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 An Introduction to Tools and Techniques Minimize

This table summarises some of the Safety Assessment Tools and Techniques available to the safety assessor. Each of these tools has its own advantages and disadvantages and the extent to which these can be used during various phases of the product lifecycle, and the degree to which they can be applied to safety assessments, vary. For a list of Advantages and Limitations of each, see Appendix A to Aircraft System Safety: Military and Civil Aeronautical Applications.

It is extremely important to note that as the complexity of the tool increases so does the degree of training required for the user and/or the need for an experienced evaluation team to conduct the evaluation. On the plus side, the data derived from the more complex methodologies may be more supportable. Unfortunately, the primary disadvantage of such tools is that "trained subject matter experts" may have limited experience in the actual operational environment and, therefore, their evaluations may not be entirely applicable to the certification process.

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 Tools and Techniques Minimize
 NameSorted By Name In Ascending OrderDescription
Test Safety AnalysisTest Safety Analysis ensures a safe environment during the conduct of systems and prototype testing. It also provides safety lessons to be incorporated into the design, as application. This approach is especially applicable to the development of new systems, and particularly in the engineering/development phase. [Tarrents, 1980]
TestsOften analysis alone cannot accurately predict precise effects or probability of failures, so it becomes essential to conduct actual tests (i.e. on rigs or in situ).

Essential in the following circumstances
  • With circuits which use integrating and differentiating functions or other processing which may be sensitive to changes in time constants.
  • In control system where it is often necessary to have cross-connections between channels in order to achieve synchronization or load sharing or cross-monitoring.
The Sequentially-Timed Events Plot Investigation System (STEP)This method is used to define systems; analyse system operations to discover, assess, and find problems; find and assess options to eliminate or control problems; monitor future performance; and investigate accidents [Tarrents, 1980]
Time/Loss Analysis For Emergency Response EvaluationThis technique is a system safety analysis-based process to semi-quantitatively analyse, measure and evaluate planned or actual loss outcomes resulting from the action of equipment, procedures and personnel during emergencies or accidents. Any airport, airline and other aircraft operators should have an emergency contingency plan to handle unexpected events can be analyzed. This approach defines organize data needed to assess the objectives, progress, and outcome of an emergency response; to identify response problems; to find and assess options to eliminate or reduce response problems and risks; to monitor future performance; and to investigate accidents. [Tarrents, 1980]
Top-Down Analysis ApproachStarts by identifying the failure condition to be investigated and then proceeds to derive those failure modes (and combinations of failure modes) which can produce it. Built on the assumption that evaluation can be best served by examining the system as a whole (its goals, objectives, operating environment, etc.), the examining the individual sub-systems or components [Garland, et at]. An example top-down approach is the Functional Hazard Analysis (FHA)
Trend(ing) AnalysisTrending is performed by sorting various characteristics of events of interest.
Uncertainty AnalysisAddresses, quantitatively and qualitatively, those factors that cause the results of an analysis to be uncertain [Tarrents, 1980]
User AnalysisHuman hazard assessment technique.

Potential system users (including maintainers and installers) are identified and characterized for each stage of the system lifecycle. The most important user population is those people who will be regular users or "operators" of the product or system.
Walk-Trough AnalysisThis technique is a systematic analysis that should be used to determine and correct root causes of unplanned occurrences related to maintenance [Tarrents, 1980]
Weibull AnalysisMost reliability analysis uses an Exponential Time To Failure (TTF) distribution, which says that the instantaneous rate of failure is constant over time, and the item is as likely to fail at one moment as another (i.e. it is "memoryless" - that is, the item is not more likely to fail the next moment simply because it has operated for a long time).
This is not good enough when considering the effect of ageing, when the failure rates are increasing. The question is: how often should this inspection be performed?
One very useful distribution for modeling TTF in the presence of aging is the Weibull Distribution, which has the advantages of:
  1. being very flexible to fit a large number of field data samples, and
  2. (collapsing to the exponential TTF distribution when the field data is fairly flat over time, and
  3. being a theoretical "limiting distribution" (which is somewhat beyond the scope of this brief).
Weibull Analysis
Weibull Analysis

In Weibull analysis, the practitioner attempts to make predictions about the life of all products in the population by "fitting" a statistical distribution to life data from a representative sample of units. The parameterized distribution for the data set can then be used to estimate important life characteristics of the product such as reliability or probability of failure at a specific time, the mean life for the product and failure rate. Life data analysis requires the practitioner to:
  • Gather life data for the product.
  • Select a lifetime distribution that will fit the data and model the life of the product.
  • Estimate the parameters that will fit the distribution to the data.
  • Generate plots and results that estimate the life characteristics, like reliability or mean life, of the product.
See ReliaSoft Weibull site.
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