Where the table indicates that the risk is currently unacceptable, action must be taken to reduce the probability of occurrence and/or
the severity of the hazard. If neither mitigating measure is available, the system clearly does not satisfy the safety objectives. In any
process where judgement is applied there will be situations where the tolerability is not clearly defined. An issue that falls into this
area of uncertainty is likely to require, before implementation, the endorsement of the individual ultimately accountable for safety
within the organisation.

Guidance on risk analysis 2
Safety Criticality Classification (JAR 25) that results in one or more of the following effects;
Catastrophic  Hazardous  Major  Minor 
Loss of the Aircraft. Multiple Fatalities  A large reduction in safety margins. Physical distress or a workload such that the flight crew cannot be relied upon to perform their tasks accurately or completely. Serious injury or death of a relatively small pro-portion of the occupants  A significant reduction in safety margins. A reduction in the ability of the flight crew to cope with adverse conditions as a result of increase in workload or as a result of conditions impairing their efficiency. Injury to occupants  Nuisance. Operating Limitations. Use of alternative or Emergency procedures 

Risk Assessment Matrix
Catastrophic  Review  Unacceptable  Unacceptable  Unacceptable  Unacceptable 
Hazardous  Review  Review  Unacceptable  Unacceptable  Unacceptable 
Major  Acceptable  Review  Review  Review  Review 
Minor  Acceptable  Acceptable  Acceptable  Acceptable  Review 
Extremely improbable  Extremely remote  remote  Reasonably probable  Frequent 

Probability of occurrence definitions
Probability of Occurrence classification  Extremely improbable  Extremely remote  Remote  Reasonably probable  Frequent 
Qualitative definition  Should virtually never occur in the whole fleet life.  Unlikely to occur when considering several systems of the same type, but nevertheless, has to be considered as being possible  Unlikely to occur during total operational life of each system but may occur several times when considering several systems of the same type  May occur once or during total operational life of a single system  May occur once or several times during operational life 
Quantitative definition  < 10-9 per flight hour  10-7 to 10-9 per flight hour  10-5 to 10-7 per flight hour  10-3 to 10-5 per flight hour  10-3 per flight hour 

The table above is reproduced from JAR 25 and is specifically related to the probability of an event occurring during flight. It is considered that the definitions are equally valid for aircraft movements at an aerodrome but with probabilities decreased as stated in the Safety Objectives in paragraph 3.
　
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本文鏈接地址：European Action Plan for the Prevention of Runway Incursions(48)