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planned destination is less than
500’ and 1 mile over the
destinations weather planning
minimums within one hour of
the planned arrival.
Provide
Enroute Flight
Advisories of
severe
weather.
Lack EFAS
warning of severe
weather to CSA
flight crew.
Early detection systems
(satellite) for severe weather.
Multiple dissemination
means. Procedures
(condition dependent)
require alternate airports.
Fuel reserve procedures.
The NAS shall detect icing
conditions greater than
moderate accretion when it
actually exists in any area of 10
miles square and at least 1000’
thick for greater than 15
minutes duration.
Tasks in the ASOR phase
Determine existing and recommended hazard controls for each hazard.
Develop requirements based on the TLS and controls.
·  Allocate the requirements so that both ground CNS/ATM and airborne systems share the
controls.
FAA System Safety Handbook, Chapter 4: Pre-Investment Decision Safety Assessments
December 30, 2000
4 -
10
4.2 COMPARATIVE SAFETY ASSESSMENT (CSA)
Comparative Safety Assessments (CSAs) are performed to assist management in the process of decision
making. The CSA is a risk assessment, in that it defines both severity and likelihood in terms of the
current risk of the system. Whereas an OSA defines the target level of safety, a risk assessment provides
an estimation of the risk associated with the identified hazards.
The first step within the CSA process involves describing the system under study in terms of the 5M
model (chapter 3). Since most decisions are a selection of alternatives, each alternative must be described
in sufficient detail to ensure the audience can understand the hazards and risks evaluated. Many times
one of the alternatives will be “no change”, or retaining the baseline system. A preliminary hazard list
(PHL) is developed and then each hazard’s risk is assessed in the context of the alternatives. After this is
done, requirements and recommendations can be made based on the data in the CSA. A CSA should be
written so that the decision-maker can clearly distinguish the relative safety merit of each alternative. An
example (with instructions) of a CSA is included in Appendix B.
4.2.1 Principles of Comparative Safety Assessments
In general, CSA should:
Be objective
Be unbiased
Include all relevant data
Use assumptions only if specific information is not available. If assumptions are made they should be
conservative and clearly identified. Assumptions should be made in such a manner that they do not
adversely affect the safety of the system.
Define risk in terms of severity and likelihood in accordance with chapter 3, paragraph 3.4. Severity is
independent of likelihood in that it can and should be defined without considering likelihood of
occurrence. Likelihood is dependent on severity. The definition of likelihood should be made on how
often an accident can be expected to occur, not how often the hazard occurs.
Compare the results of the risk assessment of each hazard for each alternative considered in order to rank
the alternatives for decision making purposes.
Assess the safety risk reduction or other benefits associated with implementation of and compliance with
an alternative under consideration.
Assess risk in accordance with the risk determination defined in Tables 3-2 and 3-3.
4.2.2 Steps in performing a CSA
Define the system under study in terms of the 5m model described in chapter 3 for the baseline system
and all alternatives.
Perform a functional analysis in accordance with the FAA System Engineering handbook. This analysis
will result in a set of hierarchical functions that the system performs.
From the functions and system description, develop a preliminary hazard list as described earlier in this
chapter.
List these PHL hazard conditions in the form contained in Appendix B
Evaluate each hazard – alternative combination for severity using the definitions contained in chapter 3.
This must be done in accordance with the principles contained in this manual, which require evaluation of
the hazard severity in the context of the worst credible conditions.
FAA System Safety Handbook, Chapter 4: Pre-Investment Decision Safety Assessments
December 30, 2000
4 -
11
Evaluate the likelihood of occurrence of the hazard conditions resulting in an accident at the level of
severity indicated in (4) above. These definitions can be found in chapter 3, Table 7 of this guidebook.
This means that the likelihood selected is the probability of an accident happening in the conditions
　
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