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

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ED Decision 2021/007/R found in: AMC-20 Amdt 23 - Airworthiness of Products, Parts and Appliances (Jan 2022)
ED Decision 2020/023/R found in: AMC-20 Amdt 21 - Airworthiness of Products, Parts and Appliances (Apr 2021)

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

ED
Decision 2021/007/R

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.

![Aviation.Bot AI suggestion, not from EASA source: **Overall Summary:** The image displays a conceptual graph illustrating the relationship between three candidate Limits of Validity (LOVs) and the progression of Widespread Fatigue Damage (WFD) over flight cycles or hours, alongside key maintenance action points.

**Detailed Description:**
The image features a black background with several white rectangular text boxes and black and red lines forming a conceptual diagram.

The horizontal axis, labeled "Flight cycles/hours" in a white rectangular box at the bottom center, represents a progression over time or usage.

Three prominent, solid red vertical lines are positioned from left to right across the diagram, each representing a "Candidate LOV". Directly above each red line, a white rectangular box contains its label:
* The leftmost red line is labeled "Candidate LOV 1".
* The middle red line is labeled "Candidate LOV 2".
* The rightmost red line is labeled "Candidate LOV 3".

To the right of the "Candidate LOV 1" line, two white rectangular boxes are vertically stacked, indicating implied Y-axis parameters:
* "Residual strength" is positioned in the upper-middle section.
* "Crack length" is positioned below "Residual strength", in the lower-middle section.

Below the horizontal axis, two white rectangular boxes indicate specific points:
* "ISP" (Inspection Start Point) is located below and slightly to the left of the "Candidate LOV 2" line.
* "SMP" (Structural Modification Point) is located below and slightly to the left of the "Candidate LOV 3" line.

On the right side of the diagram, within a large implied white rectangular area, a graph illustrates WFD behavior:
* A solid black curved line, labeled "WFD (average behaviour)" in a white rectangular box below it, starts high on the left, dips downwards, then rises, then dips again, indicating a fluctuating or progressive trend. This curve intersects the "Candidate LOV 2" line at a relatively high point and the "Candidate LOV 3" line at a lower point. It also intersects a dashed vertical line to its left.
* A solid black bell-shaped curve (resembling a normal distribution) is positioned to the right of the "Candidate LOV 3" line. Two vertical dashed black lines flank this bell curve, one to its left and one to its right, indicating its spread. The "Candidate LOV 3" line passes through this bell curve, intersecting it near its peak.
* A solid black curved line connects the "WFD (average behaviour)" curve to the bell curve. This connecting curve originates from the "WFD (average behaviour)" curve (specifically, from a point to the left of where it intersects the leftmost dashed line), curves upwards, then downwards, and intersects the bell curve at a point to the left of its peak.

**Contextual Linkage:**
The diagram visually compares different "Candidate LOVs" against the progression of "WFD (average behaviour)" over "Flight cycles/hours". It highlights how "ISP" and "SMP" are positioned relative to these LOVs and the WFD progression. The implied Y-axis labels "Residual strength" and "Crack length" suggest that these are parameters influenced by or related to the WFD progression. The bell curve and the connecting line illustrate the probabilistic aspect or distribution of WFD in relation to its average behavior.](file_tNNDI2wQkpg/image026.png)

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.

![Aviation.Bot AI suggestion, not from EASA source: The image is a two-dimensional chart illustrating required maintenance actions, specifically Inspection Start Points (ISP) and Structural Modification Points (SMP), for six distinct Widespread Fatigue Damage (WFD) susceptible structures and an overall aircraft timeline, relative to three candidate Limits of Validity (LoV1, LoV2, LoV3).

Detailed Description: The image displays a grey background with a grid structure. The vertical axis on the left is labeled "Susceptible Structure" and contains six horizontal black lines, each numerically labeled from "1" to "6" in white text within black squares, arranged vertically from top to bottom. Below these, there is an additional horizontal black line labeled "Aircraft line" in white text within a white rectangular box. The horizontal axis at the bottom is labeled "Time" in white text within a white rectangular box. Three light blue vertical lines, representing candidate Limits of Validity (LoV), are positioned across the chart. From left to right, these lines are labeled "LoV1", "LoV2", and "LoV3" in white text within light blue rectangular boxes at the top of each line. These vertical lines extend from the top of the chart down to the "Aircraft line". A legend is located at the bottom left of the chart, enclosed in a black-bordered white rectangular box. The legend defines the two types of symbols used in the chart: * A vertical red bar represents "ISP" (Inspection Start Point). * A blue upward-pointing triangle represents "SMP" (Structural Modification Point).

Various red vertical bars (ISP) and blue upward-pointing triangles (SMP) are plotted along the horizontal lines corresponding to each susceptible structure and the "Aircraft line", indicating specific time points for these actions.

Specific placements of symbols: * **Susceptible Structure 1:** One red ISP bar is located between LoV2 and LoV3. One blue SMP triangle is located to the right of LoV3. * **Susceptible Structure 2:** One red ISP bar is located between LoV2 and LoV3. One blue SMP triangle is located to the right of LoV3. * **Susceptible Structure 3:** One red ISP bar is located between LoV2 and LoV3. One blue SMP triangle is located to the right of LoV3. * **Susceptible Structure 4:** One red ISP bar and one blue SMP triangle are located to the left of LoV1. Another red ISP bar and blue SMP triangle are located between LoV1 and LoV2. A third red ISP bar and blue SMP triangle are located between LoV2 and LoV3. * **Susceptible Structure 5:** One red ISP bar is located between LoV2 and LoV3. One blue SMP triangle is located to the right of LoV3. * **Susceptible Structure 6:** One blue SMP triangle is located between LoV1 and LoV2. One red ISP bar and one blue SMP triangle are located between LoV2 and LoV3. * **Aircraft line:** One red ISP bar and one blue SMP triangle are located to the left of LoV1. One red ISP bar and one blue SMP triangle are located between LoV1 and LoV2. Three red ISP bars and three blue SMP triangles are located between LoV2 and LoV3. Two red ISP bars and two blue SMP triangles are located to the right of LoV3.

Contextual Linkage: The chart visually represents the timing of required maintenance actions (ISP and SMP) for individual WFD-susceptible structural areas (1 through 6) and for the aircraft as a whole, relative to three potential Limits of Validity (LoV1, LoV2, LoV3). The horizontal position of the ISP bars and SMP triangles indicates the "Time" at which these actions are required, with respect to the progression towards each LoV. The "Aircraft line" aggregates all maintenance actions required across all susceptible structures at different time points.](file_tNNDI2wQkpg/image027.png)

Figure 2-2: Aeroplane maintenance
actions

[Amdt 20/20]

[Amdt 20/22]

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

ED Decision 2020/023/R

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.

![Aviation.Bot AI suggestion, not from EASA source: The image is a conceptual graph illustrating the relationship between aircraft operational life (Flight cycles/hours), structural integrity parameters (Residual strength, Crack length), and Widespread Fatigue Damage (WFD) average behavior, in relation to three different candidate Limit of Validity (LOV) points.

Detailed Description:
The image displays a graph with a black background. The primary visual elements are lines and curves, along with text labels, mostly contained within a white rectangular area on the right side of the image.

Along the bottom of the graph, a white rectangular box contains the text "Flight cycles/hours", indicating the progression along the horizontal axis.

Three prominent vertical solid red lines are depicted, each representing a "Candidate LOV" and extending downwards from a white rectangular label box.
1.  The leftmost red line is labeled "Candidate LOV 1".
2.  To its right, the second red line is labeled "Candidate LOV 2".
3.  Further to the right, the third red line is labeled "Candidate LOV 3".

Within the white rectangular graph area on the right, several curves and vertical dashed lines are present:
*   A black solid curve, generally decreasing from left to right, is labeled "Residual strength". The label is in a white rectangular box to the left of the curve, connected by a horizontal line pointing to the curve.
*   A black solid curve, generally increasing from left to right, is labeled "Crack length". The label is in a white rectangular box below the "Residual strength" label and to the left of the curve, connected by a horizontal line pointing to the curve.
*   A black solid bell-shaped curve, peaking towards the right, is labeled "WFD (average behaviour)". The label is in a white rectangular box below the curve, connected by a vertical line pointing to the curve.
*   Two vertical black dashed lines are positioned relative to the "WFD (average behaviour)" curve:
    *   The left dashed line is labeled "ISP" (Inspection Start Point) in a white rectangular box below it. This line is positioned to the left of "Candidate LOV 2".
    *   The right dashed line is labeled "SMP" (Structural Modification Point) in a white rectangular box below it. This line is positioned to the right of "Candidate LOV 3".

Spatial relationships indicate that "Candidate LOV 1" is positioned significantly to the left of the "WFD (average behaviour)" curve. "Candidate LOV 2" is positioned to the right of the "ISP" dashed line but still to the left of the main part of the "WFD (average behaviour)" curve. "Candidate LOV 3" is positioned within the rising portion of the "WFD (average behaviour)" curve, to the left of its peak and to the left of the "SMP" dashed line. The "Residual strength" curve appears to intersect or be near the "Crack length" curve at some point, and both curves generally show changes over increasing "Flight cycles/hours".

Contextual Linkage:
The graph visually compares the timing of different candidate Limits of Validity (LOVs) against the progression of Widespread Fatigue Damage (WFD) in an aircraft structure, as measured by "Flight cycles/hours". The "Residual strength" and "Crack length" curves illustrate the structural integrity parameters that change over time. The "WFD (average behaviour)" curve represents the typical onset and progression of WFD. The "ISP" (Inspection Start Point) and "SMP" (Structural Modification Point) indicate critical thresholds for maintenance actions relative to the WFD behavior. The positions of "Candidate LOV 1", "Candidate LOV 2", and "Candidate LOV 3" relative to these curves and points suggest different maintenance implications or requirements at each proposed LOV.](file_wSyEACTCE2d/image021-c.png)

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.

![Aviation.Bot AI suggestion, not from EASA source: **Overall Summary:** The image displays a two-dimensional chart illustrating the timing of Inspection Start Points (ISPs) and Structural Modification Points (SMPs) for six distinct susceptible structural areas of an aeroplane, relative to three different candidate Limits of Validity (LOVs) over time.

**Detailed Description:** The image presents a horizontal bar chart on a dark grey background. The vertical axis on the left is labeled "Susceptible Structure" in a white vertical rectangle with black text. This axis lists six distinct structural areas, numbered "1" through "6", each within a separate white horizontal rectangle with black text. Horizontal black grid lines extend from each of these numbered rectangles across the chart. Below the "Susceptible Structure" axis, there is a white horizontal rectangle labeled "Aircraft line" in black text, from which another horizontal black grid line extends across the chart. The horizontal axis at the bottom is labeled "Time" in a white horizontal rectangle with black text, positioned towards the right. Three light blue vertical lines, labeled "LoV1", "LoV2", and "LoV3" respectively from left to right, are positioned across the chart. Each label is contained within a light blue horizontal rectangle centered above its corresponding vertical line. These lines divide the chart into four temporal sections.

Within the chart area, specific events are marked by symbols:
* **Red vertical bars:** Represent "ISP" (Inspection Start Point).
* **Blue upward-pointing triangles:** Represent "SMP" (Structural Modification Point). A legend at the bottom left of the chart, within a white rectangle, defines these symbols:
* A red vertical bar is shown next to the text "ISP".
* A blue upward-pointing triangle is shown next to the text "SMP".

The distribution of ISPs and SMPs across the susceptible structures and the "Aircraft line" is as follows:

* **Before LoV1:**
 * Susceptible Structure 4: One red vertical bar (ISP).
 * Aircraft line: One red vertical bar (ISP) followed by one blue upward-pointing triangle (SMP).
* **Between LoV1 and LoV2:**
 * Susceptible Structure 4: One blue upward-pointing triangle (SMP).
 * Susceptible Structure 6: One blue upward-pointing triangle (SMP).
 * Aircraft line: One red vertical bar (ISP) followed by one blue upward-pointing triangle (SMP).
* **Between LoV2 and LoV3:**
 * Susceptible Structure 1: One red vertical bar (ISP).
 * Susceptible Structure 2: One red vertical bar (ISP).
 * Susceptible Structure 3: One red vertical bar (ISP).
 * Susceptible Structure 4: One red vertical bar (ISP).
 * Susceptible Structure 5: One red vertical bar (ISP).
 * Aircraft line: Three red vertical bars (ISPs) and three blue upward-pointing triangles (SMPs) are clustered, appearing in an alternating pattern (ISP, SMP, ISP, SMP, ISP, SMP).
* **After LoV3:**
 * Susceptible Structure 1: One blue upward-pointing triangle (SMP).
 * Susceptible Structure 2: One blue upward-pointing triangle (SMP).
 * Susceptible Structure 3: One blue upward-pointing triangle (SMP).
 * Susceptible Structure 4: One red vertical bar (ISP) followed by one blue upward-pointing triangle (SMP).
 * Susceptible Structure 5: One blue upward-pointing triangle (SMP).
 * Susceptible Structure 6: One red vertical bar (ISP) followed by one blue upward-pointing triangle (SMP).
 * Aircraft line: Three red vertical bars (ISPs) and three blue upward-pointing triangles (SMPs) are clustered, appearing in an alternating pattern (ISP, SMP, ISP, SMP, ISP, SMP).

**Contextual Linkage:** The chart visually represents the maintenance actions (inspections and modifications) required for different susceptible structural areas of an aeroplane over time, depending on the chosen Limit of Validity (LOV). Each horizontal line corresponds to a specific susceptible structure, or the aggregate "Aircraft line". The red vertical bars (ISPs) indicate when an inspection must commence, and the blue triangles (SMPs) indicate when a structural modification or replacement is required. The three vertical light blue lines (LoV1, LoV2, LoV3) represent different potential LOV points, with events occurring before or after these points indicating the maintenance burden associated with each LOV. The "Aircraft line" at the bottom aggregates all required ISPs and SMPs across all susceptible structures for a given time period, providing an overall view of maintenance actions.](file_wSyEACTCE2d/image022.png)

Figure 2-2: Aeroplane
maintenance actions

[Amdt 20/20]

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

ED Decision 2021/007/R

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.

![EASA EAR image](/static/files/file_ftLzkYEblJP/image021.png)

<table cellpadding="0" cellspacing="0" width="100%"><tr><td><div><p align="center"><b>WFD</b></p><p align="center"><b>(average behaviour)</b></p></div></td></tr></table>

<table cellpadding="0" cellspacing="0" width="100%"><tr><td><div><p align="center"><b>Flight cycles/hours</b></p></div></td></tr></table>

<table cellpadding="0" cellspacing="0" width="100%"><tr><td><div><p align="center">Candidate</p><p align="center">LOV 1</p></div></td></tr></table>

<table cellpadding="0" cellspacing="0" width="100%"><tr><td><div><p align="center">Candidate</p><p align="center">LOV 2</p></div></td></tr></table>

<table cellpadding="0" cellspacing="0" width="100%"><tr><td><div><p align="center">Candidate</p><p align="center">LOV 3</p></div></td></tr></table>

<table cellpadding="0" cellspacing="0" width="100%"><tr><td><div><p><b>Crack length</b></p></div></td></tr></table>

<table cellpadding="0" cellspacing="0" width="100%"><tr><td><div><p>Residual strength</p></div></td></tr></table>

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.

![EASA EAR image](/static/files/file_ftLzkYEblJP/image023.png)

Figure 2-2: Aeroplane maintenance actions

[Amdt 20/20]

[Amdt 20/22]