Human Factors Analysis and Classification Systems (HFACS)

Introduction to HFACS

Human factors analysis and classification (HFACS) is a broad human error framework that was originally used by the US air force to investigate and analyse human factors aspects of aviation. Over the years, the application has spread to civil and general aviation as well. Although it is a highly effective tool, the model is not as widespread as desirable. HFACS is heavily based upon James Reason's swiss cheese model.

Basic HFACS framework

The HFACS framework (Image embedded from HFACS.Inc on 7 November 2011)

Unsafe acts

After an accident has occured, the "active failure" is categorised into either an error or a deviation.

An error is an unintentional mistake. Errors are further classified into perceptual, decision or skill-based errors. Perceptual errors occur when sensory inputs of the involved crew members are degraded or unusual, which causes illusions or sensory inconsistencies. Decision based errors are errors caused due to the choices or actions of the flight crew. These errors are not intentional i.e. the crew does not intend to make a mistake, but the lack of knowledge and poor choices made by the crew are the catalysts for these errors. Skill-based errors occur when the flight crew is unskilled or inexperienced in a certain environment or situation. These failures are charachterized by failure to pay attention to something or failure to remember something.

  • Decision errors: These “thinking” errors represent conscious, goal-intended behavior that proceeds as designed, yet the plan proves inadequate or inappropriate for the situation. These errors typically manifest as poorly executed procedures, improper choices, or simply the misinterpretation and/or misuse of relevant information.
  • Skill-based errors: Highly practiced behavior that occurs with little or no conscious thought. These “doing” errors frequently appear as breakdown in visual scan patterns, inadvertent activation/deactivation of switches, forgotten intentions, and omitted items in checklists. Even the manner or technique with which one performs a task is included.
  • Perceptual errors: These errors arise when sensory input is degraded, as is often the case when flying at night, in poor weather, or in otherwise visually impoverished environments. Faced with acting on imperfect or incomplete information, aircrew run the risk of misjudging distances, altitude, and descent rates, as well as of responding incorrectly to a variety of visual/vestibular illusions.

A violation is willful disregard for the rules and regulations that govern safe flight. These can be further classified into routine violations, which occur regularly and are often tolerated, even they they shouldn't be, and execptional violations, which occur sporadically and are difficult to predict.

  • Routine violations: Often referred to as “bending the rules,” this type of violation tends to be habitual by nature and is often enabled by a system of supervision and management that tolerates such departures from the rules.
  • Exceptional violations: Isolated departures from authority, neither typical of the individual nor condoned by management.

Preconditions for unsafe acts

After the accident or the mishap has been categorised into an error or a violation, the conditions can be classified into environmental factors, conditions of operations and personnel factors.

Environmental factors refer to the adequacy of the physical and technological environment. Physical environment refers to factors such as noise, temperature, working space etc. These factors encompass the physical surroundings of the flight crew. Technological environment refers to the equipment and technology used by the flight crew. This may include equipment malfunction, ergonomic design, technological complexity etc.

Conditions of operations refers to the mental states, physiological states and the physical or mental limitations of the flight crew. This may include fatigue, sickness, stress, memory capacity etc.

Personnel factors include crew resource management and personal readiness. This goes back to training of the flight crew in the area of crew resource management and how a lack of training or lack of motivation may have led to the mishap.

Environmental factors

  • Technological environment: This category encompasses a variety of issues, including the design of equipment and controls, display/interface characteristics, checklist layouts, task factors, and automation.
  • Physical environment: Included are both the operational setting (e.g., weather, altitude, terrain) and the ambient environment (e.g., as heat, vibration, lighting, toxins).

Condition of the operator

  • Adverse mental states: Acute psychological and/or mental conditions that negatively affect performance, such as mental fatigue, pernicious attitudes, and misplaced motivation.
  • Adverse physiological states: Acute medical and/or physiological conditions that preclude safe operations, such as illness, intoxication, and the myriad pharmacological and medical abnormalities known to affect performance.
  • Physical/mental limitations: Permanent physical/mental disabilities that may adversely impact performance, such as poor vision, lack of physical strength, mental aptitude, general knowledge, and a variety of other chronic mental illnesses.

Personnel factors

  • Crew resource management: Includes a variety of communication, coordination, and teamwork issues that impact performance.
  • Personal readiness: Off-duty activities required to perform optimally on the job, such as adhering to crew rest requirements, alcohol restrictions, and other off-duty mandates.

Unsafe supervision

Supervision is the act of directing or overseeing an action or a project. Unsafe supervision may be a key factor which may lead up to an accident. Supervision over an event is unsafe when factors such as inadequate supervision, improper operational planning, failure to correct a problem and violations commited by supervisors are predominant.

  • Inadequate supervision: Oversight and management of personnel and resources, including training, professional guidance, and operational leadership, among other aspects.
  • Planned inappropriate operations: Management and assignment of work, including aspects of risk management, crew pairing, operational tempo, etc.
  • Failed to correct known problems: Those instances in which deficiencies among individuals, equipment, training, or other related safety areas are “known” to the supervisor yet are allowed to continue uncorrected.
  • Supervisory violations: The willful disregard for existing rules, regulations, instructions, or standard operating procedures by managers during the course of their duties.

Organisational influences

After all the aforementioned factors are taken into account, the upper level of the organisational influences are examined to figure out the final root cause of the accident. Organisational influences are disected into resource management, organisational climate and organisational processes.

  • Organizational climate: Prevailing atmosphere/vision within the organization, including such things as policies, command structure, and culture.
  • Operational process: Formal process by which the vision of an organization is carried out including operations, procedures, and oversight, among others.
  • Resource management: How human, monetary, and equipment resources necessary to carry out the vision are managed.

Resource management refers to the human resources of the concerned organisations, monetary resources or resources devoted to equipment or technology. Organisational climate refers to structure, policies or culture the organisation. Organisational processes refers to the actual operations, procedures and oversight regulations of the organisation.

An HFACS analysis method will ideally lead to changes in organisational influences by analysing the chain of events starting from the initial unsafe act.

1. Wiegmann, D.A. & Shappell, S.A.. The Human Factors Analysis and Classification System (HFACS): Repot No. DOT/FAA/AM-00/7. Federal Aviation Administration, 2000;
2. Wiegmann, D.A. & Shappell, S.A.. Applying Reason: the human factors analysis and classification system (HFACS): Human Factors and Aerospace Safety I(I), 59-86, 2001;

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