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wiring.
This question requires an evaluation of whether the accumulation on or adjacent to
wiring can be significantly reduced. Task effectiveness criteria should include
consideration of the potential for damaging the wiring.
Though restoration tasks (e.g., cleaning) are the most likely applicable tasks, the
possibility to identify other tasks is not eliminated. A detailed inspection of a hydraulic
pipe might be assessed as appropriate if high-pressure mist from pinhole corrosion
could impinge a wire bundle and the inherent zone ventilation is low.
Step 6. “Define task and interval”
This step will define an applicable task and an effective interval. It should be included
as a dedicated task in the Systems & Powerplant section. Within MRB Reports, this
may be introduced under ATA 20 with no Failure Effect Category quoted.
It is not the intent that restoration tasks should be so aggressive as to damage the
wiring, but should be applied to a level that significantly reduces the likelihood of
combustion.
Step 7. “Is wiring close to primary and back-up hydraulic, mechanical, or electrical
flight controls?”
Where wiring is close (i.e., within 2 inches/50 mm) to both primary and back-up
hydraulic, mechanical, or electrical flight controls, this question is asked to ensure that
Step 8 logic is applied even in the absence of combustible materials in the zone.
For zones where combustible materials are present (as determined in Step 4), proximity
is addressed in the inspection level definition portion of Step 8 and this question need
not be asked.
It addresses the concern that segregation between primary and back-up flight controls
may not have been consistently achieved. Even in the absence of combustible material,
a localized wire arcing could impact continued safe flight and landing if hydraulic
pipes, mechanical cables, or wiring for fly-by-wire controls are routed in close
proximity (i.e., within 2 inches/50 mm) to a wiring harness. In consideration of the
redundancy in flight control systems, the question need be answered ‘Yes’ only if both
the primary and back-up system might be affected by wire arcing. Note that in zones
where a fire might be sustained by combustible material the enhanced logic will
automatically be followed.
PROGRAM TO ENHANCE AIRCRAFT ELECTRICAL WIRING INTERCONNECTION SYSTEM MAINTENANCE
Appendix A. Enhanced Zonal Analysis Logic Diagram and Steps 27
On all aircraft type designs, irrespective of TC date, modifications performed by an
STC holder may not have taken into account the TC holder’s design criteria. It is thus
recommended that STC holders assess their design changes with this question included
in the logic unless they can demonstrate that they followed equivalent installation
criteria. Similarly, air carriers and air operators will have to assess any modifications
that have been accomplished on their aircraft.
Step 8. “Selection of Wiring Inspection Level and Interval”
a) Inspection Level
At this point in the analysis, it is already confirmed that wiring is installed in a zone
where the presence of combustible materials is possible and/or the wiring is in close
proximity to primary and backup hydraulic or mechanical flight controls. Therefore,
some level of inspection of the wiring in the zone is required, and this step details how
the proper level of inspection and interval can be selected.
One method of selecting the proper inspection level and interval is through the use of
ratings tables which rate attributes of the zone and how the wiring is affected by, or can
affect those attributes. The precise format of this will be determined by the analyst, but
example rating tables appear in Appendix B and may be referred to for clarity.
The Inspection Level characteristics that may be included in the rating system are:
• Zone size (volume).
• Density of installed equipment within the zone.
• Potential effects of fire on adjacent wiring & systems.
Zone size will be assessed relative to the size of the aircraft, typically identified as
small, medium or large. The smaller the zone and the less congested it is, the more
likely it is that wiring degradation will be identified by GVI.
Density of installed equipment, including wiring, within the zone will be assessed
relative to the size of the zone. The density of the zone is typically identified as low,
medium or high.
Potential effects of fire on adjacent wiring and systems requires the analyst to assess the
potential effect of a localized fire on adjacent wiring and systems by considering the
potential for loss of multiple functions to the extent that continued safe operation may
　
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