Appendix 2 to AMC 20-20A Guidelines for the development of a programme to preclude the occurrence of widespread fatigue damage

1.         INTRODUCTION

The terminology and methodology in this Appendix are based upon material developed by the AAWGand lessons learned since the first issue of this AMC.

2.         DEFINITIONS

Extended service goal (ESG) is an adjustment to the design service goal established by service experience, analysis, and/or test during which the principal structure will be reasonably free from significant cracking including WFD.

Monitoring period is the period of time when special inspections of the fleet are initiated due to an increased risk of MSD/MED (ISP) and ending when the SMP is reached.

Scatter factor is a life reduction factor used in the interpretation of fatigue analysis and fatigue test results.

Test-to-structure factor is a series of factors used to adjust test results to full-scale structure. These factors could include, but are not limited to, differences in:

—        stress spectrum,

—        boundary conditions,

—        specimen configuration,

—        material differences,

—        geometric considerations, and

—        environmental effects.

Teardown inspection is the process of disassembling structure and using destructive inspection techniques or visual (magnifying glass and dye penetrant) or other non-destructive inspection (NDI) methods (eddy current, ultrasonic) to identify the extent of damage, within a structure, caused by fatigue, environmental or accidental damage.

WFD _{(average behaviour)} is the point in time when 50 % of the fleet is expected to reach WFD for a particular detail.

3.         GENERAL

The likelihood of the occurrence of fatigue damage in an aircraft’s structure increases with aircraft usage. The design process generally establishes a design service goal (DSG) in terms of flight cycles/hours for the airframe. It is expected that any cracking that occurs on an aircraft operated up to the DSG will occur in isolation (i.e. local cracking), originating from a single source, such as a random manufacturing flaw (e.g. a mis-drilled fastener hole) or a localised design detail. It is considered unlikely that cracks from manufacturing flaws or localised design issues will interact strongly as they grow.

With extended usage, uniformly loaded structure may develop cracks in adjacent fastener holes, or in adjacent similar structural details. These cracks may or may not interact, and they can have an adverse effect on the residual strength capability of the structure before the cracks become detectable. The development of cracks at multiple locations (both MSD and MED) may also result in strong interactions that can affect subsequent crack growth; in which case, the predictions for local cracking would no longer apply. An example of this situation may occur at any skin joint where load transfer occurs. Simultaneous cracking at many fasteners along a common rivet line may reduce the residual strength of the joint to less than the required levels before the cracks are detectable under the routine maintenance programme established at the time of certification.

For new type designs, certified to CS-25 Amendment 19, AMC 25.571 provides guidance on how to establish an LOV. For existing types, for which TCHs need to comply with point 26.303 of Part‑26, CS 26.303 and this AMC apply. The TCH should conduct structural evaluations to determine where and when MSD/MED may occur. Based on these evaluations, the TCH should provide additional maintenance instructions for the structure, as appropriate. The maintenance instructions include, but are not limited to inspections, structural modifications, and limits of validity of the new maintenance instructions. In most cases, a combination of inspections and/or modifications/replacements is deemed necessary to achieve the required safety level. Other cases will require modification or replacement if inspections are not viable.

There is a distinct possibility that there could be a simultaneous occurrence of MSD and MED in a given structural area. This situation is possible on some details that were equally stressed. If this is possible, then this scenario should be considered in developing appropriate service actions for structural areas.

4.         STRUCTURAL EVALUATION FOR WFD

4.1       General

The evaluation has three objectives:

(a)       Identify fatigue-critical structure that may be susceptible to MSD/MED, see paragraph 4.2;

(b)      Predict when it is likely to occur; see paragraph 4.3; and

(c)       Establish additional maintenance actions, as necessary, to ensure continued safe operation of the aircraft; see paragraph 4.4.

4.2       Structure susceptible to MSD/MED

Susceptible structure is defined as that which has the potential to develop MSD/MED. Such structure typically has the characteristics of multiple similar details operating at similar stresses where structural capability could be affected by interaction of multiple cracking at a number of similar details. The following list provides examples of known types of structure susceptible to MSD/MED. (The list is not exhaustive):

Figure A2-1: Longitudinal skin joints, frames, and tear straps (MSD/MED)

Figure A2-2: Circumferential joints and stringers (MSD/MED)

Figure A2-3: Lap joints with milled, chem-milled or bonded radius (MSD)

Figure A2-4: Fuselage frames (MED)

Figure A2-5: Stringer-to-frame attachments (MED)

Figure A2-6: Shear clip end fasteners on shear tied fuselage frame (MSD/MED)

Figure A2-9: Abrupt changes in web or skin thickness — Pressurised or unpressurised structure (MSD/MED)

Figure A2-10: Window surround structure (MSD, MED)

Figure A2-11: Over wing fuselage attachments (MED)

Figure A2-13: Skin at runout of large doubler (MSD) — Fuselage, wing or empennage

Figure A2-14: Wing or empennage chordwise splices (MSD/MED)

Figure A2-15: Rib-to-skin attachments (MSD/MED)

4.3       WFD Evaluation

Point 26.300 of Part-26 requires an LOV to be established according to specified timescales for large transport aeroplanes with MTOWs above 34 901 kg (75 000 lb). For other types, it is recommended that by the time the highest-time aircraft of a particular model reaches its DSG, the evaluation for each area susceptible to the development of WFD should be completed. A typical evaluation process is shown in Figure A2-19 below. This evaluation will establish the necessary elements to determine a maintenance programme to preclude WFD in that particular model’s aircraft fleet. These elements are developed for each susceptible area and include:

4.3.1   Identification of structure potentially susceptible to WFD

Unless already fully addressed in the existing fatigue and damage tolerance evaluation, the TCH should identify each part of the aircraft’s structure that is potentially susceptible to WFD for further evaluation. A justification should be given that supports selection or rejection of each area of the aircraft structure. DAHs for modified or repaired structure should evaluate their structure and its effect on existing structure.

Typical examples of structure susceptible to WFD are included in paragraph 4.2 of this Appendix.

4.3.2   Predicting when WFD will occur

(a)       Characterisation of events leading to WFD

The fatigue process that leads to WFD is shown in Figure A2-17. This figure is applicable to both damage that occurs in multiple sites (MSD) and damage that occurs in similar structures at more than one location (MED). For any susceptible structural area, it is not a question of whether WFD will occur, but when it will occur. In Figure A2-17, the ‘when’ is illustrated by the line titled ‘WFD _{(average behaviour)},’ which is the point when 50 % of the aeroplanes in a fleet would have experienced WFD in the considered area (note that the probability density function for flight cycles or flight hours to WFD has been depicted for reference). The WFD process includes this phase of crack initiation and a crack growth phase. During the crack initiation phase, which generally spans a long period of time, there is little or no change in the basic strength capability of the structure. The actual residual strength curve depicted in Figure A2-17 is flat, and equal to the strength of the structure in its pristine state. However, at some time after the first small cracks start to grow, residual strength begins to degrade. Crack growth continues until the capability of the structure degrades to the point of the minimum strength required by CS 25.571(b). In this context, the line in Figure A2-17 called WFD _{(average behaviour)} represents a point when 50 % of the aeroplanes in a fleet fall below the minimum strength specifications of CS 25.571(b).

Figure A2-17: Effect on residual strength of developing WFD

(b)       Determination of WFD _{(average behaviour)} in the fleet

The time in terms of flight cycles/hours defining the WFD _{(average behaviour)} in the fleet should be established for each susceptible structural area. The data to be assessed in determining the WFD _{(average behaviour)} includes:

—        a complete review of the service history of the susceptible areas, to identify any occurrences of fatigue cracking and the continuing validity of loads and mission profiles,

—        evaluation of the operational statistics of the fleet in terms of flight hours and landings,

—        significant production variants (material, design, assembly method, and any other change that might affect the fatigue performance of the detail),

—        fatigue test evidence including relevant full-scale and component fatigue and damage tolerance test data (see subparagraph 4.3.9 and Annex 1 for more details),

—        teardown inspections, and

—        any fractographic analysis available.

The evaluation of the test results for the reliable prediction of the time to when WFD might occur in each susceptible area should include appropriate test-to-structure factors. If full-scale fatigue test evidence is used, Figure A2‑20 below relates how that data might be utilised in determining WFD _{(average behaviour)}. Evaluation may be analytically determined, supported by test and, where available, service evidence.

Regardless of whether the assessment of WFD _{(average behaviour)} is based on in‑service data, full-scale fatigue test evidence, analyses, or a combination of any of these, the following should be considered:

4.3.3   Initial crack/damage scenario

This is an estimate of the size and extent of multiple cracking expected at MSD/MED initiation. This prediction requires empirical data or an assumption of the crack/damage locations and sequence plus a fatigue evaluation to determine the time to MSD/MED initiation. Alternatively, analysis can be based on either:

—        the distribution of equivalent initial flaws, as determined from the analytical assessment of flaws found during fatigue test and/or teardown inspections regressed to zero cycles; or

—        a distribution of fatigue damage determined from relevant fatigue testing and/or service experience.

4.3.4   Final cracking scenario

This is an estimate of the size and extent of multiple cracking that could cause residual strength to fall to the minimum required level as shown in A2-17. Techniques exist for 3-D elastic-plastic analysis of such problems; however, there are several alternative test and analysis approaches available that provide an equivalent level of safety. One such approach is to define the final cracking scenario as a subcritical condition (e.g. first crack at link-up). The use of a subcritical scenario reduces the complexity of the analysis and, in many cases, will not greatly reduce the total crack growth time, because the majority of the time taken to reach the critical condition is generally in the initiation phase.

4.3.5   Crack growth calculation

Progression of the crack distributions from the initial cracking scenario to the final cracking scenario should be developed. These curves can be developed:

—        analytically, typically based on linear elastic fracture mechanics, or

—        empirically, from test or service fractographic data.

4.3.6   Potential for discrete source damage (DSD)

A structure susceptible to fatigue including MSD/MED may also be affected by DSD due to an uncontained failure of high-energy rotating machinery (i.e. turbine engines). At this time, there is no specific requirement to address prior fatigue cracking in combination with DSD for certification. Nonetheless, when assessing in‑service findings of fatigue cracking, the additional threat posed by any potential DSD should be taken into account when developing the corrective actions and the timescales for its implementation.

4.3.7   Analysis methodology

Differences between multiple-site damage and multiple-element damage

Details of the approach used to characterise events leading up to WFD may be different. The differences will largely depend on whether MSD or MED is being considered. This is especially true for crack interaction.

(a)       Crack interaction

MSD has the potential for strong crack interaction, and the effect of multiple cracks on each other needs to be addressed. MED, in most cases, does not have the same potential for strong crack interaction. The differences between the interaction effects for MSD and MED are illustrated in Figure A2-18.

(b)       MSD and MED interaction

Some areas of an aeroplane are potentially susceptible to both MSD and MED. Simultaneous occurrence of MSD and MED is possible, even though it is not common. A comparison of inspection start points (ISPs) or modification start points might indicate the possibility of this occurrence. If so, the evaluation should consider the interaction between MSD and MED.

Figure A2-18: Difference between MSD and MED interaction effects

The report ‘Recommendations for Regulatory Action to Prevent Widespread Fatigue Damage in the Commercial Aeroplane Fleet’, Revision A, dated June 29, 1999 (a report of the AAWG for the ARAC’s Transport Aircraft and Engine Issues Group), discusses two Round Robin exercises developed by the TCHs to provide insight into their respective methodologies. One outcome of the exercises was an identification of key assumptions or methods that had the greatest impact on the predicted WFD behaviour. These assumptions were:

—        the flaw sizes assumed at initiation of crack growth phase of analysis;

—        material properties used (static, fatigue, fracture mechanics);

—        ligament failure criteria;

—        crack growth equations used;

—        statistics used to evaluate the fatigue behaviour of the structure (e.g. time to crack initiation);

—        methods of determining the structural modification point (SMP);

—        detectable flaw size assumed;

—        initial distribution of flaws; and

—        factors used to determine bound behaviour as opposed to mean behaviour.

(c)       MED

When considering MED, where interaction between cracks in different elements is not a factor, the following should be considered:

(1)       In a structure containing large numbers of similar elements, there is not normally a high probability that, after a crack initiates in an element, a second crack will initiate in the element right next to it. If this does happen, however, the consequences to the overall structure may be severe. This is because having two structural members fail right next to each other can completely negate any ability of the structure to tolerate additional damage. Consequently, when performing an evaluation, applicants should make conservative assumptions and assume failures to be adjacent to each other.

(2)       When an element fails completely, the load that has to be redistributed onto the non-failed structure can be large and can have a significant impact on the strength of both the cracked and uncracked structure; therefore, the effects of load redistribution must be included in the evaluation.

(d)       Establishing maintenance actions

The following parameters are developed from paragraphs 4.3.2 to 4.3.7 above, and are necessary to establish an MSD/MED maintenance programme for the area under investigation.

Fatigue damage is the gradual deterioration of a material subjected to repeated loads. This gradual deterioration is a function of use and can be statistically quantified. The term ‘WFD’ is used, and can be statistically quantified, at the end of the deterioration process when the structure is no longer able to carry the residual strength loads. WFD can never be absolutely precluded because there is always some probability, no matter how small, that it will occur. Therefore, modifying or replacing structure at a predetermined, analytically-derived time stated in flight cycles or flight hours, minimises the probability of having WFD in the fleet. Modification or replacement is the most reliable method for precluding WFD. The point at which a modification is undertaken is referred to as the ‘structural modification point (SMP)’ and it is illustrated in Figure 2-1 of Annex 2. The SMP is generally a fraction of the number representing the point in time when WFD _{(average behaviour)} will occur, and should result in the same reliability as a successful two-lifetime fatigue test. This level of reliability for setting the SMP is acceptable if MSD or MED inspections are shown to be effective in detecting cracks. If the inspections are effective, they should be implemented before the SMP. The implementation times for these inspections are known as the ‘inspection start points (ISPs)’. Repeat inspections are usually necessary to maintain this effectiveness in detecting cracks. If MSD or MED inspections are not effective in detecting cracks, then the SMP should be set at the time of ISP. For the purposes of this AMC, an inspection is effective if, when performed by properly trained maintenance personnel, it will readily detect the damage in question[9]. The SMP should minimise the extent of cracking in the susceptible structural area in a fleet of affected aeroplanes. In fact, if this point is appropriately determined, a high percentage of aeroplanes would not have any MSD or MED by the time the SMP is reached.

Due to the redundant nature of semi-monocoque structures, MED can be difficult to manage in a fleet environment. This stems from the fact that most aircraft structures are built-up in nature, and that makes the visual inspection of the various layers difficult. Also, visual inspections for MED typically rely on internal inspections, which may not be practical at the frequency necessary to preclude MED due to the time required to gain access to the structure. However, these issues are dependent on the specific design involved and the amount of damage being considered. In order to implement a viable inspection programme for MED, static stability must be maintained at all times and there should be no MED concurrent with MSD in a given structural area.

4.3.8   Inspection start point (ISP)

This is the point at which inspection starts if a monitoring period is used. Inspection is not practical for all applications and cannot replace the SMP. The ISP is determined through a statistical analysis of crack initiation based on fatigue testing, teardown, or service experience of similar structural details. It is assumed that the ISP is equivalent to a lower bound value with a specific probability in the statistical distribution of cracking events. Alternatively, the ISP may be established by applying appropriate factors to the average behaviour.

When inspections are determined to be effective, it is necessary to establish when those inspections should start. This point is illustrated in Figure 2-1. The start point is determined through a statistical analysis of crack initiation based on fatigue testing, teardown, or in-service experience of similar structure. The ISP is assumed to be equivalent to a lower-bound value with a specific probability in the statistical distribution of cracking events. Alternatively, an ISP may be established by applying appropriate factors to the number representing WFD _{(average behaviour)}. (e.g. for aluminium alloy structure, dividing the full-scale test result by a factor of 3).

For inspection intervals, see point 4.3.10.

4.3.9   Structural modification point (SMP)

The SMP should be established as a point in time when structures should be modified or replaced to prevent WFD from occurring. This is typically established by:

—        calculating when WFD would first occur in the structure (predicted using the WFD _{(average behaviour)}),

—        setting a time before the predicted occurrence of WFD to perform modifications or replacements that will prevent it.

The applicant should demonstrate that the proposed SMP established during the evaluation has the same confidence level as current regulations require for new certification. In lieu of other acceptable methods, the SMP for aluminium alloy structures can be established as a point reduced from the WFD _{(average behaviour)}, based on the viability of inspections in the monitoring period. The SMP may be determined by dividing the number representing the timing when WFD _{(average behaviour)}will occur by a factor of 2 if there are effectiveinspections, or by a factor of 3 if inspections are not effective. For other materials such as high‑strength steel alloys, larger scatter factors may be necessary to account for increased variability in fatigue performance.

An aircraft should not be operated past the SMP unless the structure is modified or replaced, or unless additional approved data is provided that would extend the SMP. However, if during the structural evaluation for WFD, a TCH/DAH finds that the flight cycles and/or flight hours SMP for a particular structural detail have been exceeded by one or more aircraft in the fleet, the TCH/DAH should expeditiously evaluate selected high-time aircraft in the fleet to determine their structural condition. From this evaluation, the TCH/DAH should notify the competent authorities and propose appropriate service actions.

A DAH may find that the SMP for a particular structural area has been exceeded by one or more aeroplanes in the fleet. In that case, the DAH should expedite the evaluation of those high-time aeroplanes to determine their structural condition, notify EASA and propose appropriate maintenance actions specific to those aeroplanes.

The initial SMP may be adjusted based on the following:

(a)       The tasks necessary to extend an SMP may include any or all of the following:

(1)       Additional fatigue or residual strength tests, or both, on a full-scale aeroplanestructure or a full-scale component followed by detailed inspections and analyses.

(2)       Fatigue tests of new structure orstructure from in-service aeroplaneson a smaller scale than full component tests (i.e. subcomponent or panel tests, or both). If a subcomponent test is used, the SMP would be extended only for that subcomponent.

(3)       Teardown inspections (destructive) on structural components that have been removed from service.

(4)       Teardowninspections (non-destructive) accomplished by selected, limited disassembly and subsequent reassembly of specific areas of high-time aeroplanes.

(5)       Analysis of in-service data (e.g. inspections) from a statistically significant number of aeroplanes.

(b)       If cracks are found in the structural detail for which the evaluation was done during either the monitoring period or the modification programme, the SMP should be re-evaluated to ensure that the SMP does in fact provide the required confidence level. If it is shown that the required confidence level is not being met, the SMP should be adjusted and the adjustment reflected in the appropriate SBs to address the condition of the fleet. Additional regulatory action may be required.

4.3.10 Inspection interval and method

An interval should be chosen to provide a sufficient number of inspections between the ISP and the SMP so that there is high confidence that no MSD/MED condition will reach the final cracking scenario without detection. The interval between inspections depends on the detectable crack size, the critical crack lengths and the probability that the cracks will be detected with the specific inspection method. Conservative scenarios should be assumed for developing the inspection interval unless other assumptions can be consistently supported by test and service experience. If the crack cannot be detected, the SMP must be re-evaluated to ensure there is a high confidence level that no aircraft will develop MSD/MED before modification.

4.4      Evaluation of maintenance actions

For all areas that have been identified as susceptible to MSD/MED, the current maintenance programme should be evaluated to determine whether adequate structural maintenance and inspection programmes exist to safeguard the structure against unanticipated cracking or other structural degradation. The evaluation of the current maintenance programme typically begins with the determination of the SMP for each area.

Each area should then be reviewed to determine the current maintenance actions and compare them to the maintenance needs established in this evaluation. Issues to be considered include the following:

(a)       Determine the inspection requirements (method, inspection start point, and repeat interval) of the inspection for each susceptible area (including that structure that is expected to arrest cracks) that is necessary to maintain the required level of safety.

(b)       Review the elements of the existing maintenance programmes already in place.

(c)       Revise and highlight elements of the maintenance programme necessary to maintain safety.

For susceptible areas approaching the SMP, where the SMP will not be increased or for areas that cannot be reliably inspected, a programme should be developed and documented that provides for replacement or modification of the susceptible structural area.

4.4.1   Period of WFD evaluation validity

At whatever point the WFD evaluation is made, it should support the LOV of the maintenance programme. Consistent with the use of test evidence to support individual SMPs, as described above in paragraph 4.3.9, the LOV of the maintenance programme should be based on fatigue test evidence. For an existing ageing aircraft type, theinitial WFD evaluation of the complete airframe will typically cover a significant forward estimation of the projected aircraft usage beyond its DSG, also known as the ‘proposed ESG’and is effectively a proposed LOV.Typically, an evaluation through an additional 25 % of the DSG would provide a realistic forecast, with reasonable planning time for necessary maintenance action. However, it may be appropriate to adjust the evaluation validity period depending on issues such as:

(a)       the projected useful life of the aircraft at the time of the initial evaluation;

(b)       current NDI technology; and

(c)       airline advance planning requirements for introduction of new maintenance and modification programmes, to provide sufficient forward projection to identify all likely maintenance/modification actions essentially as one package.

Upon completion of the evaluation and publication of the revised maintenance requirements, the ‘proposed ESG’ becomes the LOV.

Note: This assumes that all other aspects of the maintenance programme that are required to support the LOV (such as SSID, CPCP, etc.) are in place and have been evaluated to ensure they too remain valid up to the LOV.

NOTES:

1.         Fatigue cracking is defined as likely if the factored fatigue life is less than the projected ESG of the aircraft at time of WFD evaluation.

2.         The operational life is the projected ESG of the aircraft at time of WFD Evaluation. (See 4.4.1).

Figure A2-19: Aircraft WDF evaluation process

5.         DOCUMENTATION

Any person seeking approval of an LOV of an aircraft type design should develop a document containing all the necessary ISPs, inspection procedures, replacement times, SMPs, and any other maintenance actions necessary to preclude WFD, and to support the LOV. That person must revise the SSID or ALS as necessary, and/or prepare SBs that contain the aforementioned maintenance actions. Since WFD is a safety concern for all operators of older aircraft, EASA will make mandatory the identified inspection and modification programmes. In addition, EASA may consider separate AD action to address any SBs or other service information publications revised or issued as a result of in-service MSD/MED findings resulting from implementation of these programmes.

The following items should be contained in the front of the documentation supporting the LOV:

(a)       identification of the variants of the basic aircraft type to which the document relates;

(b)       summary of the operational statistics of the fleet in terms of hours and flights;

(c)       description of the typical mission, or missions;

(d)       the types of operations for which the inspection programme is considered valid;

(e)       reference to documents giving any existing inspections, or modification of parts or components; and

(f)        the LOV of the maintenance programme in terms of flight cycles or flight hours or both as appropriate to accommodate variations in usage.

The document should contain at least the following information for each critical part or component:

(a)       description of the primary structure susceptible to WFD;

(b)       details of the monitoring period (ISP, repeat inspection interval, SMP, inspection method and procedure (including crack size, location and direction) and alternatives) when applicable;

(c)       any optional modification or replacement of the structural element as terminating action to inspection;

(d)       any mandatory modification or replacement of the structural element;

(e)       SBs (or other service information publications) revised or issued as a result of in-service findings resulting from the WFD evaluations (added as a revision to the initial WFD document); and

(f)        guidance to the operator on which inspection findings should be reported to the TCH/DAH, and appropriate reporting forms and methods of submission.

6.         REPORTING REQUIREMENTS

Operators, TCHs and STCHs are required to report in accordance with various regulations (e.g. point 21.A.3A, and point 145.A.60). The regulations to which this AMC relates do not require any reporting requirements in addition to the current ones. Due to the potential threat to structural integrity, the results of inspections must be accurately documented and reported in a timely manner to preclude the occurrence of WFD. The current system of operator and TCH communication has been useful in identifying and resolving a number of issues that can be classified as WFD concerns. MSD/MED has been discovered via fatigue testing and in-service experience. TCHs have been consistent in disseminating related data to operators to solicit additional service experience. However, a more thorough means of surveillance and reporting is essential to preclude WFD.

When damage is found while conducting an approved MSD/MED inspection programme, or at the SMP where replacement or modification of the structure is occurring, the TCHs, STCHs and the operators need to ensure that greater emphasis is placed on accurately reporting the following items:

(a)       a description (with a sketch) of the damage, including crack length, orientation, location, flight cycles/hours, and condition of structure;

(b)       results of follow-up inspections by operators that identify similar problems on other aircraft in the fleet;

(c)       findings where inspections accomplished during the repair or replacement/modification identify additional similar damage sites; and

(d)       adjacent repairs.

Operators must report all cases of MSD/MED to the TCH, STCH or the competent authority as appropriate, irrespective of how frequently such cases occur. Cracked areas from in-service aircraft (damaged structure) may be needed for detailed examination. Operators are encouraged to provide fractographic specimens whenever possible. Aeroplanes undergoing heavy maintenance checks are perhaps the most useful sources for such specimens.

Operators should remain diligent in the reporting of potential MSD/MED concerns not identified by the TCH/DAH. Indications of a developing MSD/MED problem may include:

(a)       damage at multiple locations in similar adjacent details;

(b)       repetitive part replacement; or

(c)       adjacent repairs.

Documentation will be provided by the TCH and STCH as appropriate to specify the required reporting format and time frame, supporting the mandatory reporting regulations (e.g. point 21.A.3A of Part 21, point 145.A.60 of Part-145). The data will be reviewed by the TCH or STCH, operator(s), and EASA to evaluate the nature and magnitude of the problem and to determine the appropriate corrective action.

7.         WFD EVALUATION FOR STRUCTURAL MODIFICATIONS AND REPAIRS

TCHs of aeroplanes subject to the point 26.303 of Part-26 requirements for an LOV should perform WFD evaluations to assess all the applicable existing structure and the effect of future changes on the LOV.

The WFD evaluations of this AMC do not apply retroactively to existing STCH’s modifications, nor to existing repairs. Future changes and repairs need to take into account the applicable certification basis, and applicants should consider the guidance of the applicable ACJ and AMC as discussed in paragraph 8 of this AMC. The DTEs for compliance with points 26.307, 26.308, 26.333 and 26.334 of Part-26 do not have to consider WFD _{(average behaviour)}, or the related SMP and ISP.

In cases where a new DTE is performed by DAHs to comply with points 26.333 and 26.334 of Part-26 for existing changes or for new changes or repairs, according to CS 25.571 Amdt 18 or earlier amendments, the DTE and development of DTIs should take into account the cracking scenarios that could reasonably be expected to occur in the remaining operational lifetime of an aeroplane into which the repair or modification is, or may be, incorporated.

8.         RESPONSIBILITYFOR WFD EVALUATION, ESTABLISHING THE LOV AND IMPLEMENTATION OFTHE LOV AND MAINTENANCE ACTIONS

The primary responsibility is with the DAH to perform the analyses and supporting tests. However, it is expected that if extensive maintenance actions are necessary, the practicality of their implementation will be evaluated in a cooperative effort between the operators and TCHs/DAHs, with participation of EASA.

The TCH is responsible for proposing and submitting an LOV in the ALS for approval.

Note:In some cases, the ALS may already contain an LOV which is approved in accordance with a regulation of another authority. There may also be other potentially more restrictive limitations on the validity of maintenance programmes. For these cases, when the TCH needs to publish the LOV as required by point 26.303 of Part-26, this LOV and its relationship with the existing or superseded limitation should be clearly described so that no operator will exceed the most restrictive applicable limit on the general validity of the maintenance programme.

The operator is responsible for implementing the LOV in their maintenance programme.

Note:The LOV does not supersede or allow operations beyond any lower limitation applicable to the individual aeroplane and the components controlled by the maintenance programme.

[Amdt 20/2]

[Amdt 20/20]