Annex 2 to Appendix 2 to AMC 20-20A Example of how to establish an LOV

This Annex provides a simplified example of how to establish an LOV for a specified aeroplane structural configuration. The process for establishing an LOV involves four steps:

Step 1. Identifying a candidate LOV for the aeroplane structural configuration.

Step 2. Identifying WFD-susceptible structure. For this evaluation, it was determined that the aeroplane structural configuration had six areas with WFD-susceptible structures.

Step 3. Performing a WFD evaluation for each of the six areas of WFD-susceptible structure to determine whether there are inspection start points and structural modification points for the candidate LOV identified. This allows the evaluation of the candidate LOV.

Figure 2-1, shown below, shows the WFD behaviour for one WFD-susceptible area. The figure also shows three different candidate LOVs. Candidate LOV1 is at a point that occurs significantly before the WFD average behaviour line. This LOV will not require any maintenance actions. Candidate LOV2 occurs before the WFD average behaviour line, but closer to it. As a result, inspection will need to start before the LOV. Although candidate LOV3 occurs before the WFD average behaviour line, with this LOV, the probability of WFD in the fleet is unacceptable, and inspection and subsequent modification or replacement is required before the aeroplane reaches LOV3. Note that for LOV2 and LOV3, if inspections were determined to be unreliable, then the SMP would occur at the point on the chart where the ISP is. Using this example, this decision-making process needs to be repeated for all six WFD-susceptible areas.

Applicants should evaluate the candidate LOVs and the results of WFD evaluations for each susceptible area.

Figure 2-1: Comparison of WFD-susceptible structure to aircraft LOV

Step 4. Finalising the LOV. Once all the susceptible areas have been evaluated, the final step is to determine where to establish the LOV that will be proposed for compliance. Figure 2-2 shows the results of the WFD evaluation of the six WFD-susceptible areas. As it is shown, there are inspections and modifications or replacements that should be performed over time to preclude WFD. Any LOV can be valid as long as it is demonstrated that, based on its inherent fatigue characteristics and any required maintenance actions, the aeroplane model will be free from WFD up to the LOV. The example in Figure 2-2 includes three LOVs that could be proposed for compliance.

—        LOV1: Maintenance actions are not required to address WFD.

—        LOV2: Inspection and modification or replacement of area four are required to address WFD.

—        LOV3: The DAH may propose an LOV that is greater than LOV2. However, as shown in Figure 2‑2, that would result in more maintenance actions than identified for LOV2. Operators would be required to perform maintenance actions in four out of the six WFD-susceptible areas. Areas 1, 2 and 4 would have to be inspected prior to the LOV. Areas 3 and 5 are free from WFD maintenance actions. Area 4 would be required to be inspected and modified, and then the modification would be required to be inspected prior to the LOV. Area 6 would require modification prior to reaching the LOV. Some of the maintenance actions required for the LOV may have already been issued in an SB and mandated by an AD. For the rest, ADs will need to be issued.

Figure 2-2: Aeroplane maintenance actions

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