Appendix A to AMC 20-21 Enhanced Zonal Analysis Logic Diagram and Steps

ED Decision 2008/007/R

Figure 1. Enhanced Zonal Analysis Procedure

Figure 2. Step 8 - Wiring Inspection Level and Interval Selection

The following paragraphs provide further explanation of each step in the Enhanced Zonal Analyses Procedure logic, (Figures 1 and 2). It is recommended that, where possible, the analysts utilise the availability of actual aircraft to ensure they fully understand the zones being analysed. This will aid in determination of density, size, environmental issues, and accidental damage issues.

Step 1              “Identify aircraft zones, including boundaries”

The system consists of Major Zones, Major Sub Zones and Zones.

The zones, wherever possible, shall be defined by actual physical boundaries such as wing spars, major bulkheads, cabin floor, control surface boundaries, skin, etc. and include access provisions for each zone.

If the type design holder or operator has not yet established aircraft zones, it is recommended that it does 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/vapour 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 vapours, 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.

Step 3              “Zone contains wiring?”

This question serves as a means to eliminate from the EZAP 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 vapours, 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 vapour/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, its 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 localised 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?”

Most operator maintenance programmes have not included tasks directed towards removal or prevention of significant accumulations of combustible materials on or adjacent to 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 and Powerplant section. Within Maintenance Review Board (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 5 cm (2 inches)) 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 localised 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 5 cm (2 inches)) to a wiring harness. In consideration of the redundancy in flight control systems, the question needs to 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.

On all aircraft type designs, irrespective of TC date, modifications may not have taken into account the TC holder’s design and installation 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 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 back-up 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 and 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 localised fire on adjacent wiring and systems by considering the potential for loss of multiple functions to the extent that continued safe operation may not be possible.

Consideration of potential effect must also include whether wiring is in close proximity (i.e. within 5 cm (2 inches)) to both primary and back-up flight controls. A GVI alone may not be adequate if a fire caused by failure of the wiring poses a risk to aircraft controllability.

At minimum, all wiring in the zone will require a GVI at a common interval. For operators with a ZIP, this may be defined as a zonal GVI. For operators without ZIP, it shall be defined as a GVI of all wiring in the zone.

The question is asked, "Is a GVI (or zonal GVI) of all wiring in the zone at the same interval effective for all wiring in the zone?" This is to consider if there are specific items/areas in the zone that are more vulnerable to damage or contamination and thus may warrant a closer or more frequent inspection.

This determination could result in the selection of a more frequent GVI, a stand-alone GVI (for operators with a ZIP), or even a DET inspection. The intention is to select a DET of wiring only when justified by consideration of all three characteristics of the zone (size, density, and potential effect of fire). The analyst should be cautious to avoid unnecessary selection of DET where GVI is adequate. Over-use of DET dilutes the effectiveness of the inspection.

Note: The level of inspection required may be influenced by tasks identified in Steps 5 and 6. For example, if a cleaning task was selected in Step 5 and 6 that will minimise the accumulation of combustible materials in the zone, this may justify selection of a GVI in lieu of a DET for the wiring in the zone.

b.        Inspection Interval.

The selection of an effective interval can also be accomplished using a rating system. The characteristics for wiring to be rated should include the following:

—      Possibility of Accidental Damage;

—      Environmental factors.

The rating tables should be designed to define increasing inspection frequency with increasing risk of accidental damage and increasing severity of the local environment within the zone. Examples are provided in Appendix E.

The selection of inspection tasks possible in this step is specific to whether the maintenance programme includes a dedicated ZIP or not.

For ZIP programmes, the possible inspection tasks are:

—      Zonal GVI;

—      Stand-alone GVI;

—      DET.

For non-ZIP programmes, the possible inspection tasks are:

—      GVI;

—      DET.

Note: At this point the analyst will have determined the required inspection level and interval for wiring in the zone. Task consolidation in Step 9 allows consideration as to whether an inspection selected as a result of this analysis can be considered accomplished as part of the existing maintenance programme.

Step 9              “Task Consolidation”

This step in the procedure examines the potential for consolidation between the tasks derived from the EZAP and inspections that already exist in the Maintenance Programme. Consolidation requires that the inspections in the existing maintenance programme are performed in accordance with the inspection definitions provided in this AMC.

For programmes that include a ZIP:

Some GVI identified by application of the EZAP may be adequately covered by existing zonal GVI in the zone and no change or addition to the existing zonal GVI is required. This should reduce the number of new GVI that must be introduced into a programme that already includes a ZIP.

The consolidation of GVI tasks has to take into account the access requirements and the interval of each task. The Working Group may conclude that a stand-alone GVI of the wiring may be justified if the zonal GVI of the other systems within the same zone does not need to have such a frequent inspection.

Stand-alone GVI and DET identified by application of EZAP cannot be consolidated into the ZIP and must be introduced and retained as dedicated tasks in the scheduled maintenance programme under ATA 20. These tasks, along with tasks identified to reduce the accumulation of combustible materials, shall be uniquely identified to ensure they are not consolidated in the zonal programme nor deleted during future programme development. Within MSG-3 based MRB Reports, these may be introduced under ATA 20 with no Failure Effect Category quoted.

For programmes without a ZIP:

Although non-ZIP programmes may already include some dedicated inspections of wiring that may be reviewed for equivalency to new tasks identified by application of the EZAP, it is expected that a significant number of new wiring inspections will be identified for introduction as dedicated tasks in the System and Powerplant programme. All new tasks identified by application of EZAP shall be uniquely identified to ensure they are not deleted during future programme development.

The following guide can be used to determine proper consolidation between EZAP derived inspections and existing inspections that have not been specifically identified as stand-alone tasks, of the same item or area:

a.         Where the EZAP inspection interval and existing inspection interval are equal, but the inspection levels are different, the more intense inspection will take precedent (i.e. a 1C DET takes precedent over a 1C GVI).

b.        Where the EZAP inspection interval and existing inspection interval are different, but the inspection levels are equal, the more frequent inspection will take precedent (i.e. a 1C GVI takes precedent over a 2C GVI).

c.         Where the EZAP inspection interval and level are different from the existing inspection interval and level, these tasks may be consolidated only when the more frequent inspection is also the more intense (i.e. a 1C DET takes precedent over a 2C GVI). When the more frequent inspection is less intense, the tasks should not be consolidated.

For all programmes, these tasks shall be uniquely identified in the programme for future development consideration.

For EZAP-derived STC tasks, it may not be possible for the STC holder to determine whether a ZIP exists on specific aircraft that will utilise the STC. Therefore, where a ZIP exists, consolidation of EZAP-derived STC tasks into a specific operator’s ZIP will be the responsibility of the operator and subject to approval by the competent authority.

In cases where the STC holder determines a requirement for a GVI that should not be consolidated into a ZIP, this stand-alone GVI should be specifically identified as such in the EZAP derived ICAW for the STC.

[Amdt 20/4]