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An Introduction to Tools and Techniques
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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.
To hide this text and give you more room to view the table of tools and techniques, click the "minus" sign symbol at the top right of the container surrounding this introduction.
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Tools and Techniques
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| Management Oversight and Risk Tree (MORT) | MORT technique is used to systematically analyze an accident in order to examine and determine detailed information about the process and accident contributors. This is an accident investigation technique that can be applied to analyze any accident. [Tarrents, 1980] |
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| Man-Machine Integration Design and Analysis Systems (MIDAS) | MIDAS is a Silicon Graphics software tool designed to aid the application of human factors principles and performance models to the design of complex systems. It is intended for use at the earliest stages of the design process and consequently is likely to reduce some of the costs of simulation and prototyping. MIDAS describes a system's operating environment and procedures, and incorporates human performance models into the design process[ Dean, 1997]
See NASA MIDAS page and Eurocontrol. |
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| Markov Analysis (MA) | Similar to the DD and FTA, but is additionally calculates the probability of the system being in various states as a function of time. Here airworthiness is not a simple mathematical calculation, but depends on relative states of part of the system. Provides a means for analysing reliability/availability of systems whose components exhibit strong dependencies.
The Encyclopaedia Britannica defines the Markov process as "A sequence of possible dependent random variables (x1, x2, x3,...) - identified by increasing values of a parameter, commonly time - with the property that any prediction of the value xn, knowing the value x1, x2,... ,xn-1, may be based on xn-1 alone. That is, the future value of the variable depends upon the present value and no the sequence of past values".
See Mathworld Markov page and SAE ARP4761.
- Begin State 1 with full functionality.
- Study consequences of each failure. Group LRU failures.
- Assign failure states for unique consequences of phase 2.
- Connect arrows between States and add failure rate(s) of each.
- Repeat Step 2 to 4 for each state.
- Continue until equipment is totally unserviceable.
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| Master Plan Logic Diagram (MPLD) | An Outgrowth of the Master Logic Diagram to represent all the physical interrelationships among various plant systems and subsystems in a simple logic diagram. It is used for probabilistic assessments to model and integrate the relationship between all plant functions and equipment [Mauri, 2000] |
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| Materials Compatibility Analysis | Provides as assessment of materials utilized within a particular design. Any potential degradation that can occur due to material incompatibility is evaluated. Materials Compatibility Analysis in universally appropriate throughout most systems. [Tarrents, 1980] |
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| Maximum Credible Accident/Worst Case | The technique is to determine the upper bounds on a potential environment without regard to the probability of occurrence of the particular potential accident.
Similar to Scenario Analysis, this technique is used to conduct a System Hazard Analysis. The technique is universally appropriate. [Tarrents, 1980] |
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| Micro Saint | Micro-Saint is a discrete-event task network-modelling tool that can be described by flow diagrams can be analysed to test, for example, alternative solutions or options, assess workload, function allocation, and temporal analysis (albeit based on time estimates). The analysis process requires input from subject matter experts on the task under investigation, training and familiarity with using the tool, and it can be difficult and time consuming to use [Dean, 1997]
See Micro Analysis & Design and Eurocontrol. |
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| MIL-HDBK-217 | "Reliability Prediction of Electronic Equipment" - Even though this handbook is no longer being kept up to date by the US military, it remains the most widely used approach by both commercial and military analysts.
MIL-HDBK-217 has been the mainstay of reliability predictions for about 40 years but it has not been updated since 1995, and there are no plans by the military to update it in the future. For more than ten years Quanterion's Seymour Morris was DoD program manager for MIL-HDBK-217.
The handbook includes a series of empirical failure rate models developed using historical piece part failure data for a wide array of component types. There are models for virtually all electrical/electronic parts and a number of electromechanical parts as well. All models predict reliability in terms of failures per million operating hours and assume an exponential distribution (constant failure rate), which allows the addition of failure rates to determine higher assembly reliability. The handbook contains two prediction approaches: the parts stress technique and the parts count technique and covers 14 separate operational environments, such as ground fixed, airborne inhabited, etc. - As the names imply, the parts stress technique requires knowledge of the stress levels on each part to determine its failure rate, while
- the parts count technique assumes average stress levels as a means of providing an early design estimate of the failure rate.
Typical factors used in determining a part's failure rate include a temperature factor (?T), power factor (?p), power stress factor (?S), quality factor (?Q) and environmental factor (?p) in addition to the base failure rate ?b. For example, the model for a resistor is as follows: ?Resistor = ?b ?T?P?S?Q?E
See Quanterion Solutions Inc. |
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| Modelling/Simulation | There are many forms of modelling techniques that are used in system engineering. Failures, events, flows, functions, energy forms, random variables, hardware configuration, accident sequences, operational tasks, all can be modelled. [Tarrents, 1980] |
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| Modified Cooper-Harper Scale | Human Factors evaluative tool |
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