9.4.2. Root causes coverage#

Based on the classification of the root causes of failures, different system level methods are suitable, as shown in Table 9.4.1.

Table 9.4.1 Classification of failure root causes and associated system level methods.#

Failure Category

Root Cause

System Level Methods

Random failures

Unknown residual defect/weakness consistent with quality level and under normal stresses. Only moderate wear-out or degradation of parts.

Random failures on part level are mutually independent and failure rates are constant or do not change significantly over time. On system level various methods can be used, see Section 9.4.3.

Systematic Failure

Design error

Manufacturing error

Operations error

Systematic failures are the result of human error introduced during design, manufacturing or operation. The calculation of systematic failures is part of Section 9.4.9. Systematic failures are to be considered in the system reliability prediction in addition to the random failures on part level. Systematic failures could also result in Common Cause Failures, i.e. failures of several components due to the same cause. Calculation of Common Cause Failures on system level is part of Section 9.4.7.

Wear-Out Failure

Physical degradation process

Operations related

Environment related

Classified as systematic failure if early wear-out is the result of development or manufacturing error.

Failure rates of parts prone to wear-out are time-dependent. On system level wear-out failures can also be considered in the system level reliability prediction by using different probability distribution on component level for the wear-out failures (e.g. Weibull). System level analysis with wear-out on part level is considered in Section 9.4.8.

Extrinsic Failures

Space environment phenomena inducing external failure causes:

Classified as systematic failure if the root cause was underestimated, e.g. requirement or design error (see Section 1 for classification)

Quantitative assessment of extrinsic failures with the system level methods presented in Section 9.4.3 is possible. Extrinsic events can also be regarded as potential Common Cause Failure, see Section 9.4.7.