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Probability of Occurrence Classifications

Probability of Occurrence Classifications: May be expressed in qualitative terms like Extremely Improbable, Extremely Remote, Remote, Reasonably Probable or Frequent

Probability of



Extremely improbable




Reasonably probable


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 a few times during the 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

NOTE: As there are usually a number of causal factors that contribute to the risk budget of an undesired event, the probability of each item is (arbitrarily) set one or two orders of magnitude greater than in the table above.

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