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understanding of degradation and failure progression mechanisms would be paramount.
6.6 FUTURE OPPORTUNITIES.
As shown, the data process used in this study could be enhanced by including operators and
flight crews who could provide additional data and confirm occurrences. Additionally, more
centralized, careful, and complete record keeping is encouraged at the repair shops. The
methodology used in this study allows detection of aging, determination of optimal time for
maintenance, and adjustment of maintenance priorities. The methodology is preventative and
potentially can be used for regulatory compliance. It complements the safety assessment at
certification and actually estimates in-service performance. It should be included in the suite of
tools for future safety assessment.
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7. CONCLUSIONS.
The study found that the failure probabilities evaluated fell reasonably within the regulatory
requirements.
• Soft Failures: The hydraulic units could be functional, but perform sluggishly because
several subparts of the same unit were out of tolerance at the same time. Routine
functional checks might not detect sluggishness. Coupled with dependence, the ability of
an individual, sluggish, hydraulic-powered unit to provide a fail-safe load when one or
two other units failed was challenged. If this is determined to be a safety issue, future
actions should be taken.
• Minimum Corrosion: Few corrosion issues were identified. A few water-damage reports
were recorded for one of the major components, but only one report indicated significant
corrosion of internal hardware. Other corrosion cases were found in bearings and,
presumably, had increased the friction of the function. Overall, components were often
removed for more prominent failures, such as electrical faults and hydraulic leakage,
before corrosion was detected. Minor corrosion was repaired in those cases. As time
goes on, however, corrosion may arise in highly reliable units.
• Restoration Process: The refurbished parts evaluated in this study had significantly
shorter product lives than the original units. This was observed for both hydraulically
and electrically powered components. Failure modes were similar, but were more likely
to occur in refurbished parts. Review of the restoration processes is recommended.
Maintenance needs may be different for refurbished parts.
• Study Limitations: Time-failure probabilities often included unconfirmed occurrences
and are, therefore, likely to be conservative. Interviews with operators could confirm
these instances. On the other hand, records may have been incomplete due to the lack of
centralized data keeping, which caused results to be underestimated. The study was
limited by the records provided. Shop findings are essential for successful assessment;
therefore, more careful and complete record keeping is encouraged.
8. RECOMMENDATIONS.
One time in-service validation is recommended for every fleet, with revisits or revalidations
when there is a new design/upgrade or a new vendor. Emphases should be placed on areas of
potential high consequences.
The methodology used in this study allows detection of aging, helps determine the optimal time
for maintenance actions, and assists with establishing maintenance priorities. It potentially can
be used for regulatory compliance and should be included in the current suite of tools to allow
in-service validation.
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9. REFERENCES.
1. Federal Aviation Administration Advisory Circular, “System Design and Analysis,” AC
25.1309-1A, June 21, 1988.
2. Heinrich, H.W., Industrial Accident Prevention, McGraw Hill, New York, 1931.
3. Joint Aviation Regulation, “Equipment, Systems, and Installations,” JAR 25.1309.
4. Microsoft, Visual SourceSafe 6.0, 2002.
5. Microsoft, Access 2003.
6. Meeker, W.Q. and L.A. Escobar, Statistical Methods for Reliability Data, Wiley, New
York, 1998.
7. Lawless, J.F., Statistical Models and Methods for Lifetime Data, 2nd Edition, Wiley,
New York, 2002.
8. Nelson, W.B., Applied Life Data Analysis, Wiley, New York, 2003.
9. S-PLUS 7.0, Statistical Analysis, Graphics and Programming for Windows, Insightful
Corporation, 2005.
10. Lehmann, E.L. and Casella, G., Theory of Point Estimation, Springer, 2nd Edition, 1998.
11. Pawitan, Y., In All Likelihood: Statistical Modeling and Inference Using Likelihood,
Oxford University Press, 2001.
12. Society of Automotive Engineers “Guidelines and Methods for Conducting the Safety
Assessment Process on Civil Airborne Systems and Equipment,” SAE ARP 4761.
10. RELATED DOCUMENTS.
• Airbus Industrie, “Aircraft Maintenance Manual,” Section 05-23 Zonal Inspection and
　
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