Safety Objectives and Quantitative Probability Targets
Section 06: System Safety & Functional Safety
Definition
The quantitative and qualitative targets that a design must meet for each failure condition classification. For transport category aircraft under 14 CFR/CS 25.1309, the quantitative probability targets are: Catastrophic failure conditions must be extremely improbable (typically interpreted as a probability of occurrence on the order of 10^-9 or less per flight hour); Hazardous failure conditions must be extremely remote (on the order of 10^-7 per flight hour); Major failure conditions must be remote (on the order of 10^-5 per flight hour); Minor failure conditions must be probable (no specific numerical threshold, but must be shown to be acceptable). In addition to probability targets, qualitative objectives apply: no single failure should lead to a catastrophic failure condition, and the crew must be able to detect and manage failure conditions through appropriate annunciation and procedures.
Where This Shows Up
These quantitative targets are the benchmarks against which system architectures are evaluated in the safety assessment process. Fault tree analysis (FTA) and other quantitative methods are used to calculate whether a design meets the probability objectives for each failure condition. The 10^-9 per flight hour target for catastrophic conditions is one of the most widely cited numbers in aviation certification.
Primary Sources
Provides the quantitative probability targets for each failure condition classification.
EASA's equivalent guidance providing the same probability framework.
Related Terms
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