Downloaded from https://www.everyspec.com ADS-79D-HDBK Results of teardowns should be involved in validating the measured detection value to ensure that it is representative of the actual hardware condition. An example would be Hertzian Contact Fatigue for bearings. This type of fatigue generally results in spalling, which is usually easily detected (through chip detection or vibration monitoring) and also is usually associated with significant operational capability remaining from the onset of spalling.
Again, component sample sizes for validating CBM algorithms to detect faulted and unfaulted bearings should take into account the required confidence and reliability guidelines within this document when the components are flight critical. An alternative to TBO extensions employing CBM monitors and algorithms is to extend TBOs based on actual hardware condition from the field. This may be achieved by using a minimum of 5 detailed teardown inspections of components that reached the original TBO in the field. The criticality of the component and all associated failure modes should also be taken into account.
These factors will also impact the required number of satisfactory teardowns and associated TBO interval extensions. Based on the US Army‘s past experience, teardown inspections on actual field hardware, involving dimensional analysis and comparison to production and depot repair limits, ensures confidence in capturing the inherent variability that may occur with actual field usage. If the parts are determined to still be within acceptable dimensional limits (for operation and repair), a corresponding wear rate may be analyzed and a basis for a new TBO limit established with final approval of the airworthiness activity.
Therefore, it is possible to obtain TBO extensions on unmonitored aircraft based on field experience. US Army historical TBO extensions have been between 200 and 500 hours depending on the analytical results. The advantages to this alternative to CBM are: there are no additional material costs incurred to purchase components for sampling; only the costs to perform the evaluation are required since teardowns must already be conducted on fielded TBO components at the depot; and part reparability with a TBO extension is relatively easy to quantify based on the current depot information.
The disadvantages to this alternative are the time incurred to obtain components from the field that are at, or near, the TBO interval and the relatively small return on investment with the incremental maintenance benefit. 5.9.3 Transitioning to on-condition maintenance for legacy aircraft. Prior to transition to On- Condition for legacy aircraft components/assemblies, incremental TBO extensions discussed in 5.9.2 should be pursued to ensure that wear rates and failure modes associated with on-condition status are fully captured and understood.
Guidelines for obtaining on-condition status for components on monitored systems having performed data acquisition via field faults / seeded fault tests are outlined in paragraphs 5.9.3.1 and 5.9.3.2, respectively. Achieving on-condition status via field faults could take several years, therefore, incremental TBO extensions on monitored aircraft will be instrumental in increasing the chances of observing and detecting naturally occurring faults in the field. This also holds true for seeded fault selected components which have not completed all seeded fault tests required to ensure each credible, critical failure mode can be detected.
Credible critical failure modes are obtained through Failure Modes Effects Criticality Analysis (FMECA) and actual field data. Damage limits should be defined for specific components in order to classify specific hardware condition to CI/HI limits through the use of Reliability Improvement through Failure Identification and Reporting (RIMFIRE), Tear Down Analysis‘s (TDA), 2410 forms, and other available data sources. Implementation plans should be developed for each component clearly identifying goals, test requirements and schedule, initial CI/HI limits, and all work that is planned to show how the confidence and reliability levels delineated in paragraph 5.9.4 will be achieved.
The advantages of the on-condition transition approach include: providing the highest reliability (probability of detection and true negatives) since monitoring hardware and software are tested to 26