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c) the rate of exposure to the hazards. The probability of adverse consequences becomes greater
through increased exposure to the unsafe conditions. Thus, exposure may be viewed as another
dimension of probability. However, some methods of defining probability may also include the
exposure element, for example, a rate of 1 in 10 000 hours.
6.3.2 Risk is the assessed potential for adverse consequences resulting from a hazard. It is the
likelihood that the hazard’s potential to cause harm will be realized.
6.3.3 Risk assessment involves consideration of both the probability and the severity of any adverse
consequences; in other words, the loss potential is determined. In carrying out risk assessments, it is
important to distinguish between hazards (the potential to cause harm) and risk (the likelihood of that harm
being realized within a specified period of time). A risk assessment matrix (such as the one provided in
Table 6-1) is a useful tool for prioritizing the hazards most warranting attention.
6.3.4 There are many ways — some more formal than others — to approach the analytical aspects of
risk assessment. For some risks, the number of variables and the availability of both suitable data and
mathematical models may lead to credible results with quantitative methods (requiring mathematical
analysis of specific data). However, few hazards in aviation lend themselves to credible analysis solely
through numerical methods. Typically, these analyses are supplemented qualitatively through critical and
logical analysis of the known facts and their relationships.
6-4 Safety Management Manual (SMM)
6.3.5 Considerable literature is available on the types of analysis used in risk assessment. For the risk
assessments discussed in this manual, sophisticated methods are not required; a basic understanding of a
few methods will suffice.
6.3.6 Whatever methods are used, there are various ways in which risks may be expressed, for
example:
a) number of deaths, loss of revenue or loss of market share (i.e. absolute numbers);
b) loss rates (e.g. number of fatalities per 1 000 000 seat kilometres flown);
c) probability of serious accidents (e.g. 1 every 50 years);
d) severity of outcomes (e.g. injury severity); and
e) expected dollar value of losses versus annual operating revenue (e.g. U.S.$1 million loss per
U.S.$200 million revenue).
Problem definition
6.3.7 In any analytical process, the problem must first be defined. In spite of identifying a perceived
hazard, defining the characteristics of the hazard into a problem for resolution is not always easy. People
from different backgrounds and experience will likely view the same evidence from different perspectives.
Something that poses a significant risk will reflect these different backgrounds, exacerbated by normal
human biases. Thus, engineers will tend to see problems in terms of engineering deficiencies; medical
doctors as medical deficiencies; psychologists as behavioural problems; etc. The anecdote in the following
box exemplifies the multifaceted nature of defining a problem:
Charlie’s Accident
Charlie has an emotional argument with his wife and proceeds to the local bar where he
consumes several drinks. He leaves the bar and drives away in his car at high speed.
Minutes later, he loses control on the highway and is fatally injured. We know WHAT
happened; we must now determine WHY it happened.
The investigation team consists of six specialists, each of whom has a completely different
perspective on the root safety deficiency.
The sociologist identifies a breakdown in interpersonal communications within the marriage.
An enforcement officer from the Liquor Control Board notes the illegal sale of alcoholic
beverages by the bar on a “two-for-one” basis. The pathologist determines that Charlie’s
blood alcohol was in excess of the legal limit. The highway engineer finds inadequate road
banking and protective barriers for the posted speed. An automotive engineer determines
that Charlie’s car had a loose front end and bald tires. The policeman determines that the
automobile was travelling at excessive speed for the prevailing conditions.
Each of these perspectives may result in a different definition of the underlying hazard.
Chapter 6. Risk Management 6-5
6.3.8 Any or all of the factors cited in this example may be valid, underlining the nature of multicausality.
How the safety issue is defined, however, will affect the course of action taken to reduce or
eliminate the hazards. In assessing the risks, all potentially valid perspectives must be evaluated and only
the most suitable pursued.
Probability of adverse consequences
6.3.9 Regardless of the analytical methods used, the probability of causing harm or damage must be
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Safety Management Manual (SMM) 安全管理手册(46)
