Swiss Cheese Model | A Complex Accidents Can Be Understood - Aviation Professional

Swiss Cheese Model | A Complex Accidents Can Be Understood

Swiss Cheese Model |  A Complex Accidents Can Be Understood

Before explaining the Swiss Cheese model of accident causation [ originally proposed by Professor. James Reason,1990], I will explain to you The Role of Human Factors in Aviation Accidents. In aviation, the role that human factors play in accident causation has been accepted for many years.

By the early 1970s, the frequency of aviation accidents had significantly declined due to major technological advances and enhancements to safety regulations. Aviation became a safer mode of transportation, and the focus of safety endeavors was extended to include human factors, including such things as the “man/machine interface”. What I want to point is for more than twenty years in aviation, human factors tended to focus on the individual, without fully considering the operational and organizational context.

The fact is, during the period from 1970-1990 aviation safety endeavors were not extended to include failures in the management and organization of safety.


A Basic Fact of Accident Causation Model " Swiss Cheese"

The Swiss Cheese model of accident causation by Professor. James Reason (1990) was one of many accident causation models were originally developed to assist safety expert who has to investigate aviation accidents, so those aviation accidents could be investigated effectively.

 Thank you, Professor. James Reason, during the mid-1990s, in aviation, safety began to be viewed from a systemic perspective and began encompassing organizational factors as well as human and technical factors.

The notion of an “organizational accident” was introduced. This perspective considered the impact of such things as organizational culture and policies on the effectiveness of safety risk controls.

Aviation safety experts began to use new accident causation models to understand how accidents are caused. That was a useful proactive sense to identify what types of failures or errors generally cause accidents, and so action can be taken to address these failures before they have the chance to occur.


 Accident Causation Model " Swiss Cheese"

Reason's Swiss cheese model has become the dominant paradigm for analyzing human errors and aviation accidents & incidents.

Reason's Swiss cheese model illustrates that accidents involve successive breaches of multiple defenses. These breaches can be triggered by many enabling factors such as equipment failures or operational errors.

The Swiss-Cheese Model contends that complex systems such as aviation are extremely well defended by layers of defenses (otherwise known as “barriers”). A single-point failure is rarely consequential.

Breaches in safety defenses can be a delayed consequence of decisions made at the higher levels of the organization, which may remain dormant until their effects or damaging potential are activated by certain operating conditions (known as latent conditions).

Under such specific circumstances, human failures (or “Active Failures”) at the operational level act to breach the final layers of safety defense. The Reason Model proposes that all accidents include a combination of both active failures and latent conditions ("Latent failures").

Active Failures and Latent failures

The distinction between the hands-on ‘human’ failures and those made by other aspects of the organization have been described by Reason, The Swiss-Cheese Model  (1990) as ‘ACTIVEand LATENT failures.

1-      Active Failures have an immediate consequence and are usually made by front-line people such as Ground support equipment operator, maintenance technicians, and aircraft pilots. These immediately preceded and are the direct cause, of the accident.

2-      Latent failures are those aspects of the organization which can immediately predispose active failures. Common examples of latent failures include (HSE, 1999):

§  Poor design of plant and equipment;

§  Ineffective training;

§  Inadequate supervision;

§  Ineffective communications; and

§  Uncertainties in roles and responsibilities.


Latent failures are crucially important to accident prevention for two reasons:

1. If they are not resolved, the probability of repeat (or similar) accidents remains high regardless of what other action is taken;

2. As one latent failure often influences several potential errors, removing latent failures can be a very cost-effective route to accident prevention.


You may find that it's complicated to understand the Reason's Swiss Cheese Model. Let me facilitate understanding this by explaining the classification of human failures.


Classification of Human Failures (Active Failures)

The term ‘human failures’ is wide and can include a great variety of human behavior. Therefore, in attempting to define human Failures, different classification systems have been developed to describe their nature. Identifying why these Failures occur will ultimately assist in reducing the likelihood of such errors occurring.

In his classification of active failures Reason (1990) distinguishes between intentional and unintentional error. Intentional errors are described as violations, whilst unintentional errors are classified as either slips/lapses or mistakes. These types of human failure are shown in the diagram below (HSE, 1999 ).


1- Human Errors

A- Skilled-Based Error

 - Slips and Lapses

 These occur in routine tasks with a person who know the process well and are experienced in their work:

§  They are action errors that occur whilst the task is being carried out;

§  They often involved missing a step out of a sequence or getting steps in the wrong order and frequently arise from a lapse of attention;

§  Operating the wrong control through a lapse in attention or accidentally selecting the wrong gear are typical examples.

 B- Mistakes

 These are inadvertent errors and occur when the elements of a task are being considered by the person.

They are decisions that are subsequently found to be wrong, although at the time the person would have believed them to be correct. There are two types of ‘mistake’ (HSE, 1999), rule-based and knowledge-based:

§  Rule-based mistakes occur when the operation at hand is governed by a series of rules. The error occurs when an  inappropriate action is tied to a particular event

§  Knowledge-based errors occur in entirely novel situations when you are beyond your skills, beyond the provision of the rules and you have to rely entirely on adapting your basic knowledge and experience to deal with a new problem.


2- Violations

Violations are any deliberate deviation from the rules, procedures, instructions, and regulations, which are deemed necessary for the safe or efficient operation and maintenance of plant or equipment. Breaches in these rules could be accidental/unintentional or deliberate.

Violations occur for many reasons and are seldom willful acts of sabotage or vandalism. The majority stem from a genuine desire to perform work satisfactorily given the constraints and expectations that exist. Violations are divided into three categories: routine, situational and exceptional (HSE,1999).

A- Routine Violations

Are ones where breaking the rule or procedure has become the normal way of working. The violating behavior is normally automatic and unconscious but the violation is recognized as such, by the individual(s) if questioned. This can be due to cutting corners, saving time. or be due to a belief that the rules are no longer applicable.

B- Situational Violations

Occur because of limitations in the employee's immediate workspace or environment. These include the design and condition of the work area, time pressure, number of staff, supervision, equipment availability, and design and factors outside the organization's control, such as weather and time of day. These violations often occur when a rule is impossible or extremely difficult to work to in a particular situation. 

C- Exceptional Violations

Violations that are rare and happen only in particular circumstances, often when something goes wrong. They occur to a large extent at the knowledge-based level. The individual in attempting to solve a novel problem violates a rule to achieve the desired goal.



In this example, the threats to safety are represented by the holes in the slices.

Slice  1: Management level

The decision was taken three months ago (Expanding the operation network with the current human power and current maintenance capabilities) and this decision led to " Operational Pressure".

Slice  2: Reliable Maintenance

The airline suffers from " Missing Component" of reliable maintenance

Slice  3: Unsafe Acts

Undocumented Procedures

Slice  4:  Human Failures

A crew deliberately deviating from standard operating procedures followed by a lack of communication, leading to a loss of situational awareness coupled with a non-assertive behavior causing an incident or accident.



Human Error is more than front-line personnel error. Everyone can make errors no matter how well trained and motivated they are.

The Swiss Cheese model of accident causation by Professor. James Reason (1990) was one of many accident causation models were originally developed to assist safety professional who has to investigate aviation accidents, so those aviation accidents could be investigated effectively.

It is useful to distinguish between active and latent failures. Active failures are those hands-on front-line personnel errors that immediately precede an accident. Latent failures are the factors or circumstances within an organization which increase the likelihood of active failures. Latent failures lie hidden until they are triggered in the future.


Thank you for reading. Your comments are highly welcomed to improve this article.



Further reading :

-           ICAO Doc 9859- Safety Management Manual

-           Reason J (1990) Human Error, Cambridge University Press

-           HSE (1999), Reducing Error and Influencing Behaviour, HS(G)48, HSE Books



Post a Comment


  1. I am very much obliged to you that you have shared this knowledge with us. I got some different kind of info Personal Injury Medical Evaluation from your web page, and it is really helpful for everyone. Thanks

  2. The article was up to the point and described the information about education and learning. Thanks to blog author for wonderful and informative post. boaz derra