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further in this manual.
Human error
5.2.7 An error occurs when the outcome of a task being performed by a human is not the intended
outcome. The way in which a human operator approaches a task depends on the nature of the task and on
how familiar the operator is with it. Human performance may be skill-based, rule-based or knowledge-based.
Errors may be the consequence of lapses in memory, slips in doing what was intended, or the result of
mistakes which are conscious errors in judgement. A distinction should also be made between honest or
normal errors committed in the fulfilment of assigned duties, and deliberate violations of prescribed
procedures or accepted safe practices. As discussed in Chapter 4, some organizations use the concept of a
“just culture” to assist in defining what errors are “acceptable”.
System design
5.2.8 Given the complex interplay of human, material and environmental factors in operations, the
complete elimination of risk is an unachievable goal. Even in organizations with the best training
programmes and a positive safety culture, human operators will occasionally make errors. The best
designed and maintained equipment will occasionally fail. System designers must therefore take into
account the inevitability of errors and failures. It is important that the system be designed and implemented
in such a way that, to the maximum extent possible, errors and equipment failures will not result in an
accident. In other words, the system is “error-tolerant”.
5.2.9 The hardware and software components of a system are generally designed to meet specified
levels of availability, continuity and integrity. The techniques for estimating system performance in terms of
these parameters are well established. When necessary, redundancy can be built into the system to provide
alternatives in the event of failure of one or more elements of the system.
5.2.10 The performance of the human element cannot be specified as precisely; however, it is
essential that the possibility of human error be considered as part of the overall design of the system. This
requires an analysis to identify potential weaknesses in the procedural aspects of the system, taking into
account the normal shortcomings in human performance. The analysis should also take into account the fact
that accidents rarely, if ever, have a single cause. As noted earlier, they usually occur as part of a sequence
of events in a complex situational context. Therefore, the analysis needs to consider combinations of events
and circumstances in order to identify sequences that could possibly result in safety being compromised.
5.2.11 Developing a safe and error-tolerant system requires that the system contain multiple defences
to ensure that, as much as possible, no single failure or error will result in an accident, and that when a
failure or error occurs, it will be recognized and remedial action taken before a sequence of events leading
to an accident can develop. The need for a series of defences rather than just a single defensive layer arises
from the possibility that the defences themselves may not always work perfectly. This design philosophy is
called “defences-in-depth”.
5.2.12 For an accident to occur in a well-designed system, gaps must develop in all the defensive
layers of the system at the critical time when that defence should have been capable of detecting the earlier
error or failure. An illustration of how an accident event must penetrate all defensive layers is in Figure 5-1.
5-4 Safety Management Manual (SMM)
Figure 5-1. Defences-in-depth
5.3 SAFETY MANAGEMENT CONCEPTS
Cornerstones of safety management
5.3.1 In its most simple terms, safety management involves hazard identification and the closing of
any gaps in the defences of the system. Effective safety management is multidisciplinary, requiring the
systematic application of a variety of techniques and activities across the aviation spectrum. It builds upon
three defining cornerstones, namely:
a) A comprehensive corporate approach to safety. This sets the tone for the management of safety.
The corporate approach builds upon the safety culture of the organization and embraces the
organization’s safety policies, objectives and goals, and, most importantly, senior management’s
commitment to safety.
b) Effective organizational tools to deliver safety standards. Effective organizational tools are needed
to deliver the necessary activities and processes to advance safety. This cornerstone includes how
the organization arranges its affairs to fulfil its safety policies, objectives and goals, and how it
establishes standards and allocates resources. The principal focus is on hazards and their potential
　
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