The Customer: Sandia often works with the Federal Aviation
Administration (FAA) and the National Aeronautics and Space Administration (NASA) on projects involving systems reliability or risk assessment. Recently, a member of the CSR contributed to a preliminary risk assessment
of a novel airframe design for the FAA.
The Work:
The FAA, in trying
to assess the relative risks of flying a novel aircraft design, employed Sandia to apply its risk analysis techniques. Sandia examined the design to 1) evaluate its overall reliability and assess any unique
vulnerabilities associated with the design, and 2) compare the design with a conventional aircraft. The objective was met using a fault tree analysis of both hardware failures and human errors to estimate the
probability of total loss of thrust and generate a ranked list of the most likely failures. Human error analysis compared pilot error probabilities in the two designs for several single-engine failure events during
take-off and cruise scenarios.
Key Benefits:
- Systematic modeling and evaluation technique can be used for risk management
- Quantitative analysis for absolute or comparative evaluations
- Human error analysis demonstrated control and handling advantages of the novel design
- Quantitative estimates of human error could be used in conjunction with numerical estimates of hardware failure probabilities
Results:
- Government agency based licensing decisions on a scientific, quantifiable, method of evaluation
- Ranked pareto of failure modes provided insights for design upgrades
- Human error analysis substantiated developer's claim of better system performance in particular failure modes
The Customer: The Center for System Reliability has conducted several design-for-reliability studies for both industry and
government customers. The study presented here examined the hydraulic system on a commercial aircraft.
The Work:
The objectives of this study were:
- Identify system vulnerabilities.
- Provide guidance for system inspection and maintenance
- Recommend the most cost-effective upgrades for improving reliability.
Figure 1. Aircraft Hydraulic System
In high consequence systems where system failure could result in loss of life, designers try to eliminate single point failures. That is, safety systems are often designed with redundancy so that the failure of a
single component cannot cause the system to fail. A fault tree was developed for the hydraulic system to
identify possible failure mechanisms. The fault tree analysis indicated that 8 different single point failures
were possible for the hydraulic system. Figure 2 shows the top contributors to system failure probability. The 8 single point failures are all in the top 10 contributors. 
Figure 2. Top Contributors to Hydraulic System Failure
The second part of this study examined 10 possible upgrades to the hydraulic system. The objective was to
select the optimal subset of upgrades in terms of the greatest reduction in system failure probability for the lowest cost. Results of the optimal upgrades analysis are shown in Figure 3. 
Figure 3. Optimal Upgrades Analysis Key Benefits:
- Potential safety issues were identified,
- Understanding of the system was substantially improved by the analysis, and
- Substantial savings could be realized by avoiding less effective upgrades.
Results:
- The safety analysis identified 8 components whose failure would cause the system to fail. These single point failures were major contributors to system unreliability.
- The optimal upgrades analysis found that doing only 4 upgrades would provide nearly all the safety improvement of doing all 10 upgrades but at less than half the cost.
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