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must be assessed. With a clear understanding of the nature of the risks, a determination can be made as to
the “acceptability” of the risks. Those found to be unacceptable must be acted upon.
4.1.2 Safety management is centred on such a systematic approach to hazard identification and risk
management — in the interests of minimizing the loss of human life, property damage, and financial,
environmental and societal losses.
4.2 CONCEPT OF RISK
4.2.1 Since safety is defined in terms of risk, any consideration of safety must therefore involve the
concept of risk.
4.2.2 There is no such thing as absolute safety. Before any assessment can be made as to whether or
not a system is safe, it is first necessary to determine what the acceptable level of risk is for the system.
4.2.3 Risks are often expressed as probabilities; however, the concept of risk involves more than
probabilities. To illustrate this with a hypothetical example, let us assume that the probability of the
supporting cable of a 100-passenger cable car failing and allowing the cable car to fall was assessed as
being the same as the probability of a 12-passenger elevator failing and allowing the elevator to fall. While
the probabilities of the events occurring may be the same, the potential consequences of the cable car
accident are much more severe. Risk is therefore two-dimensional. Evaluation of the acceptability of a given
risk associated with a particular hazard must always take into account both the likelihood of occurrence of
the hazard and the severity of its potential consequences.
4.2.4 The perceptions of risk can be derived from the following three broad categories:
a) risks that are so high that they are unacceptable;
b) risks that are so low that they are acceptable; and
c) risks in between the two categories in a) and b), where consideration needs to be given to the
various trade-offs between risks and benefits.
4.2.5 If the risk does not meet the predetermined acceptability criteria, an attempt must always be
made to reduce it to a level that is acceptable, using appropriate mitigation procedures. If the risk cannot be
reduced to or below the acceptable level, it may be regarded as tolerable if:
4-2 Safety Management Manual (SMM)
a) the risk is less than the predetermined unacceptable limit;
b) the risk has been reduced to a level that is as low as reasonably practicable; and
c) the benefits of the proposed system or changes are sufficient to justify accepting the risk.
Note.— All three of the above criteria should be satisfied before a risk is classed as tolerable.
4.2.6 Even where the risk is classed as acceptable (tolerable), if any measures that could result in the
further reduction of the risk are identified, and these measures require little effort or resources to implement,
then they should be implemented.
4.2.7 The acronym ALARP is used to describe a risk that has been reduced to a level that is as low
as reasonably practicable. In determining what is “reasonably practicable” in this context, consideration
should be given to both the technical feasibility of further reducing the risk, and the cost; this could include a
cost-benefit study.
4.2.8 Showing that the risk in a system is ALARP means that any further risk reduction is either
impracticable or grossly outweighed by the costs. It should, however, be borne in mind that when an
individual or society “accepts” a risk, this does not mean that the risk is eliminated. Some level of risk
remains; however, the individual or society has accepted that the residual risk is sufficiently low that it is
outweighed by the benefits.
4.2.9 These concepts are illustrated diagrammatically in the Tolerability of Risk (TOR) triangle in
Figure 4-1. (In this figure, the degree of risk is represented by the width of the triangle.)
4.2.10 Additional guidance regarding risk management is contained in Chapter 6.
Figure 4-1. Tolerability of Risk (TOR) triangle
Unacceptable
region
Tolerable
region
(ALARP)
Acceptable
region
Risk
Negligible
risk
Chapter 4. Understanding Safety 4-3
4.3 ACCIDENTS VERSUS INCIDENTS
4.3.1 Annex 13 provides definitions of accidents and incidents that may be summarized as follows:
a) An accident is an occurrence during the operation of an aircraft which entails:
1) a fatality or serious injury;
2) substantial damage to the aircraft involving structural failure or requiring major repair; or
3) the aircraft is missing or is completely inaccessible.
b) An incident is an occurrence, other than an accident, associated with the operation of an aircraft
which affects or could affect the safety of operation. A serious incident is an incident involving
　
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