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the hazard analysis, failures, operating procedures, human factors, and transient conditions are included in
the list of hazard causes.
The FMECA is limited even further in that it only considers hardware failures. It may be performed
either top-down or bottom-up, usually the latter. It is generated by asking questions such as "If this fails,
what is the impact on the system? Can I detect it? Will it cause anything else to fail?" If so, the induced
failure is called a secondary failure.
Reliability predictions establish either a failure rate for an assembly (or component) or a probability of
failure. This quantitative data, at both the component and assembly level, is a major source of data for
quantitative reliability analysis. This understanding is necessary to use it correctly. In summary,
however, hazard analyses are first performed in a qualitative manner identifying risks, their causes, and
the significance of hazards associated with the risk.
8.2.4 What General Procedures Should Follow in the Performance of a Hazard Analysis?
Establish safety requirements baseline and applicable history (i.e., system restraints):
Specifications/detailed design requirements
Mission requirements (e.g., How is it supposed to operate?)
General statutory regulations (e.g., noise abatement)
Human factors standardized conventions (e.g., switches "up" or
"forward" for on)
Accident experience and failure reports
FAA System Safety Handbook, Chapter 8: Safety Analysis/Hazard Analysis Tasks
December 30, 2000
8- 6
Identify general and specific potential accident contributory factors (hazards):
In the equipment (hardware, software, and human)
Operational and maintenance environment
Human machine interfaces (e.g., procedural steps)
Operation
All procedures
All configurations (e.g., operational and maintenance)
Identify risks for each contributory factor (e.g., risks caused by the maintenance environment and the
interface hazards). An example would be performing maintenance tasks incompatible with gloves in a
very cold environment.
Assign severity categories and determine probability levels. Risk probability levels may either be
assigned qualitatively or quantitatively. Risk severity is determined through hazard analysis. This
reflects, using a qualitative measure, the worst credible accident that may result from the risk. These
range from death to negligible effect on personnel and equipment. Evaluating the safety of the system or
risk of the hazard(s), quantitatively requires the development of a probability model and the use of
Boolean algebra. The latter is used to identify possible states or conditions (and combinations thereof)
that may result in accidents. The model is used to quantify the likelihood of those conditions occurring.
Develop corrective actions for critical risks. This may take the form of design or procedural changes.
8.2.5 What Outputs Can Be Expected from a Hazard Analysis?
An assessment of the significant safety problems of the program/system
· A plan for follow-on action such as additional analyses, tests, and training
· Identification of failure modes that can result in hazards and improper usage
· Selection of pertinent criteria, requirements, and/or specifications
· Safety factors for trade-off considerations
· An evaluation of hazardous designs and the establishment of corrective/preventative action
priorities
· Identification of safety problems in subsystem interfaces
· Identification of factors leading to accidents
· A quantitative assessment of how likely hazardous events are to occur with the critical paths
of cause
· A description and ranking of the importance of risks
· A basis for program oriented precautions, personnel protection, safety devices, emergency
equipment-procedures-training, and safety requirements for facilities, equipment, and
environment
· Evidence of compliance with program safety regulations.
FAA System Safety Handbook, Chapter 8: Safety Analysis/Hazard Analysis Tasks
December 30, 2000
8- 7
8.3 Qualitative and Quantitative Analysis
Hazard analyses can be performed in either a qualitative or quantitative manner, or a combination of both.
8.3.1 Qualitative Analysis
A qualitative analysis is a review of all factors affecting the safety of a product, system, operation, or
person. It involves examination of the design against a predetermined set of acceptability parameters. All
possible conditions and events and their consequences are considered to determine whether they could
cause or contribute to injury or damage. A qualitative analysis always precedes a quantitative one.
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