Accident Causation Models FLT 241


Modern aviation safety theorists have been led by ICAO and the FAA to embrace a new way of thinking to prevent aviation accidents and increase awareness of aviation safety concepts. These experts have focused on questions such as:

Why did the accident happen?
How can we prevent future accidents and enhance aviation safety?

Purpose of Accident Modeling

  • Help explain the relationship between hazards and accidents.
  • Assist with understanding and explaining reality.
  • Aid in visualizing things that cannot be directly observed.
  • Must approximate conditions that exist in reality to be useful

Since the turn of the century, aviation accident investigations has matured from assigning blame to the following thinking regarding accident prevention: Instead of reacting to accidents and punishing the guilty party for failure to act safely, modern safety thinking has become proactive and even predictive of where and when the next accident may occur if current trends continue.

Evolution of Safety Thinking

Active Causes Vs. Root Causes

Air safety investigators must rely on active causes of accidents to find their corrective efforts ineffective if the underlying causes are not addressed. They must seek out the root causes of the accident. Anyone who wants to understand why accidents happen must learn the concept of root cause analysis and realize that there is usually much more going on than what meets the eye at first glance in an accident.

Active Cause

  • An act or condition that singly, or in combination with other causes, results in the damage or injury that occurred,
  • A deficiency that, if corrected or eliminated or avoided, would have likely prevented or mitigated the accident damage or significant injuries
  • An act, an omission, a condition, or a circumstance that either starts or sustains an accident sequence.

Root Causes

Conditions that facilitate errors by frontline operators but which are outside of the direct control of frontliners.

  • Almost all accidents are "multicausal", meaning that more than one cause produced the event. If we only focus on a single cause, then it will usually involve the pilot, who might be dead and cannot do anything to prevent a future accident.
  • Effective investigations are those that provide recommendations to deal with the root causes, since those are the recommendations that can best prevent other accidents. Recommendations crafted around active causes actually often just deal with the crew and thus often will not prevent future accidents.
  • Since accidents result from more than one cause, it is often suggested that we should not prioritize the causes. Nevertheless, people often want to know what the "primary" or "trigger" cause was for an accident.

Unforecasted Weather

  1. Inadequate wind-shear detection
  2. Insufficient wind-shear training
  3. No hint of wind-shear on the airport information system (ATIS)
  4. No warning from air traffic control
  5. Crew complacency

= Wind-Shear Accident

Skid Airways Hypothetical Accident

Reason's SWISS CHEESE Model

Features of the Swiss Cheese Model

  • Systems are protected by multiple layers of defenses that are designed to prevent hazards or system failures from cascading into accidents.
  • Each layer of protection, however can develop "holes" or flaws through safety deficiencies, resembling swiss cheese.
  • as the number and size of these holes in the defenses increase, the chances of accidents also increase.
  • When the holes in each of the layers of defenses line up, an accident occurs.

Real World Problems Using Reason's Swiss Cheese Model

Organizational Influences

  • Rapid Expansion
  • Lack of Regulation
  • Management "lip service" to safety

Unsafe Supervision

  • Risks and hazards neglected
  • Poor work scheduling- fatigue
  • Insufficient training

Preconditions for Unsafe Acts

  • High workload
  • Time pressure to perform tasks
  • Ignorance of the system

Unsafe Acts

  • Aircraft warning system disabled
  • Omission of critical checklist item
  • Over- reliance on automation

In Class Assignment- Fictional Accident

Develop a fictional accident that is the result of two holes in the "Swiss Cheese" Model.

The SHELL Model

  • Liveware to Software- The relationship between the human and supporting systems found in the workplace. Not just computer programs, these include user friendly issues in regulation, manuals and checklists.
  • Liveware to Hardware- The relationship between man and machine. Although humans adapt well to poor interfaces, they can easily cause safety hazards if not well designed.
  • Liveware to Liveware- This is the relationship between the human and other people in the workplace. Communication styles and techniques are important here.
  • Liveware to Environment- The relationship between the human and the internal and external environments.

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