Downloaded from https://www.everyspec.com ADS-79D-HDBK possible to adjust retirement times and inspection requirements based on the severity of the loads environment. Loads variability between pilots performing the same mission can be a dominant factor in establishing retirement times and inspection requirements. Feedback to the user concerning loads severity has a significant potential for reducing maintenance burden and enhancing safety. The purpose of section A.5 is to provide insight of the Army‘s expectations of utilizing a FLM system to enhance Fatigue Life Management and Component Remediation.
The Reliability Criteria for establishing maintenance actions based on a FLM system are provided in section A.6. A.5 POTENTIAL APPLICATIONS A.5.1 Updating service usage spectrums. The FLM system enhances the capability to update service usage spectrums of Army aircraft systems. Refinement with respect to prorating velocity, load factor, angle of bank, sink speed, altitude, and GW provides greater accuracy in representing service usage.
The number of aircraft required to participate in a usage survey should be statistically significant (as calculated using the statistical principals in section 3 of Appendix I).Likewise, a survey should be conducted at sufficient locations to ensure inclusion of all missions, including training locations, to ascertain appropriate usage severity. Ongoing usage monitoring (via regime recognition) is used to assess the need for an updated usage spectrum as well as the need for additional structural flight testing.
Valid regime recognition data is used during planning and conduction of the pilot interview process (it should be noted that pilot interview data is required to update a US Army aircraft usage spectrum). Usage spectrum updates are accomplished based on valid field-representative data. This data includes required pilot interview data and available regime recognition data. The updated usage spectrum provides greater accuracy of current usage. However, the updated spectrum should maintain its intended contribution to component reliability when used to compute retirement lives.
Likewise, the impact on reliability for a segment of the fleet should not be compromised through creation of an overall fleet usage distribution. An example of this would be for a small population of the fleet operating at more severe usage (example, training aircraft with more Ground Air Ground (GAG) and autorotation cycles) which is allowed to interchange components with the majority of the fleet. Lives may be calculated based on an updated worst case usage spectrum for the entire fleet, including the effect of more severe usage for a portion of the fleet.
Alternatively, the worst case life may be determined based on lives calculated in accordance with a basic usage spectrum for the majority of the fleet and a special case spectrum for a unique segment of the fleet. An example claiming to maintain required 0.999999 (six nines) reliability using updated usage spectrum from HUMS is given in reference Adams and Zhao, AHS 2009 3 for the case where: a. Design composite worst case usage spectrum was intended to reflect the 90 th percentile of total population of the anticipated usage. b. Design Top of Scatter (TOS) load was intended to reflect the 99 th percentile of total population of the anticipated load. c.
Fatigue design working curve was selected to reflect the 99.9 th percentile of total population of components. 3 D. O. Adams and J. Zhao, "Searching for the Usage Monitor Reliability Factor Using an Advanced Fatigue Reliability Assessment Model", presented at the American Helicopter Society 65th Annual Forum, Grapevine, Texas, May 27-29, 2009. 36