Appendix 5 to AMC 20-20B— Guidelines for ensuring the validity of continuing structural integrity programmes

1.         GENERAL

Point 21.A.65 of Part 21 and point 26.305 of Part-26 require a process to be established that ensures that the continuing structural integrity programme remains valid throughout the operational life of the aircraft, considering in-service experience and current operations. The intent is for the TCHs of large transport aeroplanes to review the validity of the certification assumptions upon which the maintenance programme is based and to ensure that unsafe levels of fatigue cracking or other damage will be minimised in service. This Appendix provides guidance as to what the processes should include.

This Appendix also provides interpretation, guidelines and EASA accepted means of compliance for the review of structural SBs including a procedure for selection, assessment and related recommended corrective action for ageing aircraft structures.

2.         CONTENT OF THE PROCESSES FOR COMPLIANCE WITH POINT 26.305 of PART-26

AMC1 21.A.65 andCS 26.305 establish compliance on the basis of sub-paragraphs (a) to (g), reproduced below, and consideration of the criteria of sub-paragraph (h):

(a)       a process exists, and a report that describes the process and how it is implemented is submitted to EASA; and

(b)       the process is either continuous with each service finding or is a regular review following a number of findings, or a combination of both; and

(c)       the process includes a plan to audit and report to EASA the effectiveness of the continuing structural integrity programme, including the continuing validity of the assumptions upon which it is based, prior to reaching any significant point in the life of the aircraft; and

(d)       the process includes criteria for summarising findings of fatigue environmental or accidental damage and their causes and recording them in a way that allows any potential interaction to be evaluated; and

(e)       the process includes criteria to assess and record the relevance of each potential contributing factor to the finding, including operational usage, fatigue load spectra, environmental conditions, material properties, manufacturing processes and the fatigue and damage tolerance analysis methodology and its implementation; and

(f)        the process includes criteria for establishing and revising sampling programmes to supplement the inspections and other procedures established in compliance with the applicable fatigue and damage tolerance requirements; and

(g)       the process includes criteria for establishing when structure should be modified or the inspection programme revised in the light of in-service damage findings.

(h)       Sunset criteria: The extent to which the above elements of the process require definition may be tailored to the size of the fleet and its expected useful remaining life (e.g. if less than 10 % of the fleet remains in operation worldwide at the date of applicability of point 26.305 and there is significant experience of aircraft reaching the maximum expected operating life, then additional criteria beyond the existing processes may be agreed to be unnecessary).

It should be noted that point 26.305 of Part-26 applies to all structure whose failure could contribute to a catastrophic failure, and is not limited to metallic structures and fatigue cracking, but should also encompass composite and hybrid structures.

The reporting of findings that could be relevant to the continuing structural integrity of the aeroplane should be facilitated by providing clear instructions and easy-to-use reporting means to operators that encourage and facilitate both their support and that of the customer support staff in identifying developing risks.

The intent of the audit and the associated report is to provide the TCH and EASA with a series of properly planned opportunities to assess the continuing structural integrity programme for significant systemic shortcomings and take timely action in advance of potential unsafe conditions. The audit should consider each of the parts of the continuing structural integrity programme and any links between them or with programmes for other types. In the context of this requirement, the audit should address all structures, not only metallic structures.

The audit report should summarise the processes and their evolutions. The report should describe any measures taken in the audit that are beyond the basic processes established in accordance with point 21.A.65 of Part 21 or asdescribed in the original summary report prepared in accordance with point 26.305 of Part-26 (e.g. additional field inspections or tear‑down inspections of fleet leading aircraft in terms of age and usage). The report should summarise the status of each part of the continuing structural integrity programme, the findings of the audit and proposals for the actions to be taken.

The audit should provide evidence that the processes required by point 21.A.65 of Part 21 andpoint 26.305 of Part-26 have been properly implemented, such that:

—        reported service findings have been recorded and summarised in a way that allows causes, trends and actions to be reviewed;

—        service findings are evaluated for consistency with the assumptions on which the programme is based;

—        the continuing structural integrity programme remains effective (e.g. changes have been made to the programme in response to findings, and few emergency or short-term airworthiness actions have been necessary);

—        a comparison of the loading, operational usage, fatigue methodology, design tools, and test evidence with that upon which the programme is based, shows that the programme remains valid;

—        causal analysis results have been reviewed and evaluated for evidence of repetitive themes for which no specific action has been taken; and

—        the fleet leader sampling programme has been implemented, and any findings resulting from it have been properly dispositioned.

The audit report should record how any deviations from the applicable processes, identified during the audit, have been or will be addressed.

Significant points in the aircraft life that should be considered in the audit plan include: points at which a common threshold exist for multiple damage-tolerance-based inspections, half-DSG, DSG, LOV, etc. In order to plan for a timely assessment that will allow proactive revision of the maintenance programme, the audit should take place several years before reaching the significant point.

Assumptions made at certification and subsequently regarding key operating variable parameters such as weight, fuel, payload, mission length, etc., should be evaluated on a regular basis or each time a finding indicates that the assumptions may be compromised, resulting in an adverse effect on the fatigue analysis and inspection programme.

One way to ensure confidence in the maintenance programme is by establishing a fleet leader sampling programme encompassing various operators and operations in different environments. This may, for example, be developed in coordination with the MRB and requires the cooperation of the leading operators. The sampling programme need not address all potential damage locations, and should focus on the most critical that would provide early indications of potentially erroneous assumptions. Sampling may also be beneficial where new materials or methods of construction have been introduced, especially when the extent of testing may have been limited at certification, e.g. for areas of hybrid structure where the temperature differential was not part of the full-scale fatigue test. The sampling programme may also impose more intrusive or detailed inspections and analyses of samples taken from the structure (for composites or other materials subject to environmental degradation).

The details of the sampling programme requirements and the associated reporting requirements should be established in coordination with the operators. The ICA, in compliance with Appendix H to CS-25, may need to be supplemented with this information to support the core compliance elements of the continuing structural integrity programme generated through compliance with CS 25.571.

The process for establishing when a structure should be modified or the inspection programme revised in the light of in-service damage findings should include special consideration of:

—             damage detected and reported before the inspection threshold; and

—             damage that is generally being found at or near to the critical crack size; and

—             changing damage configurations for which the reasons are not fully understood; and

—             new damage scenarios reported under existing inspection or repair procedures that could otherwise be considered to be addressed, and overlooked.

The objectives of modifying structures are to provide a reasonably high probability that the ultimate load capability will be retained over long periods of the aircraft’s life, and to significantly reduce the potential for interaction with new cracking that may develop later in the aircraft’s life.

The following guidance regarding the SB review process is retained from the original issue of this AMC for general guidance on the subject, and the criteria for bulletin selection provide useful additional factors for establishing when structures should be modified or the inspection programme revised.

3.         SB REVIEW PROCESS

SBs issued early in the life of an aircraft fleet may utilise inspections (in some cases non‑mandatory inspections) alone to maintain structural integrity. Inspections may be adequate at this early stage, when cracking is possible, but not highly likely. However, as aircraft age, the probability of fatigue cracking becomes more likely. During this later period, it is not prudent to rely only on inspections alone because there are more opportunities for cracks to be missed, and cracks may no longer occur in isolation. It is then prudent to reduce reliance strictly on inspections, with their inherent human factors limitations, and to incorporate modifications to the structure to eliminate the source of the cracking. In some cases, reliance on an inspection programme, in lieu of modification, may be acceptable through the increased use of mandatory versus non-mandatory inspections.

The TCH, in conjunction with the operators, is expected to initiate a review of all structurally related inspection and modification SBs and determine which require further actions to ensure continued airworthiness, including mandatory modification action or enforcement of special repetitive inspections.

Any aircraft primary structural components that would require frequent repeat inspections, or where the inspection is difficult to perform, taking into account the potential airworthiness concern, should be reviewed to preclude the human factors issues associated with repetitive inspections.

The SB review is an iterative process consisting of the following items:

(a)       The TCH or the TCH in conjunction with the operators at a preliminary STG meeting should review all the issued structural inspection and modification SBs to select candidate bulletins, using the following four criteria:

(i)        There is a high probability that structural cracking exists.

Related to the number and type of finding in service and from fatigue testing.

A ‘no finding’ result should be associated with the number of performed inspections.

The type of finding should include an analysis of its criticality.

(ii)       Potential structural airworthiness concern

The structural airworthiness of the aircraft is dependent on repeat inspections to verify its structural condition, and therefore on the reliability of the inspections.

A short repeat inspection interval (e.g. a short time for a crack to grow from a detectable crack to a critical length divided by a factor) will lead to increased workloads for inspectors and a possible increased risk of them missing damage.

Special attention should be paid to any single inspection tasks involving multiple repeat actions needed to verify the structural condition that may increase the risk of the inspectors missing damage (e.g. lap splice inspections).

(iii)      Damage is difficult to detect during routine maintenance (i.e. there are few additional opportunities for detection beyond the specific requirement of the SB). (Of particular concern is damage that is found when it is well-developed and closer to being critical, rather than damage which is in the early stages with several further opportunities available for detection before it becomes critical.)

The areas to be inspected are difficult to access.

NDI methods are proving unsuitable.

The human factors associated with the inspection technique are so adverse that the detection of cracks may not be sufficiently dependable to assure safety.

(iv)      There is adjacent structural damage or the potential for it.

Particular attention should be paid to areas susceptible to WFD and also to potential interaction between corrosion and fatigue cracking, e.g. between fastener damage (due to stress corrosion or other factors) and fatigue cracking.

It is recommended to consider the potential interaction of modifications or repairs usually implemented in the areas concerned to check whether the inspections are still reliable or not (operator’s input).

(b)       The TCH and operator members will be requested to submit information on their individual fleet experience related to candidate SBs. This information will be collected and evaluated by the TCH. The summarised results will then be reviewed in detail at an STG meeting (see point (c) below).

(c)       The final selection of SBs for recommendation of the appropriate corrective action to assure structural continued airworthiness, taking into account the in-service experience, will be made during an STG meeting by the voting members of the STG, either by consensus or majority vote, depending on the preference of the individual STGs.

(d)       An assessment will be made by the TCH as to whether or not any subsequent revisions to SBs affect the decisions previously made. Any subsequent revisions to the SBs previously chosen by the STG for mandatory inspection or incorporation of modification action that would affect the previous STG recommended action should be submitted to the STG for review.

(e)       The TCH should review all new structural SBs periodically to select further candidate bulletins. The TCH should schedule a meeting of the STG to address the candidates. Operator members and the competent authority will be advised of the selected candidates and provided with the opportunity to submit additional candidates.

The SB selection, review, assessment and recommendation process within the Structural Task Group (STG) is summarised in Figure A5-1. For the first SB review within an STG meeting, all the inspection SBs should be selected. Moreover, some specific modification SBs not linked to an inspection SB may also be selected for review.

The information input by operators should address the points as detailed in Figure A5-2. This information should be collected and analysed by the TCH for the STG meeting.

If, for a given selected SB, there is not sufficient in-service data available before the STG meeting that would enable a recommendation to be made, its review may be deferred until enough data is available. The TCH should then check periodically until the data becomes available.

The operators and EASA should be advised by the TCH of the SB selection list and be given the opportunity to submit additional SBs. For this purpose, the TCH should give the operators enough information in advance (e.g. 2 months) for them to be able to properly consider the proposed selection and to gather data.

When an SB is selected, it is recommended to also select, in the same package, inspection SBs that interact with it and all the related modification SBs. The main criteria for selecting SBs are defined in the following subparagraphs.

4.         STG MEETING, SB REVIEW AND RECOMMENDATIONS

It is recommended to review at the same time all SBs that can interact, the so-called SB package, in the selection process. The meeting should start with an STG agreement on the selected SB list and on those deferred. At the meeting the TCH should present their analysis of each SB utilising the collection of operator input data. The STG should then collectively review the ratings (Figure A5-2) against each criterion to reach a consensus recommendation. Such anSTG recommendation for a selected SB shall consider the following options:

(a)       to mandate a structural modification at a given threshold,

(b)       to mandate the selected inspection SB,

(c)       to revise the modification or repair actions,

(d)       to revise other SB(s) in the same area concerned by damages,

(e)       to review the inspection method and related inspection intervals,

(f)        to review ALI/MRBR or other maintenance instructions,

(g)       to defer the review to the next STG and request operators’ reports on findings for a specific SB or request an inspection sampling on the oldest aircraft.

STG recommendations for mandatory action are the responsibility of the TCH to forward to EASA for appropriate action. Other STG recommendations is information provided to the STG members. It is their own responsibility to carry them out within the appropriate framework.

Figure A5-1: SB selection process and SB review

IN-SERVICE DATA/SECTION 1

TYPE OF FINDINGS:

IN-SERVICE DATA/SECTION 2

FIGURE A5-2: Operators fleet experience

(A)      INSPECTABILITY/ACCESS RATING

OK w Inspection carried out with little or no difficulty.

Acceptable w Inspection carried out with some difficulty.

Difficulty w Inspection carried out with significant difficulty.

Note:Rating should consider difficulty of access as well as inspection technique and size of inspection area.

(B)       FREQUENCY OF REPETITIVE INSPECTIONS RATING

OK w Greater than 6 years.

Acceptable w Between 2 and 6 years.

Difficulty w Less than 2 years.

(C)       FREQUENCY OF DEFECTS NOTED RATING = % OF THOSE AEROPLANES BEYOND THRESHOLD ON WHICH DEFECTS HAVE BEEN FOUND

OK w No defect noted.

Acceptable w Defects noted but not of a significant amount (less than 10 %).

Difficulty w Substantial defects noted (greater than 10 %).

(D)      FINDING SEVERITY RATING

OK w Airworthiness not affected.

Acceptable w Damage not of immediate concern, but could progress or cause secondary damage.

Difficulty w Airworthiness affected. Damage requires immediate repair.

(E)       ADJACENT STRUCTURE DAMAGE RATING (MULTIPLE-SITE DAMAGE, MULTIPLE-ELEMENT DAMAGE, CORROSION, ETC.)

OK w Low rate of adjacent structural damage.

Acceptable w Medium rate of adjacent structural damage.

Difficulty w High rate of adjacent structural damage/Multiple service actions in area.

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[Amdt 20/20]

[Amdt 20/22]