A CMR is a required periodic task established during the design certification of the airplane as an operating limitation of the type certificate. Certification Maintenance Requirements are a subset of the tasks identified during the type certification process. CMR’s usually result from a formal, .numerical analysis conducted to show compliance with catastrophic and hazardous failure conditions.
Detects safety significant latent failure
A CMR is intended to detect safety-significant latent failures that would, in combination with one or more other specific failures or events, result in a hazardous or catastrophic failure condition.
Failure-finding tasks
It is important to note that CMR’s are derived from a fundamentally different analysis process than the maintenance tasks and intervals that result from Maintenance Steering Group (MSG-3) analysis associated with Maintenance Review Board (MRB) activities. MSG-3 analysis activity produces maintenance tasks that are performed for safety, operational, or economic reasons, involving both preventative maintenance tasks, which are performed before failure occurs (and are intended to prevent failures), as well as failure-finding tasks. CMR’s, on the other hand, are failure-finding tasks only, and exist solely to limit the exposure to otherwise hidden failures. Although CMR tasks are failure-finding tasks, use of potential failure-finding tasks, such as functional checks and inspections, may also be appropriate.
Verify failure has not occurred
CMR’s are designed to verify that a certain failure has or has not occurred and do not provide any preventative maintenance function. CMR’s “restart the failure clock to zero” for latent failures by verifying that the item has not failed, or cause repair if it has failed. Because the exposure time to a latent failure is a key element in the calculations used in a safety analysis performed to show compliance with § 25.1309, limiting the exposure time will have a significant effect on the resultant overall failure probability of the system. The CMR task interval should be designated in terms of flight hours, cycles, or calendar time, as appropriate.
Type Certification Process
The type certification process assumes that the airplane will be maintained in a condition of airworthiness at least equal to its certified or properly altered condition. The process described in this AC is not intended to establish normal maintenance tasks that should be defined through the MSG-3 analysis process. Also, this process is not intended to establish CMR’s for the purpose of providing supplemental margins of safety for concerns arising late in the type design approval process. Such concerns should be resolved by appropriate means, which are unlikely to incl~de CMR’s not established via normal safety analyses.
CMR’s should not be confused with required structural inspection programs that are developed by the type certificate applicant to meet the inspection requirements for damage tolerance, as required by § 25.571 or § 25.1529, Appendix H25.4 (Airworthiness Limitations section). CMR’s are to be developed and administered separately from any structural inspection programs.
AC 120-77 & AC 25-19
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