Arc Skills / Reliability and maintainability
Allocate a system reliability target to subsystems
This task proposes subsystem reliability budgets from a stated system success target. Arc Skills returns an auditable allocation table and exact recombination while keeping targets separate from demonstrated performance.
Use this skill
Use Arc Skills to allocate a system reliability target to its contributing elements.
Inputs:
- Controlled target, mission duration, success event and operating conditions
- System architecture, dependencies and common-cause assumptions
- Existing component evidence and allocation rules
Return proposed allocations with the success equation, target reconciliation, evidence basis, owners and unresolved assumptions. Distinguish allocations from achieved reliability and do not assume independence without justification.What you provide and what you get
| What you have | How it is used | What you get |
|---|---|---|
| Target and mission interval | Defines the design obligation | Allocation basis |
| Success logic | Sets series/parallel combination rule | Recomputation equation |
| Subsystem feasibility data | Tests whether budgets are credible | Gap to estimate |
Equal series allocation clears a 0.99 target by 0.000025
Illustrative engineering example.
A synthetic system needs both subsystem A and subsystem B to operate through one defined mission. The approved system target is 0.99 mission reliability. For illustration, propose 0.995 to each, assuming nonrepairable behavior and independent subsystem success over the same interval.
| Subsystem | Proposed mission target | Contribution | Feasibility status |
|---|---|---|---|
| A | 0.995 | Required series factor | Current estimate not supplied |
| B | 0.995 | Required series factor | Current estimate not supplied |
| Combined A × B | 0.990025 | 0.995 × 0.995; exceeds 0.99 by 0.000025 | Mathematically passes, engineering feasibility open |
The exact product is 0.990025, so this proposal exceeds the stated system target by 0.000025, or 0.0025 percentage points. It is not evidence that either subsystem can achieve 0.995. If a shared supply is required, it belongs in the success equation and consumes some of the target margin.
Equal allocation is a starting convention when no constraint data are supplied. A constrained subsystem may need a different split, provided the recombined result still meets the approved target. These are design budgets, not observed reliability estimates or safety probability objectives.
Allocate against the complete success equation
- Fix success definition, mission interval and approved target.
- Identify every required path and common series element before assigning budgets.
- Choose a feasible provisional split and recompute with the exact equation.
- Compare each target with current evidence and route the proposed allocation for approval.
Questions about this task
Why is 0.995 each enough here?
Under the stated independent two-series model, 0.995 × 0.995 = 0.990025, which is above 0.99.
Does a mathematical pass prove the design target is met?
No. It proves only that the proposed budgets recombine. Component capability, common causes and mission conditions still need evidence.
Sources and further reading
- NASA Fault Tree Handbook with Aerospace Applications: NASA-hosted guidance for fault-tree gates, cut sets and reliability block diagrams.
- NASA Reliability and Maintainability: NASA overview of reliability and maintainability practice and requirements.