Appendix C. Determination If Service Bulletin Modification of STC Requires EZAP


The EZAP provides a means for TC and STC Holders to develop improvements to EWIS maintenance programs. These improvements will be in the form of new inspections and other tasks designed to prevent significant accumulation of combustible materials on or adjacent to EWIS components that would be added to the Instructions for Continued Airworthiness or Service Bulletins (SBs) for the aircraft and STC's.

While TC Holders are required to conduct the EZAP for all zones in an aircraft, it may be determined that EZAP for an SB or STC is not necessary where the modification does not appreciably affect the zones where it is installed. The "Determination if Service Bulletin Modification or STC Requires EZAP" procedure was developed to identify modifications that sufficiently affect zone attributes to warrant re-application of EZAP to the entire zone.

This logic assumes that the aircraft TC holder has accomplished the EZAP on each zone of the aircraft without consideration of the SB modification or STC installation. The objective of this analysis is to assess whether the modification itself has affected wiring or certain zone attributes that could change the outcome of the EZAP performed by the aircraft type certificate holder.

The determination if the SB or STC requires EZAP, and re-application of the EZAP to SB or STC affected zones, is the responsibility of the respective Holder of the SB or STC. It is expected that the TC and STC Holders will collaborate with each other and Operators as necessary to obtain information required to conduct the analysis. The TC or STC Holder should communicate the results of the procedure, including when no new tasks are identified. The method of communication may be via Service Bulletin or Service Letter and ICAW Revision, or other means acceptable to the TCAA.

In situations where a previously installed STC is no longer supported by a viable STC Holder (e.g., STC Holder defunct), the responsibility for determining if the STC requires EZAP, and re-application of EZAP to any affected zones, is assigned to the individual operators who utilize the STC on their aircraft. In cases where the operator does not have experience in application of analytical logic processes, it will be necessary for the operator to gain competence in, or seek external assistance in conducting the analysis.

A record of the outcome of Operator accomplished analysis for STC's (even if no tasks are identified) should be permanently retained by the Operator. A copy of the record should be included in the aircraft records normally transferred upon change of aircraft operator.

The attached logic chart provides a means to assess whether an SB modification or STC has sufficiently affected wiring or certain other zone attributes as to require reapplication of the EZAP to the entire zone with consideration of the modification present. The section following the chart provides detailed explanations of each step in the Determination If Service Bulletin Modification or STC Requires EZAP with appropriate examples.

It is recommended that where possible, the analyst should utilize the availability of actual aircraft to ensure they fully understand the zones being analyzed. Specifically, it must be determined how installation of the modification could affect zone attributes such as density, environment, proximity of wiring to primary and backup flight controls, presence of combustible materials, and potential for accidental damage to wiring.




Determination If Service Bulletin Modification or STC Requires EZAP

Explanation of Steps

Step 1. "For each zone containing any portion of the modification installation - answer the following."

Using the aircraft manufacturer's zone identification system (typically, zone numbers), identify all zones where any portion of the modification is installed, including non-wiring portions such as plumbing, ducting, cables, etc.

For each zone containing any portion of the modification, perform the following Steps:

Step 2. "Does the modification install or route wire(s) within 2 in (50 mm) of both primary & backup hydraulic, mechanical, or electrical flight controls?"

This questions addresses the concern that segregation between primary and back-up flight controls may not have been consistently achieved in the aircraft design. Even in the absence of combustible material, a localized arc emanating from wiring could impact continued safe flight and landing if the wiring is within 2-inches/50 mm of hydraulic, mechanical, or electrical flight controls. 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 an arc from the wiring installed or routed by the modification.

If the answer to Step 2 is 'NO', bypass Steps 3 and 4 and proceed to Step 5.

If the answer is 'YES', proceed to Step 3.

Step 3. "Identify DET for wiring installed or routed by the modification in area of close proximity to flight controls. Assign interval based on Appendix C, Figure 2."

Wiring installed or routed within 2 inches/50 mm of both primary and backup flight controls will require a DET. The TC or STC Holder must identify the specific wiring for DET and use the worksheet in Figure 2 to determine the required interval. The DET and interval must be added to the SB instructions or the STC Instructions for Continued Airworthiness in accordance with CAR 525, Appendix H.

Step 4. "Include DET and Interval in Service Bulletin or STC Instructions for Continued Airworthiness."

The DET and interval identified in Step 3 must be added to the SB instructions or the STC Instructions for Continued Airworthiness.

After completion of Step 3 (and 4, if applicable), continue the analysis and proceed to Step 5.

Step 5. "Does the SB or STC modification affect key zone attributes?"

This step consists of multiple questions to determine if the modification has affected certain zone attributes that could change the outcome of the EZAP applied to the zone without the modification installed. A ‘YES' answer to any of the questions will require reapplication of EZAP to the entire zone.

Does it introduce the potential for presence of combustible materials in the zone for the first time?

This question assesses whether the modification could, by design, or failure of a single item, introduce combustible materials onto or adjacent to wiring in the zone for the first time. If yes, the full EZAP must be accomplished on the zone.

Example 1:
The modification installs a "tee" fitting in a fuel pressure line to support a new fuel pressure indication system. Prior to the modification, the fuel line transited the zone with no connections or couplings in the zone that could be potential leak sources, and the zone had no other potential sources of combustible materials. In this example, the answer to the question would be 'YES' – the newly installed "tee" fitting introduces a potential source of combustible materials in the zone for the first time.

Example 2:
The modification installs a "tee" fitting in a fuel pressure line to support a new fuel pressure indication system. The "tee" is installed at an existing coupling in the line. In this example, the answer to the question would be 'NO' because the potential for a fuel leak already existed at the fuel line connection already present in the zone.

Does it significantly affect one or more of the following zone attributes?

Zone density

Zone environment

Potential effect of fire in the zone

Frequency of maintenance access to the zone

The zone attributes listed here are considered in the EZAP performed by the aircraft TC Holder and are rated (e.g., low, medium, or high) as part of the determination of wiring inspection levels (GVI or DET) and their intervals. For example, the TC Holder could conclude that a high-density zone with an environment hostile to wiring warrants more intense and/or more frequent inspections than wiring in a less dense or hostile zone. The question is asked here assess whether the modification significantly affects any of these attributes which would require a re-analysis of the entire zone.

Zone environment – Does the modification have an effect on zone environment? For example, if the modification installs a new hot-air duct in a zone that could significantly increase the ambient temperature of the zone during any phase of operation (including ground operations), re-analysis of the zone is required.

Potential effect of fire in the zone – Does the modification significantly change the potential effect of a localized fire on adjacent wiring and systems? This question considers the potential for loss of multiple functions to the extent that continued safe operation may not be possible.

Example 1

The modification installs new Terrain Awareness Warning System wiring adjacent to existing wiring associated with primary flight instrumentation. This modification could increase the potential effect of a fire in the zone by the adding TAWS to the multiple systems contained within a single wire bundle that could be lost due to a localized arc or fire. Thus the answer to the question is "YES."

Example 2

The modification replaces existing analog flight instrumentation with new digital displays that include replacement of a single, large, multi-function wire bundle with separately routed wire bundles for the Pilot and Co-Pilot flight displays. This modification reduces the potential effect of a fire in the zone by providing greater separation of wiring for primary flight instruments. In this example, the answer is "YES," with the possibility that re-application of EZAP to the entire zone may determine that GVI rather than DET is adequate for the wiring in the zone after modification.

Frequency of maintenance access to the zone – Does the modification change the frequency of maintenance access to the zone? More frequent access to a zone offers greater potential for accidental damage to wiring in the zone. To properly answer this question, the Analyst should collaborate with the TC Holder and current operators to determine typical frequency of maintenance access to the zone.

Example 1

If the modification significantly affects any of the zone attributes noted, re-analysis of the zone is required and the analysis continues to Step 9. If the modification does not affect these attributes, the analysis continues to Step 6.

Step 6. "Was a task included in SB or STC ICAW in Box 4 above?"


This question is asked to determine whether special reporting for STCs is required. Future rule changes may require special reporting for STCs approved prior to a specified date to document compliance with EZAP requirements when no tasks are identified. If a task was identified in Step 4, the reporting requirement is satisfied by inclusion of the task in the STC ICAW. This requirement does not apply to SBs.

Step 7. "No further action."

Analysis is complete and no special reporting is required.

Step 8. "Comply with special reporting requirement for STCs if EZAP not required."

If the answer to Step 6 is ‘NO', future rulemaking may require special reporting to TCCA and operators for STCs approved prior to a specified date to document compliance with EZAP requirements when no tasks are identified.

Step 9. "EZAP must be accomplished on entire zone with consideration of the SB or STC installed."

At this point, the analyst has determined that the modification could have affected certain zone attributes that may alter the outcome of the EZAP conducted by the TC Holder without regard to the presence of the modification. The full EZAP must be applied to the zone with consideration of the modification present.

It is possible that application of the EZAP may result in no new tasks identified.


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