Appendix 4 to AMC 20-20A Guidelines for the development of a corrosion prevention control programme

1.         GENERAL

The TCH should develop a baseline CPCP, which should be reviewed by EASA. The baseline CPCP is intended to facilitate the development of a CPCP by an operator for their maintenance programme.

The operator should include a CPCP in the maintenance programme, and where a TCH baseline CPCP exists, it should be taken into account in the development of the operator’s CPCP. The operator should show that the CPCP is comprehensive in that it addresses all the corrosion likely to affect primary structure,and is systematic in that:

(a)       it provides step-by-step procedures that are applied on a regular basis to each identified task area or zone, and

(b)       these procedures are adjusted when they result in evidence that corrosion is not being controlled to an established acceptable level (Level 1 or better).

1.1       Purpose

This Appendix gives guidance to operators and DAHs who are developing and implementing a CPCP for aeroplanes maintained in accordance with an aircraft maintenance programme developed in compliance with point M.A.302 of Part-M.

CPCPs have been developed by the DAH with the assistance of aircraft operators and competent authorities. They relied heavily on service experience to establish CPCP implementation thresholds and repeat intervals. Since that time a logical evaluation process has been developed to ensure environmental damage is considered in the evaluation of aircraft structure. This process is identified in ATA MSG-3 Scheduled maintenance development document, which introduced the CPCP concept in revision 2, circa 1993.

1.2       Approval

Approval of a TCH CPCP may either be through the MRB (ISC) using existing procedures for EASA MRBR approval, or directly by EASA if no EASA-approved MRBR exists for the type. Provided that the operator has an NAA-approved aircraft maintenance programme (AMP) that controls corrosion to Level 1 or better, the operator need not follow exactly the programme offered by the TCH. However, revisions to the TCH’s approved programme should be considered by the operator for incorporation in the operator’s MP under the Part-M requirements.

2.         DEFINITIONS

—        Allowable Limit:this is the amount of material (usually expressed in the thickness of thematerial) that may be removed or blended out without affecting the ultimate design strength capability of the structural member. Allowable limits may be established by the TCH/DAH. EASA may also establish allowable limits. The DAH normally publishes allowable limits in the SRM or in SBs. Note: This revision of the AMC amends the definition of corrosion levels such that the concept of local and widespread corrosion is no longer specified. Nonetheless, when deriving allowable limits for the structure and the adjacent structure, the full extent of the damage and material removed in the finding and the previous findings affecting the same areas must be taken into account. Applicable fatigue and damage tolerance requirements must be taken into account when establishing the allowable limits.

—        Baseline CPCP:this is a CPCP developed for a specific model aeroplane. The TCH typically develops the baselineCPCP (see TCH-developed baseline CPCP below). It contains the corrosion inspection tasks, an implementation threshold, and a repeat interval for task accomplishment in each area or zone.

—        Basic task(s):this is a specific and fundamental set of work elements that should be performed repetitively in all task areas or zones to successfully control corrosion. The contents of the basic task may vary depending upon the specific requirements in an aeroplane area or zone. The basic task is developed to protect the primary structure of the aeroplane.

—        Corrosion prevention and control programme (CPCP):this is a comprehensive and systematic approach to controlling corrosion in such a way that the load carrying capability of an aircraft structure is not degraded to less than a level necessary to maintain airworthiness. It is based upon the baseline CPCP described above.A CPCP consists of a basic corrosion inspection task, task areas, defined corrosion levels, and compliance times (implementation thresholds and repeat intervals).

The CPCP also includes procedures to notify the competent authority of the findings and data associated with Level 2 and Level 3 corrosion and the actions taken to reduce future findings to Level 1.

—        Implementation threshold (IT):this is the aircraft age associated with the first time the basic corrosion inspection task should be accomplished in an area or zone.

—        Level 1 corrosionis:

Damage occurring between successive inspections that is within the allowable damage limits; or

Damage occurring between successive inspections that does not require structural reinforcement, replacement or new damage-tolerance-based inspections; or

Corrosion occurring between successive inspections that exceeds the allowable limits but can be attributed to an event not typical of operator usage of other aircraft in the same fleet; or

Light corrosion occurring repeatedly between inspections that eventually requires structural reinforcement, replacement, or new damage-tolerance-based inspections.

—        Level 2 corrosionis any corrosion finding that exceeds Level 1, requiring a review of the operator’s CPCP effectiveness, but that is not determined to be Level 3.

The operator is responsible for making the initial determination of the corrosion level, and this may subsequently be adjusted based on consultation with the DAH.

A finding of Level 2 corrosion requires repair, reinforcement, or complete or partial replacement of the applicable structure, or revised fatigue and damage tolerance inspections.

Note:A statement of fact in previously mandated CPCPs states: corrosion findings that were discovered during the corrosion inspection task accomplished at the implementation threshold, and which require repair, reinforcement, or complete or partial replacement of the applicable structure, should not be used as an indicator of the effectiveness of the operator’s CPCP. The argument is that an operator’s corrosion programme effectiveness can only be determined after a repeat inspection has been performed in a given inspection task area. This argument is valid for aircraft with mandated CPCPs introduced after the aircraft has been in service for a number of years without a CPCP. This argument, however, is not valid for aircraft that have been maintained since entry into serviceusing a CPCP that takes into account the TCH baseline CPCP and environmental deterioration (ED) programme. Consequently, corrosion findings exceeding Level 1 found on the corrosion inspection task implementation threshold may indicate that the threshold hasbeen set too high and action should be taken to adjust the implementation threshold.

—        Level 3 corrosion is that corrosion occurring during the first or subsequent accomplishments of an inspection task that the operator or subsequently the TCH or competent authority determines to be an urgent airworthiness concern.

Note: If Level 3 corrosion is determined at the implementation threshold or any repeat inspection, then it should be reported. Any corrosion that is more than the maximum acceptable to the DAH or EASA must be reported in accordance with the current regulations. This determination should be conducted jointly with the DAH.

—        Light corrosion is corrosion damage so slight that removal and blend-out over multiple repeat intervals may be accomplished before material loss exceeds the allowable limit.

—        Local corrosion. Generally, local corrosion is corrosion of a skin or web (wing, fuselage, empennage or strut) that does not exceed one frame, stringer, or stiffener bay. Local corrosion is typically limited to a single frame, chord, stringer or stiffener, or corrosion of more than one frame, chord, stringer or stiffener where no corrosion exists on two adjacent members on each side of the corroded member.

—        Operator-developed programme. In order to operate an aeroplane in compliance with the maintenance programme of Part-M and Part-26, an operator should include in their maintenance or inspection programme an approved CPCP. An operator may adopt the baseline CPCP provided by the DAH or they may choose to develop their own CPCP, or may be required to if none is available from the DAH. In developing their own CPCP, an operator may join with other operators and develop a baseline CPCP similar to a TCH‑developed baseline CPCP for use by all operators in the group. The advantages of an operator-developed baseline CPCP are that it provides a common basis for all operators in the group to develop their CPCP and it provides a broader experience base for development of the corrosion inspection tasks and identification of the task areas.

—        Repeat Interval (RI):this is the calendar time between the accomplishment of successive corrosion inspection tasks for a task area or zone.

—        Task area:this is a region of aircraft structure to which one or more corrosion inspection tasks are assigned. The task area may also be referred to as a zone.

—        TCH-developed baseline CPCP.The baseline CPCP may be developed as an integral part of the ICA or in a stand-alone section or manual. The TCHshould provide an inspection programme that includes the frequency and extent of inspections necessary to ensure the continued airworthiness of the aircraft.Furthermore, the programme should include the information needed to apply protective treatments to the structure after inspection. In order for the inspections to be effectively accomplished, the TCH should include, in the ICA, corrosion removal and cleaning procedures and reference allowable limits. The baseline CPCP is intended to facilitate the operator’s development of a CPCP for their maintenance programme.

—        Urgent airworthiness concern:this is damage that could jeopardise continued safe operation of any aircraft. An urgent airworthiness concern typically requires correction before the next flight and expeditious action to inspect the other aircraft in the operator’s fleet.

—        Widespread corrosion:this is corrosion of two or more adjacent skin or web bays (a web bay is defined by frame, stringer or stiffener spacing). Or widespread corrosion is corrosion of two or more adjacent frames, chords, stringers, or stiffeners. Or widespread corrosion is corrosion of a frame, chord, stringer, or stiffener and an adjacent skin or web bay.

—        Zone. See ‘Task area’.

3.         DEVELOPMENT OF A BASELINE CPCP

3.1.     Baseline CPCP

The objective of a baseline CPCP is to establish requirements for control of corrosion of aircraft structure to Level 1 or better for the operational life of the aircraft. The baseline CPCP should include the basic task, implementation thresholds, and repeat intervals. The baseline CPCP should also include procedures to notify the competent authority of the findings and data associated with Level 2 and Level 3 corrosion and the actions taken to reduce future findings to Level 1.

3.1.1. Baseline CPCP considerations

To establish an effective baseline CPCP, consideration of the following is necessary:

(a)       the flight and maintenance history of the aircraft model and perhaps similar models;

(b)       the corrosion properties of the materials used in the aircraft structure;

(c)       the protective treatments used;

(d)       the general practice applied during construction and maintenance; and

(e)       local and widespread corrosion* (see Figure A4-1).

*Note: In some existing CPCPs, the concept of local and widespread corrosion is directly related to the corrosion level definitions, and for those programmes, those definitions remain applicable. The alignment of a programme with the corrosion level definitions of this amendment of the AMC may require a reassessment of the allowable limits and the way they are presented in the applicable ICA. This is because the assumptions made to determine the allowable limits may not have taken into account the fatigue and damage tolerance requirements that are now applicable through retroactive rulemaking and the updated certification basis. In addition, programmes that addressed widespread corrosion within the allowable limits as Level 2 corrosion may have addressed the derivation of the allowable limits without assuming that the maximum material loss would occur over the whole area.

When determining the detail of the corrosion inspection tasks, the implementation threshold and the repeat interval, a realistic operational environment should be considered. Technical representatives of both the TCH and the operators should participate in evaluating the service history and operational environment for the aircraft model. For new aircraft models and for aircraft models that have been in operation for only a short time, technical representatives of operators of similar aircraft models should be invited to participate.

Figure A4-1

3.1.2. TCH-developed baseline CPCP

During the design development process, the TCH should provide a baseline CPCP as part of the ICA. The TCH initially evaluates the service history of corrosion available for aircraft of similar design used in the same operational environment. Where no similar design with service experience exists, those structural features concerned should be assessed using the environmental damage approach of ATA MSG-3. The TCH develops a preliminary baseline CPCP based on this evaluation. The TCH then convenes a working group consisting of operator technical representatives and representatives of the participating competent authorities. The working group reviews the preliminary baseline CPCP to assure that the tasks, implementation thresholds and repeat intervals are practical and assure the continued airworthiness of the aircraft. Once the working group review is complete, the TCH incorporates the baseline CPCP into the ICA (see Figure A4-2).

Figure A4-2: TCH-developed baseline CPCP

3.1.3   Operator-developed CPCP

Exceptionally, there may be instances where the TCH does not provide a baseline CPCP. In such instances, an operator may develop their CPCP without using a baseline CPCP, as long as the operator-developed CPCP is consistent with the requirements.It would be beneficial for an operator developing its own CPCP to consult other operators of the same or similar aircraft models in order to broaden the service experience available for use in preparing their programme. When a TCH‑prepared baseline CPCP is unavailable, a group of operators may prepare a baseline CPCP from which each operator in the group will develop their CPCP.

(a)       Operator-developed baseline CPCP

An operator-developed baseline CPCP should particularly focus on the areas of the aircraft prone to corrosion, such as:

(i)        exhaust trail areas,

(ii)       battery compartments and battery vent openings,

(iii)      areas surrounding lavatories, buffets and galleys,

(iv)      bilges,

(v)       fuselage internal lower structure,

(vi)      wheel wells and landing gear,

(vii)     external skin areas,

(viii)    water entrapment areas,

(ix)      engine frontal areas and cooling air vents,

(x)       electronic or avionics compartments, and

(xi)      flight control cavities open during take-off and landing.

Note:CPCPs for large transport aeroplanes were developed based on a triad amongst the Airworthiness Authorities, DAHs, and the operators for the particular aeroplane model. If operator(s) were to develop a CPCP, they may want to follow the example of the large transports.

Figure A4-3: Operator-developed baseline CPCP

(b)       Individual operator-developed CPCP

An operator may develop their CPCP without reference to a baseline CPCP only when a baseline CPCP does not exist.The CPCP should be consistent with the requirements of the applicable operating rules. Any operator who develops their own CPCP without a baseline CPCP, should review all available corrosion-related service data on the individual aircraft model and similardesign details in similar aircraft models when the operator’s data and the service difficulty report data show no entries.

3.1.4. Continuous analysis and surveillance

The operator’s continuous analysis and surveillance system should contain procedures to review corrosion inspection task findings and establish corrosion levels. These procedures should provide criteria for determining if findings that exceed allowable limits are an isolated incident not typical of the operator’s fleet. The operator’s programme should also provide for notifying the competent authority whenever a determination of Level 2 or Level 3 corrosion is made. Due to the potential urgent airworthiness concern associated with a Level 3 finding, the operator’s procedures should provide for notification as soon as possible but no later than 3 calendar days after the Level 3 determination has been made.

3.2.     Baseline CPCP documentation

The baseline CPCP documentation should include instructions to implement the baseline CPCP. It may be in a printed form or other form acceptable to the competent authority. It should also be in a form that is easy to revise. The date of the last revision should be entered on each page. The baseline CPCP documentation should be clearly identified as a baseline CPCP. The aircraft make, model and the person who prepared the documentation should also be identified.

3.2.1. Purpose and background

This section of the documentation should state the purpose of the baseline CPCP, which is to establish minimum requirements for preventing and controlling corrosion that may jeopardise continuing airworthiness of the aircraft model fleet. The section should further state that an operator should include an effective CPCP in their maintenance or inspection programme.

3.2.2. Introduction

The introduction should include a general statement that corrosion becomes more widespread as aircraft age and that it is more likely to occur in conjunction with other damage such as fatigue cracking. The introduction should also indicate that it is not the intent of a CPCP to establish rigid requirements to eliminate all corrosion in the fleet, but to control corrosion at or below levels that do not jeopardise continued airworthiness. However, due to the unpredictability of corrosion, it must be removed and the structure repaired and corrosion prevention treatment reapplied.

3.2.3. Programme application

For a programme to be fully effective, it is essential that a corrosion inspection task be applied to all areas where corrosion may affect primary structure. This section should recommend that priority for implementing the CPCP be given to older aeroplanes and to areas requiring significant changes to previous maintenance procedures in order to meet corrosion prevention and control requirements. This section should allow an operator to continue its current corrosion control procedures in a given task area or zone where there is documentation to show that corrosion is being consistently controlled to Level 1.

3.2.4. Baseline CPCP

This section should fully describe the baseline CPCP. It should include the basic task, corrosion inspection task areas, implementation thresholds, and repeat intervals.

3.2.5. Reporting system

Procedures to report findings of Level 2 and Level 3 corrosion as necessaryto the TCH and the competent authority should be clearly established in this section. The TCH should indicate any specific requirement they have for reporting on corrosion levels that may be needed to revise the baseline CPCP. The information on Level 2 corrosion may be needed in a form acceptable to the competent authority responsible for approval of any revision to the maintenance programme resulting from a Level 2 finding. The timing of reporting should take into account the processes for the periodic review (see 3.2.6). All Level 2 and Level 3 findings should be reported in accordance with any applicable AD, operator’s service difficulty reporting procedures or reporting required by other competent authorities. Procedures for alerting the competent authority of Level 3 findings should be established that expedite such reporting. This report to the competent authority shall be made after the determination of the corrosion level.

3.2.6   Periodic review

This section should establish a period for the TCH (or lead operator) and participating operators to meet with EASA and review the reported Level 2 and Level 3 findings. The purpose of this review is to assess the baseline CPCP and make adjustments if necessary. This may be accomplished through maintenance programme reviews conducted via the Maintenance Programme Industry Steering Committees (MRB Structures Working Group or equivalent meetings) for the model.

3.2.7. Corrosion-related airworthiness directives

This section should include a list of all ADs that contain requirements related to known corrosion-related problems. This section should state that these ADs are in addition to and take precedence over the operator’s CPCP.

3.2.8. Development of the baseline CPCP

This section should identify the actions taken in preparing the baseline CPCP. It should include a description of the participants, the documents (e.g. SBs, service letters, ADs, service difficulty reports, accident and incident reports) reviewed, and the methodology for selecting and categorising the corrosion-prone areas to be included in the baseline CPCP. The selection criteria for corrosion-prone areas should be based on areas having similar corrosion exposure characteristics and inspection access requirements. Some corrosion-prone areas that should be considered are the main wing box, the fuselage crown, the bilge, areas under lavatories and galleys, etc. This section should state that the implementation threshold was selected to represent the typical aircraft age beyond which an effective corrosion inspection task should be implemented for a given task area.

3.2.9. Procedures for recording corrosion inspection findings

EASA has not imposed a requirement for additional record-keeping for an operator’s CPCP. However, the operator should maintain adequate records to substantiate any proposed programme adjustments. For example, an operator should maintain records to enable the operator to determine the amount of damage that has occurred during the repeat interval for each corrosion inspection task. Such data should be maintained for multiple repeat intervals in order to determine whether the damage remains constant or is increasing or decreasing. Such records are necessary when an operator is seeking approval for interval extension (escalation) or task reduction.

3.2.10.Glossary

This section should define all terms specifically used in the baseline CPCP documentation.

3.2.11.Application of the basic task

This section should describe in detail the basic task. It should provide procedures describing how to accomplish the following actions:

(a)       Removal of all systems equipment and interior furnishings to allow access to the area.

(b)       Cleaning of the area as required.

(c)       Visual inspection of all task areas and zones listed in the baseline CPCP.

(d)       Removal of all corrosion, damage evaluation, and repair of structure as necessary.

(e)       Unblocking holes and gaps that may hinder drainage.

(f)        Application of corrosion protective compounds.

(g)       Reinstallation of dry insulation blankets, if applicable.

3.2.12. Determination of corrosion levels based on findings

This section should describe how the corrosion level definitions are used in evaluating the corrosion findings and assigning a corrosion level. This section should also instruct the operator to consult the DAH or the competent authority for advice in determining corrosion levels.

3.2.13. Typical actions following determination of corrosion levels

This section should establish criteria for evaluating whether or not Level 2 or Level 3 corrosion is occurring on other aircraft in the operator’s fleet. Criteria to be considered include: cause of the corrosion problem, past maintenance history, operating environment, production build standard, years in service, and inspectability of the corroded area. These and any other identified criteria should be used in identifying those aircraft that should be included in a fleet campaign. The results of the fleet campaign should be used to determine necessary adjustments in the operator’s CPCP. The following instructions should also be included in this section:

(a)       If corrosion exceeding the allowable limit is found during accomplishment of the corrosion inspection task implementation threshold for a task area, it may be necessary to adjust the CPCP. (See ‘NOTE’ under ‘Level 2 corrosion’ definition)

(b)       A single isolated occurrence of corrosion between successive inspections that exceeds Level 1 does not necessarily warrant a change in the operator’s CPCP. If the operator experiences multiple occurrences of Level 2 or Level 3 corrosion for a specific task area, then the operator should implement a change to the CPCP.

(c)       The operator should not defer maintenance actions for Level 2 and Level 3 corrosion. These maintenance actions should be accomplished in accordance with the operator’s maintenance manual.

(d)       The operator may implement changes such as the following to improve the effectiveness of the programme:

(1)       reduction of the repeat interval,

(2)       multiple applications of corrosion treatments,

(3)       additional drainage provisions,

(4)       incorporation of DAHs service information, such as SBs and service letters.

3.2.14. Programme implementation

This section should state that each task is to be implemented on each aircraft when the aircraft reaches the age represented by the implementation threshold for the task. It should also describe procedures to be used for establishing a schedule for implementation where the aircraft age exceeds the implementation threshold for individual tasks. Finally,it should state that once a task is implemented in an area, subsequent tasks are to be accomplished at the repeat interval in that task area.

4.         DEVELOPMENT OF OPERATORS PROGRAMME

4.1.     Baseline CPCP available

If a baseline CPCP is available, the operator should use it as a basis for developing their CPCP. In addition to adopting the basic task, task areas, implementation thresholds and repeat intervals of the baseline CPCP, the operator should make provisions for:

(a)       aeroplanes that have exceeded the implementation threshold for certain tasks,

(b)      aeroplanes being removed from storage,

(c)       unanticipated scheduling adjustments,

(d)      corrosion findings made during non-CPCP inspections,

(e)      adding newly acquired aircraft, and

(f)       modifications, configuration changes, and operating environment.

4.1.1. Provisions for aircraft that have exceeded the implementation threshold

The operator’s CPCP must establish a schedule for accomplishing all corrosion inspection tasks in task areas where the aircraft age has exceeded the implementation threshold (see main text of AMC paragraph 12).

4.1.2. Aeroplanes being removed from storage

Corrosion inspection task intervals are established based on elapsed calendar time. Elapsed calendar time includes time out of service. The operator’s CPCP should provide procedures for establishing a schedule for accomplishment of corrosion inspection tasks that have accrued during the storage period.

The schedule should result in accomplishment of all accrued corrosion inspection tasks before the aircraft is placed in service.

4.1.3. Unanticipated scheduling adjustments

The operator’s CPCP should include provisions for adjustment of the repeat interval for unanticipated schedule changes. Such provisions should not exceed 10 % of the repeat interval. The CPCP should include provisions for notifying the competent authority when an unanticipated scheduling adjustment is made.

4.1.4. Corrosion findings made during non-CPCP inspections

Corrosion findings that exceed allowable limits may be found during any scheduled or unscheduled maintenance or inspection activities. These findings may be indicative of an ineffective CPCP. The operator should make provision in their CPCP to evaluate these findings and adjust their CPCP accordingly.

4.1.5. Adding newly acquired aircraft

Before adding any aircraft to the fleet, the operator should establish a schedule for accomplishing all corrosion inspection tasks in all task areas that are due. This schedule should be established as follows:

(a)       For aircraft that have previously operated under an approved maintenance programme, the initial corrosion inspection task for the new operator must be accomplished in accordance with the previous operator’s schedule or in accordance with the new operator’s schedule, whichever would result in the earliest accomplishment of the corrosion inspection task.

(b)       For aircraft that have not previously been operated under an approved maintenance programme, each initial corrosion task inspection must be accomplished either before the aircraft is added to the operator’s fleet, or in accordance with the schedule approved by the competent authority. After each corrosion inspection task has been performed once, the subsequent corrosion task inspections should be accomplished in accordance with the new operator’s schedule.

4.1.6. Modifications, configuration changes and operating environment

The operator must ensure that their CPCP takes account of any modifications, configurations changes and the operating environment applicable to them, that were not addressed in the baseline CPCP documentation.

4.2.     Baseline CPCP not available

If there is no baseline CPCP available for the operator to use in developing their CPCP, the operator should develop their CPCP using the provisions listed in Paragraph 3 of this Appendix for a baseline CPCP as well as the provisions listed in subparagraphs 4.1.1 through 4.1.6 of this paragraph.

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