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Swiss Cheese Model

swiss cheese slice

The Human Factors Analysis and Classification System—HFACS

Cover and Documentation
1. Unsafe Acts
2. Preconditions for Unsafe Acts
3. Unsafe Supervision
4. Organizational Influences

HFACS and Wildland Fatality Investigations

Hugh Carson wrote this article a few days after the Cramer Fire

Bill Gabbert wrote this article following the release of the Yarnell Hill Fire ADOSH report

A Roadmap to a Just Culture: Enhancing the Safety Environment

Cover and Contents
Forward by James Reason
Executive Summary
1. Introduction
2. Definitions and Principles of a Just Culture
3. Creating a Just Culture
4. Case Studies
5. References
Appendix A. Reporting Systems
Appendix B. Constraints to a Just Reporting Culture
Appendix C. Different Perspectives
Appendix D. Glossary of Acronyms
Appendix E. Report Feedback Form

Rainbow Springs Fire, 1984 — Incident Commander Narration

Years Prior
April 25th
Fire Narrative
Lessons Learned

U.S. Forest Service Fire Suppression: Foundational Doctrine

Tools to Identify Lessons Learned

An FAA website presents 3 tools to identify lessons learned from accidents. The site also includes an animated illustration of a slightly different 'Swiss-cheese' model called "defenses-in-depth."

The Human Factors Analysis and Classification System–HFACS

The “Swiss cheese” model
of accident causation
February 2000

Drawing upon Reason’s (1990) concept of latent and active failures, HFACS describes four levels of failure: 1) Unsafe Acts, 2) Preconditions for Unsafe Acts, 3) Unsafe Supervision, and 4) Organizational Influences. A brief description of the major components and causal categories follows, beginning with the level most closely tied to the accident, i.e. unsafe acts.

2. Preconditions for Unsafe Acts

Arguably, the unsafe acts of pilots can be directly linked to nearly 80 % of all aviation accidents. However, simply focusing on unsafe acts is like focusing on a fever without understanding the underlying disease causing it. Thus, investigators must dig deeper into why the unsafe acts took place. As a first step, two major subdivisions of unsafe aircrew conditions were developed: substandard conditions of operators and the substandard practices they commit (Figure 3).

Figure 3. Categories of preconditions of unsafe acts.

Substandard Conditions of Operators

Adverse mental states. Being prepared mentally is critical in nearly every endeavor, but perhaps even more so in aviation. As such, the category of Adverse Mental States was created to account for those mental conditions that affect performance (Table 2). Principal among these are the loss of situational awareness, task fixation, distraction, and mental fatigue due to sleep loss or other stressors. Also included in this category are personality traits and pernicious attitudes such as overconfidence, complacency, and misplaced motivation.

TABLE 2. Selected examples of Unsafe Aircrew Conditions (Note: This is not a complete listing)

Substandard Conditions of Operators

Adverse Mental States

  • Channelized attention
  • Complacency
  • Distraction
  • Mental Fatigue
  • Get-home-itis
  • Haste
  • Loss of situational awareness
  • Misplaced motivation
  • Task saturation

Adverse Physiological States

  • Impaired physiological state
  • Medical illness
  • Physiological incapacitation
  • Physical fatigue

Physical/Mental Limitation

  • Insufficient reaction time
  • Visual limitation
  • Incompatible intelligence/aptitude
  • Incompatible physical capability

Substandard Practice of Operators

Crew Resource Management

  • Failed to back-up
  • Failed to communicate/coordinate
  • Failed to conduct adequate brief
  • Failed to use all available resources
  • Failure of leadership
  • Misinterpretation of traffic calls

Personal Readiness

  • Excessive physical training
  • Self-medicating
  • Violation of crew rest requirement
  • Violation of bottle-to-throttle requirement

Predictably, if an individual is mentally tired for whatever reason, the likelihood increase that an error will occur. In a similar fashion, overconfidence and other pernicious attitudes such as arrogance and impulsivity will influence the likelihood that a violation will be committed. Clearly then, any framework of human error must account for preexisting adverse mental states in the causal chain of events.

Adverse physiological states. The second category, adverse physiological states, refers to those medical or physiological conditions that preclude safe operations (Table 2). Particularly important to aviation are such conditions as visual illusions and spatial disorientation as described earlier, as well as physical fatigue, and the myriad of pharmacological and medical abnormalities known to affect performance.

The effects of visual illusions and spatial disorientation are well known to most aviators. However, less well known to aviators, and often overlooked are the effects on cockpit performance of simply being ill. Nearly all of us have gone to work ill, dosed with over-the-counter medications, and have generally performed well. Consider however, the pilot suffering from the common head cold. Unfortunately, most aviators view a head cold as only a minor inconvenience that can be easily remedied using over-the counter antihistamines, acetaminophen, and other non-prescription pharmaceuticals. In fact, when confronted with a stuffy nose, aviators typically are only concerned with the effects of a painful sinus block as cabin altitude changes. Then again, it is not the overt symptoms that local flight surgeons are concerned with. Rather, it is the accompanying inner ear infection and the increased likelihood of spatial disorientation when entering instrument meteorological conditions that is alarming - not to mention the side-effects of antihistamines, fatigue, and sleep loss on pilot decision-making. Therefore, it is incumbent upon any safety professional to account for these sometimes subtle medical conditions within the causal chain of events.

Physical/Mental Limitations. The third, and final, substandard condition involves individual physical/ mental limitations (Table 2). Specifically, this category refers to those instances when mission requirements exceed the capabilities of the individual at the controls. For example, the human visual system is severely limited at night; yet, like driving a car, drivers do not necessarily slow down or take additional precautions. In aviation, while slowing down isn’t always an option, paying additional attention to basic flight instruments and increasing one’s vigilance will often increase the safety margin. Unfortunately, when precautions are not taken, the result can be catastrophic, as pilots will often fail to see other aircraft, obstacles, or power lines due to the size or contrast of the object in the visual field.

Similarly, there are occasions when the time required to complete a task or maneuver exceeds an individual’s capacity. Individuals vary widely in their ability to process and respond to information. Nevertheless, good pilots are typically noted for their ability to respond quickly and accurately. It is well documented, however, that if individuals are required to respond quickly (i.e., less time is available to consider all the possibilities or choices thoroughly), the probability of making an error goes up markedly. Consequently, it should be no surprise that when faced with the need for rapid processing and reaction times, as is the case in most aviation emergencies, all forms of error would be exacerbated.

In addition to the basic sensory and information processing limitations described above, there are at least two additional instances of physical/mental limitations that need to be addressed, albeit they are often overlooked by most safety professionals. These limitations involve individuals who simply are not compatible with aviation, because they are either unsuited physically or do not possess the aptitude to fly. For example, some individuals simply don’t have the physical strength to operate in the potentially high-G environment of aviation, or for anthropometric reasons, simply have difficulty reaching the controls. In other words, cockpits have traditionally not been designed with all shapes, sizes, and physical abilities in mind. Likewise, not everyone has the mental ability or aptitude for flying aircraft. Just as not all of us can be concert pianists or NFL linebackers, not everyone has the innate ability to pilot an aircraft – a vocation that requires the unique ability to make decisions quickly and respond accurately in life threatening situations. The difficult task for the safety professional is identifying whether aptitude might have contributed to the accident causal sequence.

Substandard Practices of Operators

Clearly then, numerous substandard conditions of operators can, and do, lead to the commission of unsafe acts. Nevertheless, there are a number of things that we do to ourselves that set up these substandard conditions. Generally speaking, the substandard practices of operators can be summed up in two categories: crew resource mismanagement and personal readiness.

Crew Resource Mismanagement. Good communication skills and team coordination have been the mantra of industrial/organizational and personnel psychology for decades. Not surprising then, crew resource management has been a cornerstone of aviation for the last few decades (Helmreich & Foushee, 1993). As a result, the category of crew resource mismanagement was created to account for occurrences of poor coordination among personnel. Within the context of aviation, this includes coordination both within and between aircraft with air traffic control facilities and maintenance control, as well as with facility and other support personnel as necessary. But aircrew coordination does not stop with the aircrew in flight. It also includes coordination before and after the flight with the brief and debrief of the aircrew.

It is not difficult to envision a scenario where the lack of crew coordination has led to confusion and poor decision making in the cockpit, resulting in an accident. In fact, aviation accident databases are replete with instances of poor coordination among aircrew. One of the more tragic examples was the crash of a civilian airliner at night in the Florida Everglades in 1972 as the crew was busily trying to troubleshoot what amounted to a burnt out indicator light. Unfortunately, no one in the cockpit was monitoring the aircraft’s altitude as the altitude hold was inadvertently disconnected. Ideally, the crew would have coordinated the trouble-shooting task ensuring that at least one crewmember was monitoring basic flight instruments and “flying” the aircraft. Tragically, this was not the case, as they entered a slow, unrecognized, descent into the everglades resulting in numerous fatalities.

Personal Readiness. In aviation, or for that matter in any occupational setting, individuals are expected to show up for work ready to perform at optimal levels. Nevertheless, in aviation as in other professions, personal readiness failures occur when individuals fail to prepare physically or mentally for duty. For instance, violations of crew rest requirements, bottle-to-brief rules, and self-medicating all will affect performance on the job and are particularly detrimental in the aircraft. It is not hard to imagine that, when individuals violate crew rest requirements, they run the risk of mental fatigue and other adverse mental states, which ultimately lead to errors and accidents. Note however, that violations that affect personal readiness are not considered “unsafe act, violation” since they typically do not happen in the cockpit, nor are they necessarily active failures with direct and immediate consequences.

Still, not all personal readiness failures occur as a result of violations of governing rules or regulations. For example, running 10 miles before piloting an aircraft may not be against any existing regulations, yet it may impair the physical and mental capabilities of the individual enough to degrade performance and elicit unsafe acts. Likewise, the traditional “candy bar and coke” lunch of the modern businessman may sound good but may not be sufficient to sustain performance in the rigorous environment of aviation. While there may be no rules governing such behavior, pilots must use good judgment when deciding whether they are “fit” to fly an aircraft.

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