Probability of Occurrence Classifications May be expressed in qualitative terms like Extremely Improbable, Extremely Remote, Remote, Reasonably Probable or Frequent. Extremely Improbable (Qualitative definition) Should virtually never occur in the whole fleet life; (Quantitative definition) < 109 per flight hour. Extremely Remote (Qualitative definition) Unlikely to occur when considering several systems of the same type, but nevertheless, has to be considered as being possible; (Quantitative definition) 107 to 109 per flight hour. Remote (Qualitative definition) Unlikely to occur during total operational life of each system but may occur several times when considering several systems of the same type; (Quantitative definition) 105 to 107 per flight hour. Reasonably Probable (Qualitative definition) May occur once or a few times during the total operational life of a single system; (Quantitative definition) 103 to 105 per flight hour. Frequent (Qualitative definition) May occur once or several times during operational life; (Quantitative definition) 103 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