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include access provisions for each zone.
If the type design holder or operator has not already established aircraft zones it is
recommended that it do so. Whenever possible zones should be defined using a
consistent method such as ATA iSpec 2200 (formerly ATA Spec 100), varied only to
accommodate particular design constructional differences.
Step 2. “List of details of zone”
An evaluation will be carried out to identify system installations, significant
components, L/HIRF protection features, typical power levels in any installed wiring
bundles, combustible materials (present or possible accumulation), etc.
With respect to power levels the analyst should be aware whether the bundle consists
primarily of main generator feeder cables, low voltage instrumentation wiring or
standard bus wiring. This information will later be used in determining the potential
effects of deterioration.
The reference to combustible materials highlights the need to assess whether the zone
might contain material/vapor that could cause a fire to be sustained in the event of an
ignition source arising in adjacent wiring. Examples include the possible presence of
fuel vapors, dust/lint accumulation and contaminated insulation blankets. See also
under Step 4 for further information.
For aircraft types whose design directives may not have excluded the possibility of
inadequate segregation between systems, the analyst should identify locations where
both primary and back-up flight controls are routed within 2 inches/50 mm of a wiring
harness. This information is required to answer the question in Step 7.
PROGRAM TO ENHANCE AIRCRAFT ELECTRICAL WIRING INTERCONNECTION SYSTEM MAINTENANCE
Appendix A. Enhanced Zonal Analysis Logic Diagram and Steps 25
Step 3. “Zone contains wiring?”
This question serves as a means to eliminate from the enhanced zonal analysis
procedure those zones that do not contain any wiring.
Step 4. “Combustible materials in zone?”
This question requires an evaluation of whether the zone might contain combustible
material that could cause a fire to be sustained in the event of an ignition source arising
in adjacent wiring. Examples include the possible presence of fuel vapors, dust/lint
accumulation, and contaminated insulation blankets.
With respect to commonly used liquids (e.g., oils, hydraulic fluids, corrosion
prevention compounds) the analyst should refer to the product specification in order to
assess the potential for combustibility. The product may be readily combustible only in
vapor/mist form and thus an assessment is required to determine if conditions might
exist in the zone for the product to be in this state.
Although liquid contamination of wiring by most synthetic oil and hydraulic fluids
(e.g., Skydrol) may not be considered combustible, it is a cause for concern if it occurs
in a zone where it causes significant adherence of dust and lint.
The analyst should assess what sources of combustible products may contaminate the
zone following any single failure considered likely from in-service experience.
Unshrouded pipes having connections within the zone should be considered as potential
contamination sources. Inherent ventilation in the zone should be taken into account
when determining the potential for subsequent combustion. This influences the
response to the question of how near to the harness the source should be for there to be
a concern.
Avionics and instruments located in the flight compartment and equipment bays tend to
attract dust, etc. In view of the heat generated by these components and the relatively
tightly packed installations, the analyst should consider these zones as having potential
for combustible material. Thus the enhanced logic should always be used for these
zones.
Note: Although moisture (whether clean water or otherwise) is not combustible, it’s
presence on wiring is a cause for concern because it may increase the probability of
arcing from small breaches in the insulation which could cause a localized fire in the
wire bundle. The risk of a sustained fire caused by moisture induced arcing is
mitigated in Step 5 by identification of a task to reduce the likelihood of accumulation
of combustible material on or adjacent to the wiring.
Step 5. “Is there an effective task to significantly reduce the likelihood of accumulation of
combustible materials?”
PROGRAM TO ENHANCE AIRCRAFT ELECTRICAL WIRING INTERCONNECTION SYSTEM MAINTENANCE
Appendix A. Enhanced Zonal Analysis Logic Diagram and Steps 26
Most operator maintenance programs have not included tasks directed towards removal
or prevention of significant accumulations of combustible materials on or adjacent to
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