Tools › Reliability › System Models › System Reliability › Example
MIL-HDBK-338B §6.3.2–6.3.3: equal and ARINC apportionment of a 0.90 requirement
| Tool | System Reliability |
| Source | U.S. Department of Defense (1998). Electronic Reliability Design Handbook. MIL-HDBK-338B, 1 October 1998. Sections 6.3.2 (Example 5) and 6.3.3 (Example 6). |
| Level | Undergraduate |
Problem
A system of three series subsystems with predicted failure rates 0.003, 0.001 and 0.004 per hour must have a reliability of 0.90 for a 20-hour mission. Allocate the requirement to the subsystems by the ARINC weighting method; also show the equal apportionment of a 0.729 requirement to three identical subsystems.
Inputs to Enter
- Components: S1 exp 0.003; S2 exp 0.001; S3 exp 0.004; structure S1 and S2 and S3
- Mission time 20 hours; apportionment target R 90 %
Answers
- Required system rate λ* = −ln 0.9/20 (handbook rounds to 0.005)
- Weights w1 = 0.375, w2 = 0.125, w3 = 0.5; λ1* = 0.001875 with the handbook's λ*
- Allocated R1*(20) = 0.96, R2*(20) = 0.99, R3*(20) = 0.95; their product is exactly 0.90
- Example 5: equal apportionment of 0.729 to three subsystems gives (0.729)^(1/3) = 0.90 each
Notes
Replace the default components and structure and set the mission time to 20 hours; the apportionment select shows the equal and ARINC methods together.