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boards and hydraulically powered components shared redundancy issues (discussed below).
Their performance depended on vendor and parts upgrades. Upgrades sometimes changed the
prominent failure modes and introduced new failure modes.
5.2 DEPENDENCE BETWEEN REDUNDANT PATHS.
Several reviews revealed varying degrees of dependence in the designs of redundant paths. For
example, the backup motor, although not powered, could be subject to the same mechanical wear
and tear by following the same mechanism as the active motor. Independent actuators powered
by separate hydraulic systems sharing the same mechanical linkages could also suffer the same
aging effects. Additionally, dependence could come from functioning under the same conditions
(vibration, weather, etc.).
These cases were confirmed in the study. It was evident that working conditions and certain
design-related dependencies were unavoidable. Common practice in the initial safety assessment
should not generally assume complete independence. However, it often assumes that the loss of
one redundancy would not affect the system or the aircraft. This study should complement the
Common Mode Analysis of Society of Automotive Engineers (SAE) Aerospace Recommended
Practice (ARP) 4761, “Guidelines and Methods for Conducting the Safety Assessment Process
on Civil Airborne Systems and Equipment” [12], by estimating the probabilities of potential
common component failures using actual in-service data. If any common component failures are
determined to be safety-related, future assessment should address these issues.
5.3 SOFT FAILURES.
System descriptions also showed that hydraulically supplied components were often designed to
serve as the active backup of other units in the same subsystem. As a group, they were designed
to overcome loss of redundancy or failures of other units. The failure modes could be benign
(e.g., internal hydraulic leak) or more critical (e.g., valve jamming).
This study found that the units could be functional but perform sluggishly. For example, they
could have a combination of worn swivel bearings, out-of-tolerance control valves, and partial
internal leakage and still pass the routine functional checks. This brought up the issue of
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whether the units were able to overcome other failures and provide fail-safe load. The scope of
existing functional checks might need to be expanded or changed to address component
degradation that previously was not determined significant for continued airworthiness. Routine
safety assessments assume optimal health as long as the systems and subsystems pass required
functional checks. In the case of soft failures, sluggish performance, and similar issues,
functional checks should be revisited.
5.4 ZERO TIME ASSUMPTION CHALLENGED ON REFURBISHED ASSEMBLIES.
A zero time condition was assumed for refurbished assemblies (the LRUs of this study).
Although normally assumed for rebuilt engines, in this case, there was a question about whether
the refurbished assemblies of the system could be considered as good as new. The refurbished
assemblies evaluated in this study had significantly shorter product lives than the original units.
This was observed for both hydraulically and electrically powered components. They had failure
modes similar to those of their respective original components. In those cases where the same
subcomponent part is the cause for removal and is replaced during successive repairs, the repair
and acceptance process should be re-evaluated. In contrast, simple mechanical units were found
to be extremely reliable. There has not been enough removals and refurbishments to evaluate the
zero time assumption for those units.
5.5 AGING ISSUES.
Unlike an aircraft’s structure, mechanical components of a mechanical system are LRUs. They
are subject to aging within their own units. Their performance, in general, has little or nothing to
do with the age of the aircraft. Proper monitoring and optimal maintenance should result in a
long-term healthy system. Aging effects were observed in all units evaluated; some effects were
evident sooner than others. Electrical and hydraulic failures seemed to precede mechanical wear
and corrosion. Electrical failures showed a random failure trend over time, while hydraulic and
mechanical failures showed a wearing out failure trend.
5.6 BEARING WEAR.
Bearing wear was consistently prevalent across the evaluated components. Two failure
indications, bearing free play and inefficient mechanical feedback, were often observed with the
actual single failure mode of bearing wear, which was well addressed during the certification
process.
Safety issues can arise under a common mode failure condition (see section 5.2) and if bearing
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