RAM Performance Optimization & Availability 3 — Questions and Answers
Question 1: What does the 'bathtub curve' depict in the context of RAM analysis?
- System availability over a mission timeline
- Failure rate variation across a product's lifecycle (infant mortality, useful life, wearout) (Correct answer)
- Cost of maintenance versus system age
- Spare parts demand rate over operational time
Correct answer: Failure rate variation across a product's lifecycle (infant mortality, useful life, wearout)
The bathtub curve illustrates how failure rates are high initially (infant mortality), decrease and stabilize (useful life), then increase again as components wear out.
Question 2: Which redundancy configuration provides fault tolerance without requiring switching logic?
- Active (hot) parallel redundancy (Correct answer)
- Standby (cold) redundancy
- k-out-of-n redundancy
- Series configuration
Correct answer: Active (hot) parallel redundancy
Active parallel redundancy operates all units simultaneously so that failure of one unit does not interrupt system function — no switching is needed.
Question 3: In a 2-out-of-3 redundant system where each unit has reliability R, what is the system reliability?
- 3R² − 2R³
- R³ + 3R²(1−R)
- 3R² − 2R³ = R²(3 − 2R) (Correct answer)
- R + R(1−R)
Correct answer: 3R² − 2R³ = R²(3 − 2R)
For 2-out-of-3: Rs = 3R² − 2R³, which accounts for all combinations where at least two of the three units succeed.
Question 4: The Weibull distribution is preferred over the exponential distribution for reliability modeling primarily because:
- It assumes a constant failure rate, simplifying calculations
- Its shape parameter allows modeling of infant mortality, constant, and wearout failure modes (Correct answer)
- It requires fewer data points to fit accurately
- It only applies to electronic components
Correct answer: Its shape parameter allows modeling of infant mortality, constant, and wearout failure modes
The Weibull shape parameter β allows the distribution to model decreasing (β<1), constant (β=1), or increasing (β>1) failure rates, covering all lifecycle phases.
Question 5: Which preventive maintenance strategy is based on monitoring actual equipment condition rather than fixed time intervals?
- Time-directed maintenance (TDM)
- Condition-based maintenance (CBM) (Correct answer)
- Failure-finding maintenance (FFM)
- Run-to-failure maintenance
Correct answer: Condition-based maintenance (CBM)
Condition-based maintenance (CBM) uses real-time monitoring data (vibration, temperature, etc.) to schedule maintenance only when degradation indicators reach thresholds.
Question 6: How does increasing system complexity (more components in series) generally affect system reliability?
- Reliability increases because more components provide redundancy
- Reliability remains unchanged if each component meets its specification
- Reliability decreases because each additional series element reduces the product of component reliabilities (Correct answer)
- Reliability improves due to load sharing among components
Correct answer: Reliability decreases because each additional series element reduces the product of component reliabilities
For series systems, Rs = R1 × R2 × … × Rn; adding components always reduces overall reliability since each Ri ≤ 1.
Question 7: What is the effect of 'burn-in testing' on field reliability performance?
- It extends the wearout phase by consuming component life
- It shifts operating life into the wearout region prematurely
- It screens out infant-mortality failures before fielding, improving early-life reliability (Correct answer)
- It has no measurable effect on fielded unit failure rates
Correct answer: It screens out infant-mortality failures before fielding, improving early-life reliability
Burn-in testing stresses components under controlled conditions to precipitate and remove early-life (infant mortality) failures before the product is delivered.
RAM Performance Optimization & Availability 3
What does the 'bathtub curve' depict in the context of RAM analysis?