Defining accidents

Before any safety-related activities can start, the definition of an accident needs to be agreed upon by the system customer and other stakeholders. This definition, in essence, establishes the goals for the safety effort. 

Defining accidents in TCAS is straightforward—only one is relevant, a midair collision. Other more interesting examples are shown in chapter 7. 

Basically, the criterion for specifying events as accidents is that the losses are so important that they need to play a central role in the design and tradeoff process. In the outer planets explorer example in chapter 7, some of the losses involve the mission goals themselves while others involve losses to other missions or a negative impact on our solar system ecology. 

Priorities and evaluation criteria may be assigned to the accidents to indicate how conflicts are to be resolved, such as conflicts between safety goals or conflicts between mission goals and safety goals and to guide design choices at lower levels. The priorities are then inherited by the hazards related to each of the accidents and traced down to the safety-related design features. 

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Fault trees are a deductive method for identifying ways in which hazards can lead to accidents. The approach starts with a well-defined accident, or top event, and works backward toward the various scenarios that can cause the accident. 

Occupational safety legislation and people in industry have traditional recognized the adverse role of the accident in the workplace and have accepted the need to identify causation for the purpose of avoiding repetition. However, the statutes have generally resisted an attempt to provide a workable definition of an accident or have defined accident in terms of an outcome, i.e. injury, near miss, dangerous occrrrrence, incident, etc. 

A formalized deductive technique that works backward from a defined accident to I identify and graphicaliy display combinations of equipment failure and operator error that can lead to the accident. Probabilities are tissigned to each failure and error and an overall probability for the event is determined. 

Acceptable Daily Intake (ADI) The dally intake of a chemical that is considered without appreciable risk on the basis of all the facts known at the time it is defined. Accident An uncontrolled event which has tlie potential... 

The legal definition of accident according to the Merriam-Webster Legal Dictionary, allows for this as it defines accident as an unexpected usually sudden event that occurs without intent or volition although sometimes through carelessness, unawareness, ignorance, or a combination of causes and that produces an unfortunate result (as an injury) . However, even this definition includes the term unexpected . 

Defence in depth is one of the most important principles, since it underlies the safety technology employed in nuclear power plants. All safety activities, whether organisational, behavioural or equipment-related, are subject to layers of overlapping provisions which are designed to ensure that if a failure should occur it would be compensated for or corrected without causing harm to individuals or the public at large. This idea of multiple levels of protection is the central feature of defence in depth, and it is repeatedly used in the specific safety principles applied in nuclear power plants. Two (related) principles of defence in depth are defined accident prevention and accident mitigation [1]. 

Many dictionaries define accident as an event occurring by chance or unintentionally. An accident is also defined as an unplanned event that results in a harmful outcome for example, death, injury, occupational illness, or major damage to or loss of property (System Safety Handbook Practices and Guidelines for Conducting System Safety Engineering and Management, Federal Aviation Administration [FAA], December 2000). An accident event is undesired, unintentional, and results in negative consequences. 

Kunkel (1973) defines accident proneness as an "interindividually differing, personality constant, time-stable tendency to become involved into accidents", (p. 27). The proneness results in an interindividually different accident risk. Farmer Chambers (1926) reached the conclusion that the results of accident statistics allow to differentiate between accident proneness and accident liability. According to them accident proneness is a narrower term compared to accident liability and means a personal tendency predisposing the individual to a relatively high accident rate. Accident liability includes all the factors determining accident rates technical, organizational, and personal factors. 

In order to understand the terminology used in Table 4.1, we need to define accident severity and the accident probability terms. 

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