Chapter 1 - Use of Probability Concept
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Chapter 1 - Use of Probability Concept

1 General

1.1 The safety assessment process is based on the principle that an inverse relationship should exist between the probability of an occurrence and the severity of its effect. This principle is illustrated in table 1, relating the category of effect to acceptable levels of probability.

1.2 To ensure consistency in the application of the safety assessment process the following definitions apply.

2 Definitions

2.1 Common cause means an occurrence that affects several elements which are otherwise considered independent or redundant.

2.2 Failure is a loss of function or a malfunction of a system or part of a system.

2.3 Failure condition is a condition with an effect on the craft and its occupants caused by one or more failures, taking into account relevant adverse operational or environmental conditions. A failure condition is classified according to the severity of its effects.

2.4 Failure effect is the consequence of a failure condition at craft, system or item level. Failure effects are categorized as follows:footnote

  • .1 Minor effect means the effect of failure conditions that does not significantly reduce craft safety, and which involve crew actions that are well within their capabilities. Failure conditions with a minor effect may include, for example, a slight reduction in safety margins or functional capabilities, a slight increase in crew workload, or some inconvenience to occupants.

  • .2 Major effect means the effect of failure conditions that reduces the capability of the craft or the ability of the crew to cope with adverse operating conditions to the extent that there would be, for example, a significant reduction in safety margins or functional capabilities, a significant increase in crew work load or in conditions impairing crew efficiency, or discomfort to occupants, possibly including injuries.

  • .3 Hazardous effect means the effect of failure conditions that reduces the capability of the craft or the ability of the crew to cope with adverse operating conditions to the extent that there would be, for example, a large reduction in safety margins or functional capabilities, physical distress or higher workload such that the flight crew cannot be relied upon to perform their tasks accurately or completely, or serious or fatal injuries to a relatively small number of occupants.

  • .4 Catastrophic effect means the effect of failure conditions that leads to a loss of the craft and/or multiple fatalities.

  • .5 Hazard is a potentially unsafe condition resulting from failures, malfunctions, external events, errors, or a combination of these.

2.5 Probability level means an acceptable probability range and should be established as the risk per hour in ground effect operation, based on the expected mean operating time for the craft. Five probability levels are distinguished:

  • .1 Extremely improbable failure conditions would be unlikely to arise in the entire operational life of all craft of one type (at worst 10-9).

  • .2 Extremely remote failure conditions are unlikely to occur when considering the total operational life of all craft of one type, but nevertheless have to be considered as being possible (at worst 10-7).

  • .3 Remote failure conditions are unlikely to occur to each craft during its total life but may occur several times when considering the total operational life of a number of craft of a type (at worst 10-5).

  • .4 Reasonably probable failure conditions could arise several times in the life of a craft (between 10-5 and 10-3).

  • .5 Frequent failure conditions are those having a probability greater than 10-3.

2.6 Risk means the frequency (probability) of occurrence and the associated level of hazard.

2.7 Safety assessment means a systematic evaluation of the craft functions and the design of systems performing these functions. It uses recognized methods to identify failure conditions, establish safety objectives and requirements and evaluate the implemented system.

2.8 Safety requirement means a statement in a specification that can be validated and against which an implementation can be verified.

Parent topic: Part C - Safety Assessment and Safety Management
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