Downloaded from https://www.everyspec.com ADS-79D-HDBK capture all failure modes of a component; and providing the fewest false positives/negatives as a result of the validated reliability and confidence levels in the on-condition design. The disadvantages of the on-condition transition approach involve:
relatively higher (if not highest) costs and lengthiest schedule required to test the sample sizes necessary for validation limitations of current condition monitoring designs which may not be capable of, or optimal for, capturing the onset of component failure modes for legacy aircraft; and the potential limited application for components with life limited parts not tracked via regime recognition monitoring. 5.9.3.1 Seeded fault testing. Seeded fault testing may dramatically reduce the timeline for achieving on-condition maintenance status because it requires less time to seed and test a faulted component than to wait for a naturally occurring fault in the field.
However, if during the seeded fault test program a naturally occurring fault is observed and verified, it can also be used as a data point to help reduce the required testing. Test plans will be developed, identifying each of the credible, critical failure modes and corresponding seeded fault tests required to reliably show that each credible, critical failure mode can be detected. The seeded fault test plan should include requirements for ensuring the test is representative of the aircraft.
Also, on aircraft ground testing may be required to confirm the detectability of seeded faults provided there is sufficient time between detection and component failure to maintain an acceptable level of risk to the aircraft and personnel. To be eligible for on-condition status using seeded fault testing for critical components, a statistically significant sample size for faulted and unfaulted components should take into account the required confidence and reliability guidelines within this document (see Appendix I). TDA‘s will be ongoing for components exceeding initially established CI/HI limits.
Once the capability of the monitoring system has been validated, based on successful test results from the sample specimens for each credible, critical failure mode, increased TBO intervals may be modified to on condition status and approved for use by the airworthiness authority. 5.9.3.2 Field fault analysis. The guidance for achieving on condition status via the accumulation of field faults is essentially the same as those identified in paragraph 5.9.3.1. Incremental TBO extensions will play a bigger role utilizing this approach based on the assumption that fault data will take much longer to obtain if no seeded fault testing is performed.
To be eligible for on-condition status using field fault analysis, a statistically significant sample size (see Appendix I) for faulted and unfaulted components should take into account the required confidence and reliability guidelines within this document for flight critical components. TDA‘s will be ongoing for components exceeding initially established CI/HI limits. Once the capability of the monitoring system has been validated by successful test results from the sample specimens for each credible, critical failure mode, increased TBO intervals may be modified to on condition status and approved for use by the airworthiness authority.
5.9.3.3 Alternatives to transitioning to on-condition for legacy aircraft . During the Reliability Centered Maintenance analysis for justifying pursuit of an On Condition CBM approach, it may become evident that other alternatives to the on condition transition approach are more feasible and should be considered. Two of these alternatives are discussed, herein. One alternative is component redesign and requalification using current on-condition designs that could be built into the component.
For example, an unmonitored, grease filled gear box, that is time limited in the field by the life of the grease, may benefit from a redesign using an oil filled gear box and incorporating a chip detector. Sometimes a simple redesign of a seal may be all that is needed to increase time on wing for a gearbox versus implementing an on-condition maintenance approach. The advantages of a redesign and requalification alternative include: being able to design a specific form of monitoring tailored to capture all failure modes versus a limited number of failure modes (e.g. chip detector versus vibration monitor on a complex gearbox); sustaining fewer false 27