27 Required function (mission profile) • Set up the reliability block diagram FMEA where (RBD), by performing a redundancy appears Eliminate reliability weaknesses • Determine the component stresses • component/material selection • Compute the failure rate Ai of each • derating component • screening • Compute R(t) at the assembly level • redundancy • Check the fulfillment of reliability design rules • Perform a preliminary design review no yes Go to the next assembly or to the next integration level Figure 2. 1 Reliability analysis procedure at assembly level Taking account of the above considerations, Fig. 2. 1 shows the reliability analysis procedure used in practical applications at assembly level. The proce­ dure of Fig. 2. 1 is based on the part stress method discussed in Section 2. 2. 4 (see Section 2. 2. 7 for the part count method). Also included are a failure modes and effect analysis (FMEA/FMECA), to check the validity of the assumed failure modes, and a verification of the adherence to design guidelines for relia­ bility in a preliminary design review (Section 5. 1, Appendices A3. 3. 5 & A4). Verification of the assumed failure modes is mandatory where redundancy appears, in particular because of the series element in the reliability block diagram (see for instance Example 2. 6, Sections 2. 3. 6 for elements with more than one failure mode & 6. 8. 7 for common cause failures, and Figs. 2. 8- 2. 9 & 6. 17- 6.