety hazard).
  • Supervisory Violation: Refers to those instances when existing rules and regulations are willfully disregarded by supervisors (e.g. enforcement of rules and regulations, authorized unnecessary hazard, inadequate documentation).

Level 4: Organizational Influences[edit]

The Organizational Influences level is divided into three categories.

  • Resource Management: Refers to the organizational-level decision-making regarding the allocation and maintenance of organizational assets (e.g. human resources, monetary/budget resources, equipment/facility recourse).
  • Organizational Climate: Refers to the working atmosphere within the organization (e.g. structure, policies, culture).
  • Operational Process: Refers to organizational decisions and rules that govern the everyday activities within an organization (e.g. operations, procedures, oversight).

See also[edit]


  1. ^ a b c "The Human Factors Analysis and Classification System (HFACS)," Approach, July - August 2004. Accessed July 12, 2007.
  2. ^ Reason, J.[1990]Human Error.Cambridge University Press
  3. ^ HFACS Analysis of Military and Civilian Aviation Accidents: A North American Comparison.ISASI,2004
  4. ^ Wiegmann, D. A., & Shappell, S. A. (2003). A human error approach to aviation accident analysis: The human factors analysis and classification system. Burlington, VT: Ashgate Publishing, Ltd.
  5. ^ "US Department of Defense HFACS,"