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Table 3-3.
Table 3-2: Severity Definitions for FAA AMS Process
Catastrophic Results in multiple fatalities and/or loss of the system
Hazardous
Reduces the capability of the system or the operator ability to cope
with adverse conditions to the extent that there would be:
Large reduction in safety margin or functional capability
Crew physical distress/excessive workload such that operators
cannot be relied upon to perform required tasks accurately or
completely
(1) Serious or fatal injury to small number of occupants of aircraft
(except operators)
Fatal injury to ground personnel and/or general public
Major
Reduces the capability of the system or the operators to cope with
adverse operating condition to the extent that there would be –
Significant reduction in safety margin or functional capability
Significant increase in operator workload
Conditions impairing operator efficiency or creating significant
discomfort
Physical distress to occupants of aircraft (except operator)
including injuries
Major occupational illness and/or major environmental damage,
and/or major property damage
Minor
Does not significantly reduce system safety. Actions required by
operators are well within their capabilities. Include
Slight reduction in safety margin or functional capabilities
Slight increase in workload such as routine flight plan changes
Some physical discomfort to occupants or aircraft (except
operators)
Minor occupational illness and/or minor environmental damage,
and/or minor property damage
No Safety Effect Has no effect on safety
FAA System Safety Handbook, Chapter 3: Principles of System Safety
December 30, 2000
3- 6
Table 3-3: Likelihood of Occurrence Definitions
Probable Qualitative: Anticipated to occur one or more times during the entire
system/operational life of an item.
Quantitative: Probability of occurrence per operational hour is greater that 1 x
10-5
Remote Qualitative: Unlikely to occur to each item during its total life. May occur
several time in the life of an entire system or fleet.
Quantitative: Probability of occurrence per operational hour is less than 1 x 10-5
, but greater than 1 x 10-7
Extremely
Remote
Qualitative: Not anticipated to occur to each item during its total life. May
occur a few times in the life of an entire system or fleet.
Quantitative: Probability of occurrence per operational hour is less than 1 x 10-7
but greater than 1 x 10-9
Extremely
Improbable
Qualitative: So unlikely that it is not anticipated to occur during the entire
operational life of an entire system or fleet.
Quantitative: Probability of occurrence per operational hour is less than 1 x 10-9
MIL-STD-882 Definitions of Severity and Likelihood
An example taken from MIL-STD-882C of the definitions used to define Severity of Consequence and
Event Likelihood are in Tables 3-4 and 3-5, respectively.
Table 3-4: Severity of Consequence
Description Category Definition
Catastrophic I Death, and/or system loss, and/or severe
environmental damage.
Critical II Severe injury, severe occupational illness, major
system and/or environmental damage.
Marginal III Minor injury, minor occupational illness, and/or
minor system damage, and/or environmental
damage.
Negligible IV Less then minor injury, occupational illness, or lee
then minor system or environmental damage.
FAA System Safety Handbook, Chapter 3: Principles of System Safety
December 30, 2000
3- 7
Table 3-5: Event Likelihood (Probability)
Description Level Specific Event
Frequent A Likely to occur frequently
Probable B Will occur several times in the life of system.
Occasional C Likely to occur some time in the life of the
system.
Remote D Unlikely but possible to occur in the life of the
system.
Inprobable E So unlikely, it can be assumed that occurrence
may not be experienced.
3.3.3 Comparison of FAR and JAR Severity Classifications
Other studies have been conducted to define severity and event likelihood for use by the FAA. A
comparison of the severity classifications for the FARs and JARs from one such study2 is contained in
Table 3-6. JARs are the Joint Aviation Regulations with European countries.
2 Aircraft Performance Comparative Safety Assessment Model (APRAM), Rannoch Corporation, February 28, 2000
FAA System Safety Handbook, Chapter 3: Principles of System Safety
December 30, 2000
3- 8
Probability
(Quantitative) 1.0 10 -3 10 -5 10 -7 10 -9
Probability
(Descriptive)
FAR Probable Improbable Extremely Improbable
JAR Reasonably
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