Appendix 1 to AMC 20-20AGuidelines for the development of a supplementary structural inspection programme

1.         GENERAL

1.1       Purpose

This Appendix 1 gives interpretations, guidelines and acceptable means of compliance for the SSIP actions.Aeroplanes addressed by point 26.302 of Part-26 require damage tolerance inspections (DTIs) and other procedures to ensure freedom from catastrophic failure due to fatigue throughout the operational life of the aircraft. Compliance can be demonstrated by developing an SSIP or DT-based ALS. Other aircraft may benefit from an SSIP, and some TCHs have already developed programmes for general aviation types that should also be implemented under Part-M requirements.

1.2       Background

Service experience has demonstrated that there is a need to have continually updated knowledge concerning the structural integrity of aircraft, especially as they become older, to ensure they continue to meet the level of safety intended by the certification specifications.In addition, earlyfatigue requirements, such as ‘fail safe’ regulations, did not provide for timely inspection of an aircraft’s critical structure to ensure that damaged or failed components could be dependably identified and then repaired or replaced before hazardous conditions developed.

In 1978 the damagetolerance concept was adopted for transport category aeroplanes in the USA as Amendment 25-45 to 14 CFR 25.571. This amended rule required damagetolerance analyses as part of the type design of transport category aeroplanes for which application for typecertification was received after the effective date of the amendment. In 1980 the requirement for damagetolerance analyses was also included in JAR 25.571 Change 7.

One prerequisite for the successful application of the damage tolerance approach for managing fatigue is that crack growth and residual strength can be anticipated with sufficient precision to allow inspections to be established that will detect cracking before it reaches a size that will degrade the strength to less thana specified level. When damage is discovered, airworthiness is ensured by repair or revised maintenance action. Evidence to date suggests that when all critical structure is included, fatigue and damage‑tolerance‑based inspections and procedures (including modification and replacement when necessary) provide the best approach to address aircraft fatigue.

Pre-14 CFR Part 25 Amendment 25-45 (JAR-25 Change 7) aeroplanes were built to varying standards that embodied fatigue and fail-safe requirements. These aeroplanes, as certified, had no specific mandated requirements to perform inspections for fatigue. Following the amendment of 14 CFR Part 25 to embody damagetolerance requirements, the FAA published Advisory Circular 91-56A. That AC was applicable to pre‑Amendment 25-45 aeroplanes with a maximum gross weight greater than 75 000 lb (34 019 kg). According to the AC, the TCH, in conjunction with operators, was expected to initiate development of an SSIP for each aeroplane model.

AC 91-56A provided guidance material for the development of such programmes based on damagetolerance principles. Many TCHs of large aeroplanes developed SSIPs for their pre-Amendment 25-45 aeroplanes. The documents containing the SSIP are designated SSIDs or SIDs.

The competent authorities have in the past issued a series of ADs requiring compliance with these SSIPs. Generally, these ADs require the operators to incorporate the SSIPs into their maintenance programmes. Under Part-M requirements, it is expected that an operator will automatically incorporate the SSID into their maintenance programme once it is approved by EASA, unless already mandated by an AD.

For post-Amendment 25-45 aeroplanes (JAR-25 Change 7), it was required that inspections or other procedures should be developed based on the DTEs required by 14 CFR 25.571, and included in the maintenance data. In Amendment 25-54 to 14 CFR 25 and change 10 to JAR-25, it was required to include these inspections and procedures in the ALS of the ICA required by 25.1529. At the same amendment, 25.1529 was changed to require applicants for typecertificates to prepare ICA in accordance with Appendix H to FAR/JAR-25. Appendix H requires that the ICA must contain a section titled airworthiness limitations that is segregated and clearly distinguishable from the rest of the document. This section shall contain the information concerning inspections and other procedures as required by FAR/JAR/CS 25.571.

The content of the ALS of the ICA is designated by some TCHs as airworthiness limitations instructions (ALI). Other TCHs have decided to designate the same items as ALI.

Part-M requires the ALS to be incorporated into the operator’s maintenance programme.

2.         SUPPLEMENTAL STRUCTURAL INSPECTION PROGRAMME (SSIP)

Increased utilisation, longer operational lives, and the high safety demands imposed on the current fleet of transport aeroplanes indicate the need for a programme to ensure a high level of structural integrity for all aeroplanes in the transport fleet.

This AMC is intended to provide guidance to TCHs and other DAHs to develop or review existing inspection programmes for effectiveness. SSIPs are based on a thorough technical review of the damagetolerance characteristics of the aircraft structure using the latest techniques and changes in operational usage. They lead to revised or new inspection requirements primarily for structural cracking and replacement or modification of structure where inspection is not practical.

Whether the aircraft was originally certified to be damage-tolerant or not, the TCH should review its operational usage on a regular basis and ensure that it remains in accordance with the assumptions made at certification or when the SSIP was first developed. Factors such as the payload, fuel at take-off and landing, flight profile, etc. should be addressed. For large transport aeroplanes, the requirement of point 26.305 of Part-26 stipulates that a process must be in place to ensure that the continuing structural integrity programme remains valid, considering service experience and current operations.

Large transport aeroplanes that were certified according to 14 CFR 25.571 Amendment 25-45 or JAR 25 Change 7 or later are damage-tolerant. The maintenance instructions and airworthiness limitations arising from the fatigue and damage tolerance evaluations that have been specified as mandatory are included in the ALS (and/or ADs). Other maintenance instructions are usually part of the MRB Report, as required by ATA MSG-3. However, for pre‑ATA MSG-3 rev 2 aeroplanes there are no requirements for regular MRB Report review and for post-ATA MSG-3 rev 2 aeroplanes there is only a requirement for regular MRB Report review in order to assess whether the CPCP is effective. Concerning ageing aircraft activities, it is important to regularly review the part of the MRB Report containing the structural inspections resulting from the fatigue and damagetolerance analysis for effectiveness.

2.1       Pre-Amendment 25-45 aeroplanes

The TCH is expected to initiate development of an SSIP for each aeroplane model. Such a programme must be implemented before analysis, test and/or service experience indicate that a significant increase in inspection and or modification is necessary to maintain structural integrity of the aeroplane. This should ensure that an acceptable programme is available to the operators when needed. The programme should include procedures for obtaining service information, and assessment of service information, available test data, and new analysis and test data.

An SSID should be developed in accordance with Paragraph 3 of this Appendix 1. The recommended SSIP, along with the criteria used and the basis for the criteria, should be submitted by the TCH to EASA for approval. The SSIP should be adequately defined in the SSID and presented in a manner that is effective. The SSID should include the type of damage being considered, and likely sites; inspection access, threshold, interval method and procedures; applicable modification status and/or life limitation; and types of operation for which the SSID is valid.

The review of the SSID by EASA will include both engineering and maintenance aspects of the proposal. In the event an acceptable SSID cannot be obtained on a timely basis, the competent authority may impose service life, operational, or inspection limitations to assure structural integrity.

The TCH should check the SSID periodically against current service experience. This should include an evaluation of current methods and findings. Any unexpected defect occurring should be assessed as part of the continuing assessment of structural integrity to determine a need for revision to the document.

2.2.     Post-Amendment 25-45 aeroplanes

Aeroplanes certified to FAR 25.571 Amendment 25-45, JAR 25.571 Change 7 and CS-25 or later amendments are damage-tolerant. The airworthiness limitations including the inspections and procedures established in accordance with FAR/JAR/CS 25.571 shall be included in the ICA, ref. FAR/JAR/CS 25.1529. Further guidance for the actual contents is incorporated in FAR/JAR/CS-25 Appendix H.

To maintain the structural integrity of these aeroplanes, it is necessary to follow up the effectiveness of these inspections and procedures. The DAH should therefore check this information periodically against current service experience. Any unexpected defect occurring should be assessed as part of the continuing assessment of structural integrity to determine a need for revision to this information. The revised data should be developed in accordance with the same procedures as at type certification giving consideration to any additional test or service data available and changes to aeroplanes operating patterns.

3.         GUIDELINES FOR DEVELOPMENT OF THE SUPPLEMENTAL STRUCTURAL INSPECTION DOCUMENT

This paragraph is based directly on Appendix 1 to FAA AC 91-56B which applies to transport category aeroplanes that were certified prior to Amendment 25-45 of 14 CFR 25 or equivalent requirement.

3.1.     General

Amendment 25-45 to § 25.571 of 14 CFR Part 25 introduced wording which emphasises damage-tolerant design. However, the structure to be evaluated, the type of damage considered (fatigue, corrosion, service, and production damage), and the inspection and/or modification criteria should, to the extent practicable, be in accordance with the damagetolerance principles of the current § 25.571 of 14 CFR Part 25 standards. An acceptable means of compliance can be found in AC 25.571-1C (‘Damage-Tolerance and Fatigue Evaluation of Structure’, dated April 29, 1998) or later revision.

It is essential to identify the structural parts and components that contribute significantly to carrying flight, ground, pressure, or control loads, and whose failure could affect the structural integrity necessary for the continued safe operation of the aeroplane. The damagetolerance of these parts and components must be established or confirmed. Following the guidance material of AMC 25.571, it is essential that the inspections provided in the SSIP or ALS are practical and effective in maintaining airworthiness. Where this is not the case, modifications or replacements should be considered.

Analyses made in respect of the continuing assessment of structural integrity should be based on supporting evidence, including test and service data. This supporting evidence should include consideration of the operating loading spectra, structural loading distributions, and material behaviour. Appropriate allowance should be made for the scatter in life to crack initiation and rate of crack propagation in establishing the inspection threshold, inspection frequency, and, where appropriate, retirement life. Alternatively, an inspection threshold may be based solely on a statistical assessment of fleet experience, if it can be shown that equal confidence can be placed in such an approach.

An effective method of evaluating the structural condition of older aeroplanes is selective inspection with intensive use of non-destructive techniques, and the inspection of individual aeroplanes, involving partial or complete dismantling (‘teardown’) of available structure.

The effect of repairs and modifications approved by the TCH should be considered. In addition, it may be necessary to consider the effect of non-TCH repairs and modifications on individual aircraft. The operator has the responsibility for ensuring notification and consideration of any such aspects in conjunction with the DAH. Guidance on the EASA’s requirements for the DT of repairs and modifications is found in Appendix 3 to this AMC, and further guidance for the WFD evaluation of repairs and modifications is provided in Section 7 of Appendix 2.

3.2.     Damage-tolerant structures

The damagetolerance assessment of the aircraft structure should be based on the best information available. The assessment should include a review of analysis, test data, operational experience, and any special inspections related to the type design.

A determination should then be made of the site or sites within each structural part or component considered likely to crack, and the time or number of flights at which this might occur.

The growth characteristics of damage and interactive effects on adjacent parts in promoting more rapid or extensive damage should be determined. This determination should be based on study of those sites that may be subject to the possibility of crack initiation due to fatigue, corrosion, stress corrosion, disbonding, accidental damage, or manufacturing defects in those areas shown to be vulnerable by service experience or design judgement. The damage tolerance certification specification of CS 25.571 requires not only fatigue damage to be addressed but also accidental and environmental damage. Some types of accidental damage (e.g. scribe marks) can not be easily addressed by the MSG process and require specific inspections based on fatigue and damage tolerance analysis and tests. Furthermore, some applicants may choose to address other types of accidental damage and environmental damage in the SSID or ALS by modelling the damage as a crack and performing a fatigue and damage tolerance analysis. The resulting inspection programme may be tailored to look for the initial type of damage or the resulting fatigue cracking scenario, or both.

The minimum size of damage that is practical to detect and the proposed method of inspection should be determined. This determination should take into account the number of flights required for the crack to grow from detectable to the allowable limit, such that the structure has a residual strength corresponding to the conditions stated under CS 25.571.

Note: In determining the proposed method of inspection, consideration should be given to visual inspection, non-destructive testing, and analysis of data from built-in load and defect monitoring devices.

The continuing assessment of structural integrity may involve more extensive damage than might have been considered in the original fail-safe evaluation of the aircraft, such as:

(a)       a number of small adjacent cracks, each of which may be less than the typically detectable length, developing suddenly into a long crack;

(b)      failures or partial failures in other locations following an initial failure due to redistribution of loading causing a more rapid spread of fatigue; and

(c)       concurrent failure or partial failure of multiple load path elements (e.g., lugs, planks, or crack arrest features) working at similar stress levels.

3.3.     Information to be included in the assessment

The continuing assessment of structural integrity for the particular aircraft type should be based on the principles outlined in paragraph 3.2 of this Appendix. The following information should be included in the assessment and kept by the TCH in a form acceptable to EASA:

(a)       the current operational statistics of the fleet in terms of hours or flights;

(b)      the typical operational mission or missions assumed in the assessment;

(c)       the structural loading conditions from the chosen missions; and

(d)      supporting test evidence and relevant service experience.

In addition to the information specified above, the following should be included for each critical part or component:

(a)       the basis used for evaluating the damagetolerance characteristics of the part or component;

(b)      the site or sites within the part or component where damage could affect the structural integrity of the aircraft;

(c)       the recommended inspection methods for the area;

(d)      for damage-tolerant structures, the maximum damage size at which the residual strength capability can be demonstrated and the critical design loading case for the latter; and

(e)      for damage-tolerant structures, at each damage site the inspection threshold and the damage growth interval between detectable and critical, including any likely interaction effect from other damage sites.

Note: Where re-evaluation of fail-safety or damagetolerance of certain parts or components indicates that these qualities cannot be achieved, or can only be demonstrated using an inspection procedure whose practicability or reliability may be in doubt, replacement or modification action may need to be defined.

3.4.     Inspection programme

The purpose of a continuing airworthiness assessment in its most basic terms is to adjust the current maintenance inspection programme, as required, to assure continued safety of the aircraft type.

In accordance with Paragraphs 1 and 2 of this Appendix, an allowable limit of the size of damage should be determined for each site such that the structure has a residual strength for the load conditions specified in CS 25.571. The size of damage that is practical to detect by the proposed method of inspection should be determined, along with the number of flights required for the crack to grow from detectable to the allowable limit.

The recommended inspection programme should be determined from the data described above, giving due consideration to the following:

(a)       fleet experience, including all scheduled maintenance checks;

(b)      confidence in the proposed inspection technique; and

(c)       the joint probability of reaching the load levels described above and the final size of damage in those instances where probabilistic methods can be used with acceptable confidence.

Inspection thresholds for supplemental inspections should be established. These inspections would be supplemental to the normal inspections, including the detailed internal inspections.

(a)       For structure with reported cracking, the threshold for inspection should be determined by analysis of the service data and available test data for each individual case.

(b)      For structure with no reported cracking, it may be acceptable, provided sufficient fleet experience is available, to determine the inspection threshold on the basis of analysis of existing fleet data alone. This threshold should be set such as to include the inspection of a sufficient number of high-time aircraft to develop added confidence in the integrity of the structure (see Paragraph 1 of this Appendix).

3.5.     The supplemental structural inspection document (SSID)

The SSID should contain the recommendations for the inspection procedures and replacement or modification of parts or components necessary for the continued safe operation of the aircraft up to the LOV. Where an LOV is not provided as a result of needing to meet a specific requirement for an LOV, the applicant may establish an LOV or must consider all the likely fatigue scenarios up to an operational life beyond which it is highly unlikely that the aircraft will remain in service. This may be either conservatively set based on experience or provided as a limitation in the ICA/SSID. The document should be prefaced by the following information:

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

(b)      reference to documents giving any existing inspections or modifications of parts or components;

(c)       the types of operations for which the inspection programme are considered valid;

(d)      a list of SBs (or other service information publication) revised as a result of the structural reassessment undertaken to develop the SSID, including a statement that the operator must account for these SBs;

(e)      the type of damage which is being considered (i.e. fatigue, corrosion and/or accidentaldamage); and

(f)       guidance to the operator on which inspection findings should be reported to the TCH.

The document should contain at least the following information for each critical part or component (PSE and FCS):

(a)       a description of the part or component and any relevant adjacent structure, including means of access to the part;

(b)      relevant service experience;

(c)       likely site(s) of damage;

(d)      inspection method and procedure, and alternatives;

(e)      minimum size of damage considered detectable by the method(s) of inspection;

(f)       SBs (or other service information publication) revised or issued as a result of in‑service findings resulting from implementation of the SSID (added as revision to the initial SID);

(g)       initial inspection threshold;

(h)      repeat inspection interval;

(i)        reference to any optional modification or replacement of part or component as terminating action to inspection;

(j)        reference to the mandatory modification or replacement of the part or component at given life, if fail-safety by inspection is impractical; and

(k)       information related to any variations found necessary to ‘safe lives’ alreadydeclared.

The SSID should be compared from time to time against current service experience. Any unexpected defect occurring should be assessed as part of the continuing assessment of structural integrity to determine the need for revision of the SSID. Future structural SBs should state their effect on the SSID.

[Amdt 20/2]

[Amdt 20/20]