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maintenance intervals are not optimal. For example, excessive free play of the swivel bearings
simultaneously in all of the servocontrol actuators can lead to oscillation or vibration of the
aircraft rudder in flight. Bearing wear can also result in activation of the redundant path of the
electronic boards and cause switchover to the backup channel, which might cause a discrepancy
between motor input and transducer output. The prevalence of bearing wear reported on the
OEM records indicated the importance of optimal maintenance intervals.
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5.7 CORROSION ISSUES.
In repair records reviewed for all components, few corrosion issues were identified. There were
a few water damage reports recorded in one of the major components, but only one report
indicated significant corrosion of internal hardware. This particular component was removed for
predominately electrical faults and corrosion was investigated, however, the corrosion only
appeared once. Electrical failure was the driving risk factor. Other corrosion cases were found
in the swivel bearings of the servocontrol actuators and, presumably, had increased the friction of
the servocontrol actuation. Overall, components were often removed for failures such as
electrical faults and hydraulic leakage, which were more prominent and preceded corrosionrelated
issues. Minor corrosion was repaired in the cases where it was observed. Corrosion
prevention and control processes seemed to be under control at the time of evaluation.
Determination of long-term effects will rely on continued monitoring. If future upgrades of
electronic boards permit long-term reliability, corrosion issues may arise as a result.
5.8 OPTIMAL MAINTENANCE INTERVALS.
In a few instances, maintenance intervals were found to be more than optimal, but in other
instances, they were less than optimal. Validation of current maintenance practices using inservice
data may be beneficial.
5.9 MINIMUM PERFORMANCE DIFFERENCES AMONG OPERATORS.
Among the major components investigated, the study found minimum performance differences
among operators. The manufacturer’s original designs and maintenance requirements were key
to performance of the system. Variations in operator functional check schedules did not seem to
make a significant difference.
5.10 FUNCTIONAL CHECKS.
Functional and operational checks are performed for degradation or loss of functions. They are
designed to detect failures when they occur or are about to occur. These checks are carried out at
defined intervals of a certain number of flight hours, flight cycles, or calendar time. If well
defined, these checks can detect progression to failures and eliminate loss of function; however,
if they are not well defined, and the failures require immediate attention, the failures most likely
will occur in the air. This was shown in one instance, despite frequent functional checks. Over
time, check instructions and routines can be evaluated against removal and repair indications
reported by the operators and, if necessary, the routines (instructions or schedules) can be
revised.
6. SUMMARY.
6.1 SOCIETY OF AUTOMOTIVE ENGINEERS STANDARDS.
A guiding document for safety assessment and aerospace recommended practice is SAE ARP
4761, “Guidelines and Methods for Conducting the Safety Assessment Process on Civil Airborne
Systems and Equipment” [12]. It is primarily associated with showing compliance with
CFR 25.1309. The processes and methods recommended in the document are commonly known
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tools: function hazard assessments, SSA, FTAs, failure modes and effect analysis (FMEA),
FMESs, etc. Additionally, common cause analysis (CCA) is provided as a tool to verify
independence or to identify specific dependencies among functions, systems, or items. These
tools are comprehensive and include process descriptions of the design and development cycle of
the aircraft. They also include aircraft, system, and item design requirements and verifications,
as well as safety objectives and implementations. Two areas relevant to this study are discussed
below.
For those quantitative tools using probabilities (e.g., FTA and FMEA), SAE ARP 4761 states
“Probability calculations for civil aircraft certifications are based on average probabilities. . . .
For the purpose of these analyses, the failure rates are usually assumed to be constant over time
and are estimates of mature failure rates after infant mortality and prior to wear-out. If wear-out
or infant mortality is a consideration then other methods would need to be employed . . . .
[O]ther distribution functions (e.g., Weibull) have to be applied . . . .” This agrees with the
methodology of this study. Because this study was an aging study, infant mortality and wear-out
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