Airline/Manufacturer

MAINTENANCE PROGRAM DEVELOPMENT

DOCUMENT

MSG-3

Revision 2

Originally Issued September 30, 1980

Revision 1: March 31, 1988

Revision 2: September 12, 1993

September 1993

Prepared By:

Maintenance Steering Group - 3 Task Force

Air Transport Association of America
IMPORTANT INFORMATION ABOUT THIS DOCUMENT

READ BEFORE USING THIS DOCUMENT

This document contains recommended specifications that have been developed for the

covered topics. ATA does not mandate their use. You must decide whether or not to use the

recommendations in this document. You may choose to use them in whole, in part, or not at

all.

There may be practices, standards and/or regulatory requirements applicable to your

operations that exceed the recommendations in this document. You are solely responsible for

determining if such practices, standards or requirements exist and whether they apply to your

activities, and for complying with those that are applicable. Such practices, standards and

requirements can change significantly over time.

ATA does not guarantee, promise or warrant that the specifications in this document will meet

the needs of your operations. This is a determination that you must make and for which ATA

is not responsible.

Comments about or suggested changes to this document should be sent to:

Publications

Air Transport Association of America

1301 Pennsylvania Avenue, N.W.

Washington, DC 20004-1707

Copyright ©1996 by Air Transport Association of America (ATA). All rights reserved. No

part of this document may be reproduced or transmitted by any means, electronic or

mechanical, including photocopying and recording, or by any information storage or

retrieval system, except as may be expressly permitted in writing by the publisher.
Oct 07 1993

AIRLINE/MANUFACTURER MAINTENANCE PROGRAM DEVELOPMENT

DOCUMENT (MSG-3)

Mr. Michael F. Rioux

Vice President, Engineering,

Maintenance and Materiel

Air Transport Association of America

Washington, DC 20004-1707

Dear Mr. Rioux:

The Maintenance Review Board (MRB) Policy Board chairman has reviewed the

Airline/Manufacturer Maintenance Program Development Document MSG-3, Revision 2,

dated September 12, 1993. Minor discrepancies have been discussed with Mr. Dave Nakata.

The Federal Aviation Administration's Flight Standards Service, hereby, accepts MSG-3,

Revision 2, dated September 12, 1993, as the guideline document for the development of

future MRB reports.

Sincerely,

Frederick J. Leonelli

Manager, Aircraft Maintenance Division
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

MSG PREFACE

Page i

September 12, 1993

Airline and manufacturer experience in developing scheduled maintenance programs for new aircraft has shown

that more efficient programs can be developed through the use of logical decision processes.

In July, 1968, representatives of various airlines developed Handbook MSG-1, "Maintenance Evaluation and

Program Development," which included decision logic and inter-airline/manufacturer procedures for developing

a maintenance program for the new Boeing 747 aircraft.

Subsequently, it was decided that experience gained on this project should be applied to update the decision logic

and to delete certain 747 detailed procedural information so that a universal document could be made applicable

for later new type aircraft. This was done and resulted in the document, entitled, "Airline/ Manufacturer

Maintenance Program Planning Document," MSG-2. MSG-2 decision logic was used to develop scheduled

maintenance programs for the aircraft of the 1970's.

In 1979, a decade after the publication of MSG-2, experience and events indicated that an update of MSG

procedures was both timely and opportune in order for the document to be used to develop maintenance

programs for new aircraft, systems or powerplants.

An ATA Task Force reviewed MSG-2 and identified various areas that were likely candidates for improvement.

Some of these areas were the rigor of the decision logic, the clarity of the distinction between economics and

safety, and the adequacy of treatment of hidden functional failures. Additionally:

A. The development of new generation aircraft provided a focus, as well as motivation, for an

evolutionary advancement in the development of the MSG concept.

B. New regulations which had an effect on maintenance programs had been adopted and therefore

needed to be reflected in MSG procedures. Among those were new damage tolerance rules for

structures and the Supplemental Structural Inspection program for high time aircraft.

C. The high price of fuel and the increasing cost of materials created trade-off evaluations which had

great influences on maintenance program development. As a result, maintenance programs required

careful analysis to ensure that only those tasks were selected which provided genuine retention of

the inherent designed level of safety and reliability, or provided economic benefit.

MSG-3, ORIGINAL REVISION: Against this background, ATA airlines decided that a revision to existing

MSG-2 procedures was both timely and appropriate. The active participation and combined efforts of the FAA,

CAA/UK, AEA, U.S. and European aircraft and engine manufacturers, U.S. and foreign airlines, and the U.S.

Navy generated the document, MSG-3. As a result there were a number of differences between MSG-2 and

MSG-3, which appeared both in the organization/presentation of the material and in the detailed procedural

content. However, MSG-3 did not constitute a fundamental departure from the previous version, but was built

upon the existing framework of MSG-2 which had been validated by ten years of reliable aircraft operation using

maintenance programs based thereon.

The following reflects some of the major improvements and enhancements generated by MSG-3 as compared to

MSG-2.

1. Systems/Powerplant Treatment:

MSG-3 adjusted the decision logic flow paths to provide a more rational procedure for task definition and a

more straightforward and linear progression through the decision logic.

MSG-3 logic took a "from the top down" or consequence of failure approach. At the outset, the functional

failure was assessed for consequence of failure and was assigned one of two basic categories:

A. SAFETY

B. ECONOMIC
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

MSG PREFACE

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September 12, 1993

Further classification determined sub-categories based on whether the failure was evident to or hidden from

the operating crew. (For structures, category designation was "significant" or "other" structure, and all

functional failures were considered safety consequence items).

With the consequence category established for systems/powerplants, only those task selection questions

pertinent to the category needed to be asked. This eliminated unnecessary assessments and expedited the

analysis. A definite applicability and effectiveness criteria was developed to provide more rigorous selection

of tasks. In addition, this approach helped to eliminate items from the analytical procedure whose failures

had no significant consequence.

Task selection questions were arranged in a sequence such that the most preferred, most easily accomplished

task, was considered first. In the absence of a positive indication concerning the applicability and

effectiveness of a task, the next task in sequence was considered, down to and including possible redesign.

Structures Treatment:

Structures logic evolved into a form which more directly assessed the possibility of structural deterioration

processes. Considerations of fatigue, corrosion, accidental damage, age exploration programs and others,

were incorporated into the logic diagram and were routinely considered.

2. MSG-3 recognized the new damage tolerance rules and the supplemental inspection programs, and provided

a method by which their intent could be adapted to the Maintenance Review Board (MRB) process instead

of relying on type data certificate restraints. Concepts such as multiple failures, effect of failure on adjacent

structure, crack growth from detectable to critical length, and threshold exploration for potential failure,

were covered in the decision logic of the procedural material.

3. The MSG-3 logic was task-oriented and not maintenance process oriented (MSG-2). This eliminated the

confusion associated with the various interpretations of Condition Monitoring (CM), On-Condition (OC),

Hardtime (HT) and the difficulties encountered when attempting to determine what maintenance was being

accomplished on an item that carried one of the process labels.

 By using the task-oriented concept, one would be able to view the MRB document and see the initial

scheduled maintenance program reflected for a given item (e.g., an item might show a lubrication task at the

"A" frequency, and inspection/functional check at the "C" frequency and a restoration task at the "D"

frequency).

4. Servicing/Lubrication was included as part of the logic diagram to ensure that this important category of task

was considered each time an item was analyzed.

5. The selection of maintenance tasks, as output from the decision logic, was enhanced by a clearer and more

specific delineation of the task possibilities contained in the logic.

6. The logic provided a distinct separation between tasks applicable to either hidden or evident functional

failures; therefore, treatment of hidden functional failures was more thorough than that of MSG-2.

7. The effect of concurrent or multiple failure was considered. Sequential failure concepts were used as part of

the hidden functional failure assessment (Systems/Powerplant), and multiple failure was considered in

structural evaluation (Structures).

8. There was a clear separation between tasks that were economically desirable and those that were required for

safe operation.

9. The structures decision logic no longer contained a specific numerical rating system. The responsibility for

developing rating systems was assigned to the appropriate manufacturer with approval of the Industry

Steering Committee.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

MSG PREFACE

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September 12, 1993

MSG-3, REVISION 1: In 1987, after using MSG-3 procedures on a number of new aircraft and powerplants in

the first half of the 1980's, it was decided that the benefits of the experience so gained should be used to improve

the document for future application; thus, Revision 1 was undertaken.

This revised document includes changes developed by American and European airframe manufacturers,

American and European airworthiness authorities, supplemented and agreed to by the Air Transport Association

of America and other airline representatives.

The major improvements and enhancements reflected in items one through nine above were basically unchanged

and remain applicable to this revised document.

The following are some of the more noteworthy revisions that have been incorporated:

1. Table of Contents and a List of Effective Pages: ADDED.

2. Clarification that MSG-3 is used to develop an "initial scheduled maintenance program."

3. The task - "Operating Crew Monitoring": DELETED.

4. Section addressing "Threshold Sample": REVISED.

5. Section addressing "Program Development Administration": DELETED.

6. "Top-down approach" - explanation of process: ADDED.

7. "Visual Check" added to "Operational Check" task.

8. System/Powerplant and Structures logic diagrams: REVISED.

9. Task selection criteria table: ADDED.

10. Inspections:

 Detailed Inspection - REVISED.

 Directed Inspection - DELETED.

 External Surveillance Inspection - DELETED.

 General Visual Inspection - REVISED.

 Internal Surveillance Inspection - DELETED.

 Special Detailed Inspection - UNCHANGED.

 Walk Around Check Inspection - DELETED.

11. Clarification of hidden functional failure: "one additional failure."

12. Inspection/Functional Check task question revised.

13. Reference to a "User's Guide" for procedures related to administration and forms added.

14. Reference to "off-aircraft" deleted.

15. Operating Crew Normal Duties - "Normal Duties" revised to delete pre-flight and post-flight check list;

added "on a daily basis" for frequency of usage with respect to normal crew duties.

16. Added that procedures for handling composite of other new materials may have to be developed.

17. Reference to specific U.S. Federal Air Regulations: DELETED.

18. Definition of "Operating": REVISED.

19. Defined logic for failures which may affect dispatch capability or involve the use of abnormal or emergency

procedures. Failure-effect Category 6 is now identified as "Operational - Evident".

20. Noted that each MSI and SSI should be recorded for tracking purposes whether or not a task was derived

therefrom.

MSG-3, Revision 2: In 1993, MSG-3 Revision 2 was incorporated. The most significant changes introduced

were:

1. To adapt MSG-3 logic procedures to assure development of tasks/intervals associated with the aircraft's

certificated operating capabilities.

2. To provide guidelines which ensure that a consistent approach be taken with respect to tasks/intervals

required to maintain compliance with Type Certification requirements.

3. To provide guidelines on the development of Corrosion Prevention and Control Programs.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

MSG PREFACE

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September 12, 1993

4. To introduce procedures to determine the appropriate scheduled maintenance requirements for composite

structure.

5. To revise inspection task definitions.

MSG-3 Section 2.4 and its respective logic diagrams have been revised to add an evaluation process to insure the

Corrosion Prevention and Control Program (CPCP) is considered in the evaluation of each Structural Significant

Item (SSI) and every zone.

Damage Sources Section 2.4.3.1 now includes a discussion of non-metallic materials (composites).

Procedures Section 2.4.4.1 has been revised to add Procedure and Decision blocks for the CPCP evaluation and

edited to produce a more ordered flow of the Procedure and Decision block numbers.

The Glossary - Appendix A Inspection Level Definitions have been revised to apply to Systems, Powerplants and

Structures, and definitions related to CPCP have been added.

It is suggested, in order to fully comprehend the MSG-3 concept, that the entire MSG-3 document be reviewed

and considered prior to accepting or modifying its approaches to maintenance programs development. A User's

Guide or Policies and Procedures Handbook may be adopted with guidance and approval of the Industry Steering

Committee.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

1. GENERAL

1.1 OBJECTIVE

It is the objective of this document to present a means for developing a maintenance program which

will be acceptable to the regulatory authorities, the operators, and the manufacturers. The

maintenance program details will be developed by coordination with specialists from the operators,

manufacturers, and the Regulatory Authority of the country of manufacture. Specifically, this

document outlines the general organization and decision processes for determining scheduled

maintenance requirements initially projected for the life of the aircraft and/or powerplant.

Historically, the initial scheduled maintenance program has been specified in Maintenance Review

Board (MRB) Reports. MSG-3 is intended to facilitate the development of initial scheduled

maintenance programs. The remaining maintenance, that is, non-scheduled or non-routine

maintenance, consists of maintenance actions to correct discrepancies noted during scheduled

maintenance tasks, other non-scheduled maintenance, normal operation, or data analysis.

This document addresses the development of a maintenance program using the MSG-3 analysis

procedure. Any additional requirements developed, using different ground rules and procedures

from MSG-3, must be submitted with selection criteria to the Industry Steering Committee for

consideration and inclusion in the MRB Report recommendation.

1.2 SCOPE

For the purpose of developing an MRB report, MSG-3 is to be used to determine initial scheduled

maintenance requirements. The analysis process identifies all scheduled tasks and intervals based on

the aircraft's certificated operating capabilities.

1.3 ORGANIZATION

The organization to carry out the maintenance program development for a specific type aircraft shall

be staffed by representatives of the airline operators purchasing the equipment, the prime

manufacturers of the airframe and powerplant, and the Regulatory Authority.

1.3.1 Industry Steering Committee

The management of the maintenance program development activities shall be accomplished

by an Industry Steering Committee composed of members from a representative number of

operators and representatives of the prime airframe and engine manufacturers. It shall be

the responsibility of this committee to establish policy, set initial goals for scheduled

maintenance check intervals, direct the activities of Working Groups or other working

activity, carry out liaison with the manufacturer and other operators, prepare the final

program recommendations and represent the operators in contacts with the Regulatory

Authority. The ISC should see that the MSG-3 process identifies 100% accountability for

all Maintenance Significant Items (MSI's) and Structural Significant Items (SSI's), whether

or not a task has been derived from the analysis.

1.3.2 Working Groups

One or more Working Groups, consisting of specialist representatives from the participating

operators, the prime manufacturer, and the Regulatory Authority, may be constituted. The

Industry Steering Committee, alternatively, may arrange some other means for obtaining the

detailed technical information necessary to develop recommendations for maintenance

programs in each area. Irrespective of the organization of the working activity, written

technical data must be provided that supports its recommendations to the Industry Steering

Committee. After approval by the Industry Steering Committee, these analyses and

recommendations shall be consolidated into a final report for presentation to the Regulatory

Authority.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2. DEVELOPMENT OF MAINTENANCE PROGRAMS

2.1 PROGRAM REQUIREMENTS

It is necessary to develop a maintenance program for each new type of aircraft prior to its

introduction into airline service.

2.1.1 Purpose

The primary purpose of this document is to develop a proposal to assist the Regulatory

Authority in establishing an initial scheduled maintenance program for new types of aircraft

and/or powerplant. The intent of this program is to maintain the inherent safety and

reliability levels of the equipment. This program becomes the basis for the first issue of

each airline's maintenance requirements to govern its initial maintenance policy. Initial

adjustments may be necessary to address operational and/or environmental conditions

unique to the operator. As operating experience is accumulated, additional adjustments

may be made by the operator to maintain an efficient maintenance program.

2.1.2 Approach

It is desirable, therefore, to define in some detail:

a) The objectives of an efficient maintenance program.

b) The content of an efficient maintenance program.

c) The method by which an efficient maintenance program can be developed.

2.1.2.1 Maintenance Program Objectives

The objectives of an efficient airline maintenance program are:

a) To ensure realization of the inherent safety and reliability levels of the

equipment.

b) To restore safety and reliability to their inherent levels when deterioration

has occurred.

c) To obtain the information necessary for design improvement of those items

whose inherent reliability proves inadequate.

d) To accomplish these goals at a minimum total cost, including maintenance

costs and the costs of resulting failures.

These objectives recognize that maintenance programs, as such, cannot correct

deficiencies in the inherent safety and reliability levels of the equipment. The

maintenance program can only prevent deterioration of such inherent levels. If the

inherent levels are found to be unsatisfactory, design modification is necessary to

obtain improvement.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2.1.2.2 Maintenance Program Content

The content of the maintenance program itself consists of two groups of tasks:

a) A group of scheduled tasks to be accomplished at specified intervals.

The objective of these tasks is to prevent deterioration of the inherent

safety and reliability levels of the equipment. The tasks in a scheduled

maintenance program may include:

(1) Lubrication/Servicing (LU/SV)

(2) Operational/Visual Check (OP/VC)

(3) Inspection/Functional Check (IN/FC)

(4) Restoration (RS)

(5) Discard (DS)

 and

b) A group of non-scheduled tasks which result from:

(1) The scheduled tasks accomplished at specified intervals.

(2) Reports of malfunctions (usually originated by the operating

crew).

(3) Data analysis.

The objective of these non-scheduled tasks is to restore the equipment to an

acceptable condition.

An efficient program is one which schedules only those tasks necessary to meet the

stated objectives. It does not schedule additional tasks which will increase

maintenance costs without a corresponding increase in reliability protection.

2.1.2.3 Method for Maintenance Program Development

This document describes the method for developing the scheduled maintenance

program. Non-scheduled maintenance results from scheduled tasks, normal

operation or data analysis.

Maintenance programs will be developed via use of a guided logic approach and

will result in a task-oriented program. The logic's flow of analysis is failure-effect

oriented.

Items that, after analysis, have no scheduled task(s) specified, may be monitored by

an operator's reliability program.

2.2 DIVISIONS OF MSG-3 DOCUMENT

The working portions of MSG-3 are contained in the next three sections. Systems/Powerplant,

including components and APU's, are considered in Section 2.3. Aircraft Structures is considered in

Section 2.4, and Zonal Inspections in Section 2.5. Each section contains its own explanatory

material and decision logic diagram (as appropriate); therefore, it may be used independently of

other MSG-3 sections.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

SYSTEMS/POWERPLANT

LOGIC DIAGRAM

FIGURE 1
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

SYSTEMS/POWERPLANT

LOGIC DIAGRAM

FIGURE 1 (CONT'D.)
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2.3 AIRCRAFT SYSTEMS/POWERPLANT ANALYSIS METHOD

The method for determining the maintenance program for systems/powerplant, including

components and APU's, uses a progressive logic diagram. A glossary of terms and definitions used

in the logic diagram is listed in Appendix A. This logic is the basis of an evaluation technique

applied to each maintenance significant item (system, sub-system, module, component, accessory,

unit, part, etc.), using the technical data available. Principally, the evaluations are based on the

item's functional failures and failure causes.

Before the actual MSG-3 logic can be applied to an item, the aircraft's significant systems and

components must be identified. This process of identifying Maintenance Significant Items (MSI's) is

a conservative process (using engineering judgment) based on the anticipated consequences of

failure.

MSI's are those items identified by the manufacturer whose failure:

a) could affect safety (on ground or in flight), and/or,

b) could be undetectable or are not likely to be detected during operations, and/or,

c) could have significant operational impact, and/or,

d) could have significant economic impact.

The initial list of MSI's is prepared by the manufacturer and submitted to the ISC for distribution to

the appropriate Working Groups.

The top-down approach is a system of identifying the significant items on the aircraft. An acceptable

process follows:

a) Partition the aircraft into major functional areas: ATA Systems and Sub-Systems.

b) Continue the process until sub-components which are not replaced on-aircraft are

identified.

c) A candidate MSI is usually a system or sub-system and is, in most cases, one level above

the lowest (on-aircraft) level identified in step "b." This level is considered the highest

manageable level, i.e., one which is high enough to avoid unnecessary analysis, but low

enough to be properly analyzed and ensure that all functions, failures, and causes are

covered.

After the MSI's have been selected, the following must be identified for each MSI:

a) Functions(s) - the normal characteristic actions of an item

b) Functional Failure(s) - how an item fails to perform its function

c) Failure Effect(s) - what is the result of a functional failure

d) Failure Cause(s) - why the functional failure occurs

Tasks and intervals required in the maintenance program are identified using the procedures set forth

herein. Both the economic and safety related tasks are included so as to produce an initial scheduled

maintenance program.

2.3.1 Task Analysis Procedure

Prior to applying the MSG-3 logic diagram to an item, a preliminary work sheet will be

completed that clearly defines the MSI, its function(s), functional failure(s), failure

effect(s), failure cause(s) and any additional data pertinent to the item; e.g., ATA chapter

reference, fleet applicability, manufacturer's part number, a brief description of the item,

expected failure rate, hidden functions, need to be on M.E.L., redundancy (may be unit,

system or system management), etc. This work sheet is to be designed to meet the user's

requirements and will be included as part of the total MSG-3 documentation for the item.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2.3.1 Task Analysis Procedure (cont’d.)

The approach taken in the following procedure is to provide a logic path for each functional

failure. Each functional failure and failure cause must be processed through the logic so

that a judgment will be made as to the necessity of a task. The resultant tasks and intervals

will form the initial scheduled maintenance program.

2.3.2 Logic Diagram

The decision logic diagram (see figure 1) is used for analysis of systems/powerplant items.

The logic flow is designed whereby the user begins the analysis at the top of the diagram,

and answers to the "YES" or "NO" questions will dictate direction of the analysis flow.

2.3.2.1 Levels of Analysis

The decision logic has two levels (see Figure 1):

a) Level 1 (questions 1, 2, 3 and 4) requires the evaluation of each

FUNCTIONAL FAILURE for determination of the Failure Effect Category;

i.e., safety, operational, economic, hidden safety or hidden non-safety.

b) Level 2 (questions 5,6,7,8 and 9, "A" through "F", as applicable) then takes the

FAILURE CAUSE(S) for each functional failure into account for selecting the

specific type of task(s).

At level 2, the task selection section, paralleling and default logic have been

introduced. Regardless of the answer to the first question regarding

"Lubrication/Servicing", the next task selection question must be asked in all

cases. When following the hidden or evident safety effects path, all subsequent

questions must be asked. In the remaining categories, subsequent to the first

question, a "YES" answer will allow exiting the logic.

NOTE: At the user's option, advancement to subsequent questions after

deriving a "YES" answer is allowable, but only until the cost of the

task is equal to the cost of the failure prevented.

Default logic is reflected in paths outside the safety effects areas by the

arrangement of the task selection logic. In the absence of adequate information

to answer "YES" or "NO" to questions in the second level, default logic

dictates a "NO" answer be given and the subsequent question be asked. As

"NO" answers are generated the only choice available is the next question,

which in most cases provides a more conservative, stringent and/or costly task.

2.3.3 Procedure

This procedure requires consideration of the functional failures, failure causes, and the

applicability/ effectiveness of each task. Each functional failure processed through the logic

will be directed into one of five Effect categories.
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2.3.4 Consequences of Failure (First Level)

The decision logic diagram (see Figure 1) facilitates the identification of the tasks required.

There are four first level questions.

2.3.4.1 Evident or Hidden Functional Failure

Question 1:

This question asks if the operating crew will be aware of the loss (failure) of the

function during performance of normal operating duties. Question 1 must be asked

for each functional failure of the item being analyzed. The intent is to segregate the

evident and hidden functional failures. The operating crew consists of qualified

cockpit and cabin attendant personnel who are on duty. Normal duties are those

duties associated with the routine operation of the aircraft on a daily basis.

A "YES" answer indicates the functional failure is evident; proceed to Question 2

(2.3.4.2).

A "NO" answer indicates the functional failure is hidden; proceed to Question 3

(2.3.4.3).

2.3.4.2 Direct Adverse Effect on Safety

Question 2:

For a "YES" answer the functional failure must have a direct adverse effect on

operating safety.

Direct: To be direct the functional failure or resulting secondary damage must

achieve its effect by itself, not in combination with other functional failures (no

redundancy exists and it is a primary dispatch item).

Adverse Effect on Safety: This implies that the consequences are extremely serious

or possibly catastrophic and might cause the loss of aircraft or injury to occupants.

Operating: This is defined as the time interval during which passengers and crew

are on board for the purpose of flight.

A "YES" answer indicates that this functional failure must be treated within the

Safety Effects category and task(s) must be developed in accordance with section

2.3.5.1.

A "NO" answer indicates the effect is either operational or economic and Question

4 (2.3.4.4) must be asked.

IS THE OCCURRENCE OF A FUNCTIONAL FAILURE EVIDENT TO

THE OPERATING CREW DURING THE PERFORMANCE OF NORMAL

DUTIES?

DOES THE FUNCTIONAL FAILURE OR SECONDARY DAMAGE

RESULTING FROM THE FUNCTIONAL FAILURE HAVE A DIRECT

ADVERSE EFFECT ON OPERATING SAFETY?
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September 12, 1993

2.3.4.3 Hidden Functional Failure Safety Effect

Question 3:

This question is asked of each hidden functional failure which has been identified in

Question 1.

The question takes into account failures in which the loss of the one hidden function

(whose failure is unknown to the operating crew) does not of itself affect safety;

however, in combination with an additional functional failure (system related or

intended to serve as a back-up) has an adverse effect on operating safety.

For protective safety/emergency systems or equipment, the additional failure is the

event for which the system or equipment is designed.

If a "YES" answer is determined, there is a safety effect and task development must

proceed in accordance with 2.3.5.4.

A "NO" answer indicates that there is a non-safety effect and will be handled in

accordance with 2.3.5.5.

2.3.4.4 Operational Effect

Question 4:

This question asks if the functional failure could have an adverse effect on operating

capability:

a) requiring either the imposition of operating restrictions or correction

prior to further dispatch; or

b) requiring flight crew use of abnormal or emergency procedures.

This question is asked of each evident functional failure not having a direct adverse

effect on safety. The answer may depend on the type of operation.

If the answer to this question is "YES", the effect of the functional failure has an

adverse effect on operating capability, and task selection will be handled in

accordance with 2.3.5.2.

A "NO" answer indicates that there is an economic effect and should be handled in

accordance with 2.3.5.3.

DOES THE COMBINATION OF A HIDDEN FUNCTIONAL FAILURE

AND ONE ADDITIONAL FAILURE OF A SYSTEM RELATED OR

BACK-UP FUNCTION HAVE AN ADVERSE EFFECT ON OPERATING

SAFETY?

DOES THE FUNCTIONAL FAILURE HAVE A DIRECT ADVERSE

EFFECT ON OPERATING CAPABILITY?
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2.3.5 Failure Effect Categories (First Level)

Once the analysts have answered the applicable first level questions, they are directed to

one of the five Effect Categories:

a) Evident Safety (5),

b) Evident Operational (6),

c) Evident Economic (7),

d) Hidden Safety (8),

e) Hidden Non-Safety (9).

2.3.5.1 Evident Safety Effects (category 5)

The Evident Safety Effect category must be approached with the understanding that

a task is required to assure safe operation. All questions in this category must be

asked. If no effective task(s) results from this category analysis, then redesign is

mandatory. The following is the logic progression for functional failures that have

Evident Safety Effects.
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September 12, 1993

2.3.5.2 Evident Operational Effects (category 6)

A task(s) is desirable if it reduces the risk of failure to an acceptable level. Analysis

of the failure causes through the logic requires the first question

(Lubrication/Servicing) to be answered. Either a "YES" or "NO" answer of

question "A" still requires movement to the next level; from this point on, a "YES"

answer will complete the analysis and the resultant task(s) will satisfy the

requirements. If all answers are "NO", then no task has been generated. In such a

case, the appropriate item is forwarded to the Industry Steering Committee for

review with the MRB. If operational penalties are severe, a redesign may be

desirable. The following is the logic progression for functional failures that have

Evident Operational Effects.
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September 12, 1993

2.3.5.3 Evident Economic Effects (category 7)

A task(s) is desirable if the cost of the task is less than the cost of repair. Analysis

of the failure causes through the logic requires the first question

(Lubrication/Servicing) to be answered. Either a "YES" or "NO" answer to

question "A" still requires movement to the next level; from this point on, a "YES"

answer will complete the analysis and the resultant task(s) will satisfy the

requirements. If all answers are "NO", no task has been generated. If economic

penalties are severe, a redesign may be desirable. The following is the logic

progression for functional failures that have Evident Economic Effects.
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2.3.5.4 Hidden Function Safety Effects (category 8)

The Hidden Function Safety Effect requires a task(s) to assure the availability

necessary to avoid the safety effect of multiple failures. All questions must be

asked. If there are no tasks found effective, then redesign is mandatory. The

following is the logic progression for functional failures that have Hidden Function

Safety Effects.
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2.3.5.5 Hidden Function Non-Safety Effects (category 9)

The Hidden Function Non-Safety Effect category indicates that a task(s) may be

desirable to assure the availability necessary to avoid the economic effects of

multiple failures. Movement of the failure causes through the logic requires the first

question (Lubrication/Servicing) to be answered. Either a "YES" or "NO" answer

still requires movement to the next level; from this point on, a "YES" answer will

complete the analysis and the resultant tasks(s) will satisfy the requirements. If all

answers are "NO", no task has been generated. If economic penalties are severe, a

redesign may be desirable. The following is the logic progression for functional

failures that have Hidden Function Non-Safety Effects.
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2.3.6 Task Development (Second Level)

Task development is handled in a similar manner for each of the five Effect categories. For

task determination, it is necessary to apply the failure causes for the functional failure to the

second level of the logic diagram. There are six possible task resultant questions in the

Effect categories as follows:

2.3.6.1 Lubrication/Servicing (ALL CATEGORIES)

Any act of lubrication or servicing for the purpose of maintaining inherent design

capabilities.

Applicability Criteria:

The replenishment of the consumable must reduce the rate of functional

deterioration.

Effectiveness Criteria - Safety:

The task must reduce the risk of failure.

Effectiveness Criteria - Operational:

The task must reduce the risk of failure to an acceptable level.

Effectiveness Criteria - Economic:

The task must be cost-effective.

IS A LUBRICATION OR SERVICING TASK APPLICABLE AND

EFFECTIVE?
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2.3.6.2 Operational/Visual Check (HIDDEN FUNCTIONAL FAILURE

CATEGORIES ONLY)

An operational check is a task to determine that an item is fulfilling its intended

purpose. The check does not require quantitative tolerances. This is a failure

finding task.

A visual check is an observation to determine that an item is fulfilling its intended

purpose. The check does not require quantitative tolerances. This is a failure

finding task.

Applicability Criteria:

Identification of failure must be possible.

Effectiveness Criteria - Safety:

The task must ensure adequate availability of the hidden function to reduce

the risk of a multiple failure.

Effectiveness Criteria - Economic:

The task must ensure adequate availability of the hidden function in order to

avoid economic effects of multiple failures and must be cost-effective.

IS A CHECK TO VERIFY OPERATION APPLICABLE AND

EFFECTIVE?
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2.3.6.3 Inspection/Functional Check (ALL CATEGORIES)

An inspection is:

A. DETAILED INSPECTION

An intensive visual examination of a specific structural area, system,

installation or assembly to detect damage, failure or irregularity. Available

lighting is normally supplemented with a direct source of good lighting at

an intensity deemed appropriate by the inspector. Inspection aids such as

mirrors, magnifying lenses, etc. may be used. Surface cleaning and

elaborate access procedures may be required.

OR

B. GENERAL VISUAL (SURVEILLANCE) INSPECTION

A visual examination of an interior or exterior area, installation or

assembly to detect obvious damage, failure or irregularity. This level of

inspection is made under normally available lighting conditions such as

daylight, hangar lighting, flashlight or drop-light and may require removal

or opening of access panels or doors. Stands, ladders or platforms may be

required to gain proximity to the area being checked.

OR

C. SPECIAL DETAILED INSPECTION

An intensive examination of a specific item(s), installation, or assembly to

detect damage, failure or irregularity. The examination is likely to make

extensive use of specialized Inspection Techniques and/or equipment.

Intricate cleaning and substantial access or disassembly procedure may be

required.

A functional check is a quantitative check to determine if one or more functions of

an item performs within specified limits.

Applicability Criteria:

Reduced resistance to failure must be detectable, and there exists a

reasonably consistent interval between a deterioration condition and

functional failure.

Effectiveness Criteria - Safety:

The task must reduce the risk of failure to assure safe operation.

Effectiveness Criteria - Operational:

The task must reduce the risk of failure to an acceptable level.

Effectiveness Criteria - Economic:

The task must be cost-effective; i.e., the cost of the task must be less

than the cost of the failure prevented.

IS A INSPECTION OR FUNCTIONAL CHECK TO DETECT

DEGRADATION OF FUNCTION APPLICABLE AND EFFECTIVE?
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2.3.6.4 Restoration (ALL CATEGORIES)

That work necessary to return the item to a specific standard.

Since restoration may vary from cleaning or replacement of single parts up to a

complete overhaul, the scope of each assigned restoration task has to be specified.

Applicability criteria:

The item must show functional degradation characteristics at an

identifiable age and a large proportion of units must survive to that age.

It must be possible to restore the item to a specific standard of failure

resistance.

Effectiveness Criteria - Safety:

The task must reduce the risk of failure to assure safe operation.

Effectiveness Criteria - Operational:

The task must reduce the risk of failure to an acceptable level.

Effectiveness Criteria - Economic:

The task must be cost-effective: i.e., the cost of the task must be less

than the cost of the failure prevented.

IS A RESTORATION TASK TO REDUCE FAILURE RATE APPLICABLE

AND EFFECTIVE?
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2.3.6.5 Discard (ALL CATEGORIES)

The removal from service of an item at a specified life limit.

Discard tasks are normally applied to so-called single celled parts such as

cartridges, canisters, cylinders, engine disks, safe-life structural members, etc.

Applicability Criteria:

The item must show functional degradation characteristics at an

identifiable age and a large proportion of units must survive to that age.

Effectiveness Criteria - Safety:

A safe-life limit must reduce the risk of failure to assure safe operation.

Effectiveness Criteria - Operational:

The task must reduce the risk of failure to an acceptable level.

Effectiveness Criteria - Economic:

An economic-life limit must be cost-effective: i.e., the cost of the task

must be less than the cost of the failure prevented.

2.3.6.6 Combination (SAFETY CATEGORIES ONLY)

Since this is a safety category question and a task is required, all possible avenues

must be analyzed. To do this, a review of the task(s) that are applicable is

necessary. From this review the most effective task(s) must be selected.

IS A DISCARD TASK TO AVOID FAILURES OR TO REDUCE THE

FAILURE RATE APPLICABLE AND EFFECTIVE?

IS THERE A TASK OR COMBINATION OF TASKS APPLICABLE AND

EFFECTIVE?
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2.3.6.7 Task Selection Criteria

TASK APPLICABILITY

SAFETY

EFFECTIVENESS

OPERATIONAL

EFFECTIVENESS

ECONOMIC

EFFECTIVENESS

LUBRICATION

OR

SERVICING

The replenishment of

the consumable must

reduce the rate of

functional deterioration.

The task must reduce

the risk of failure.

The task must reduce

the risk of failure to an

acceptable level.

The task must be cost

effective.

OPERATIONAL

OR

VISUAL CHECK

Identification of failure

must be possible.

The task must ensure

adequate availability of

the hidden function to

reduce the risk of a

multiple failure.

Not applicable. The task must ensure

adequate availability of

the hidden function in

order to avoid economic

effects of multiple

failures and must be

cost effective.

INSPECTION

OR

FUNCTIONAL

CHECK

Reduce resistance to

failure must be

detectable, and there

exists a reasonably

consistent interval

between a deterioration

condition and functional

failure.

The task must reduce

the risk of failure to

assure safe operation.

The task must reduce

the risk of failure to an

acceptable level.

The task must be cost

effective; i.e., the cost

of the task must be less

than the cost of the

failure prevented.

RESTORATION The item must show

functional degradation

characteristics at an

identifiable age, and a

large proportion of units

must survive to that age.

It must be possible to

restore the item to a

specific standard of

failure resistance.

The task must reduce

the risk of failure to

assure safe operation.

The task must reduce

the risk of failure to an

acceptable level.

The task must be cost

effective; i.e., the cost

of the task must be less

than the cost of the

failure prevented.

DISCARD The item must show

functional degradation

characteristics at an

identifiable age and a

large proportion of units

must survive to that age.

The safe life limit must

reduce the risk of

failure to assure safe

operation.

The task must reduce

the risk of failure to an

acceptable level.

An economic life limit

must be cost effective;

i.e., the cost of the task

must be less than the

cost of the failure

prevented.
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2.3.7 Setting Task Frequencies/Intervals

Determine whether real and applicable data are available which suggest an effective interval

for task accomplishment. Appropriate information may consist of one or more of the

following:

a) Prior knowledge from other aircraft systems/power-plants that show a scheduled

maintenance task has offered substantial evidence of being effective and

economically worthwhile.

b) Manufacturer's test data which indicates that a scheduled maintenance task will be

effective for the item being evaluated.

c) If there is no prior knowledge from other aircraft systems/powerplant, or if there is

insufficient similarity between the previous and current systems, the task

interval/frequency can only be established initially by experienced working group

and steering committee personnel using good judgment and operating experience in

concert with accurate data (reliability, redundancy, dispatch, etc.).

2.3.7.1 Threshold Sample

The threshold sample is an examination of a specified number of items in order to

verify design calculations while attaining in-service experience with the items.

Thresholds may be established for the MRB defined items.
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2.4 AIRCRAFT STRUCTURAL MAINTENANCE PROGRAM DEVELOPMENT

This section contains guidelines for developing scheduled maintenance tasks for aircraft structure.

These are designed to relate the scheduled maintenance tasks to the consequences of structural

damage remaining undetected. Each structural item is assessed in terms of its significance to

continuing airworthiness, susceptibility to any form of damage, and the degree of difficulty involved

in detecting such damage. Once this is established, a structural maintenance program can be

developed which can be shown to be effective in detecting and preventing structural degradation due

to fatigue, environmental deterioration, or accidental damage throughout the operational life of the

aircraft. The structural maintenance task(s) developed as part of the structural maintenance program

are used to satisfy aircraft type certification and MRB requirements.

Mandatory replacement times for structural safe-life parts are included in the Airworthiness

Limitations, required by the regulatory authorities as part of the Instructions for Continued

Airworthiness. Some of the items requiring fatigue related inspections may also be included, as well

as specific Corrosion Prevention and Control Program (CPCP) tasks which subsequently warrant

inclusion, based on the in-service experience of the operators.

Requirements for detecting Accidental Damage (AD), Environmental Deterioration (ED), Fatigue

Damage (FD), and procedures for preventing and/or controlling corrosion form the basis for the

MRB structural maintenance program. However, all FD inspection requirements may not be

available when the aircraft enters service. In such cases the manufacturer shall propose, prior to the

entry of the aircraft into service, an appropriate time frame for completing the FD inspection

requirements.

Procedures should be developed for composite or other new materials because damage

characteristics may not follow those accepted for metallic structures.

2.4.1 Structural Maintenance Program Requirements

The primary objective of a structural maintenance program is to maintain the inherent

airworthiness throughout the operational life of the aircraft in an economical manner. To

achieve this, the inspections in this program must meet the detection requirements from

each of the AD, ED and FD assessments. Full account may be taken of all applicable

inspections occurring in the fleet.

Inspections related to detection of AD/ED are applicable to all aircraft when they first enter

service. Changes or adjustments can be made to these inspections based on individual

operator experience, when approved by their local regulatory authority.

Additional maintenance tasks (related to the ED Program) to control corrosion to Level 1 or

better are applicable at a threshold which is established during the aircraft type certification

process. These are based on manufacturer and operator experience with similar aircraft

structure, taking into consideration differences in relevant design features e.g. choice of

material, assembly process, corrosion protection systems, galley and toilet design etc. See

also Section 2.4.1.5 entitled Corrosion Prevention and Control Program.

Inspections related to FD detection are applicable after a threshold, which is established

during the aircraft type certification process. At the time the fatigue related inspections are

implemented, a sampling program can be used, where such a program is applicable and

effective. The fatigue related inspections are based directly on the manufacturer's approved

damage tolerance evaluations and changes or adjustments by the operators require use of an

approved procedure.

Where no service experience exists with similar structure, the structural maintenance

requirements shall be based on manufacturer's recommendations.

Proposed initial scheduled maintenance checks, to be used as the basis for the structural

maintenance program, are established for each aircraft type by the Industry Steering

Committee on the basis of:
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2.4.1 Structural Maintenance Program Requirements (cont’d.)

a. Operator experience

b. Manufacturer's proposals

c. Considerations of systems analysis requirements

2.4.1.1 Structural Maintenance Tasks

As part of the structural maintenance program development procedure, applicable

and effective structural maintenance tasks are selected for each deterioration

process of the SSI. To assure a direct correlation between the structural damage

tolerance evaluations and the structural maintenance program, it is necessary to

describe each task.

To all extents possible, the inspection methods specified in the tasks should use the

standard set of definitions included in the MSG-3 glossary. Changes and/or

additions to the inspection methods and definitions must be approved by the

Industry Steering Committee.

2.4.1.2 Inspection Thresholds

The inspection threshold for each SSI inspection task is a function of the source of

damage as follows:

a. Accidental Damage -

 The first inspection (threshold) for accidental damage normally corresponds to

a period equal to the defined repeat inspection interval, from the time of first

entry into service.

b. Environmental Deterioration -

 The initial inspection thresholds for all levels of inspection are based on

existing relevant service experience, manufacturers recommendations, and/or a

conservative age exploration program.

c. Fatigue Damage -

Inspections directly related to fatigue damage detection will occur after a

threshold(s) to be established by the manufacturer and approved by the

appropriate regulatory authority Thresholds are normally established as part of

the damage tolerance certification requirements. These are subject to change as

service experience, additional testing, or analysis work is obtained.

2.4.1.3 Repeat Inspection Intervals

After each inspection has been conducted, the repeat interval sets the period until

the next inspection.

a. Accidental Damage -

 The repeat interval should be based on operator and manufacturer experience

with similar structure. Selected intervals will normally correspond to single or

multiple levels of the scheduled maintenance check intervals.

b. Environmental Deterioration -

The repeat interval for detection/prevention/control of ED (corrosion, stress

corrosion, etc.) should be based on existing relevant service experience and/or

manufacturers recommendations.
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2.4.1.3 Repeat Inspection Intervals (cont’d.)

c. Fatigue Damage - The repeat intervals for fatigue related inspections are based

on the damage tolerance evaluations. These are used to demonstrate that

applicable and effective inspections provide sufficient probability of detecting

fatigue damage for each SSI.

2.4.1.4 Fatigue Related Sampling Inspection Programs

Transport aircraft with the highest number of flight cycles are most susceptible to

initial fatigue cracking in the fleet. This means that adequate inspections on such

aircraft will provide the greatest benefits for timely detection of fatigue damage.

Such sampling inspection programs are developed on the basis of appropriate

statistical variables, including:

a. The number of aircraft inspected.

b. The inspection methods and repeat intervals.

c. The number of flight cycles completed.

A list of SSIs that are suitable for a fatigue related sampling inspection program(s)

will be established by the Structures Working Group and submitted to the Industry

Steering Committee for approval and inclusion in the MRB report proposal. Full

details of the fatigue related sampling inspection program(s) will be established by a

joint operator/ manufacturer task force, based on the manufacturer's technical

evaluations, prior to aircraft exceeding the fatigue damage threshold(s).

2.4.1.5 Corrosion Prevention and Control Programs (CPCP)

A Corrosion Prevention and Control Program should be established to maintain the

aircraft's resistance to corrosion as a result of systematic (e.g. age related)

deterioration through chemical and/or environmental interaction.

The program is expected to allow control of the corrosion on the aircraft to

Corrosion Level 1 or better. The CPCP should be based on the ED analysis,

assuming an aircraft operated in a typical environment. If corrosion is found to

exceed Level 1 at any inspection time, the corrosion control program for the

affected area must be reviewed by the operator with the objective to ensure

Corrosion Level 1 or better.
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2.4.1.6 Age Exploration Programs

An age exploration program may be desirable to verify the aircraft's resistance to

corrosion deterioration before the Corrosion Prevention and Control Program Task

Thresholds.

Guidelines for age exploration programs should be established by the Structures

Working Group and submitted to the Industry Steering Committee for approval and

inclusion in the structural maintenance program.

2.4.1.7 Zonal Inspections

Some parts of the inspection requirements for SSIs and most of the items

categorized as Other Structure can be provided by the zonal program (see Section

2.5).

Tasks and intervals included in the zonal program should be based on operator and

manufacturer experience with similar structure. For structure containing new

materials and/or construction concepts, tasks and intervals may be established based

on assessment of the manufacturer's recommendations.

2.4.1.8 Inspection Results

The type certificate holder (manufacturer) and the operators will implement a

satisfactory system for the effective collection and dissemination of service

experience from the structural maintenance program.

This process will supplement the system which is required by existing regulations

for reporting occurrences of failures, malfunctions or defects (e.g. Service Difficulty

Reports).

2.4.2 Aircraft Structure Defined

Aircraft structure consists of all load carrying members including wings, fuselage,

empennage, engine mountings, landing gear, flight control surfaces and related points of

attachment. The actuating portions of items such as landing gear, flight controls, doors, etc.

will be treated as systems components and will be analyzed as described in Section 2.3.

Attachment of the actuators to the airframe will be treated as structure.

2.4.2.1 Significant and Other Structure

Structure can be subdivided into items according to the consequences of their

failure to aircraft safety as follows:

a. A Structural Significant Item (SSI) is any detail, element or assembly, which

contributes significantly to carrying flight, ground, pressure or control loads,

and whose failure could affect the structural integrity necessary for the safety of

the aircraft.

b. Other Structure is that which is judged not to be a Structural Significant Item.

It is defined both externally and internally within zonal boundaries.

2.4.3 Damage Sources and Inspection Requirements

This section describes the damage sources and inspection requirements to be considered

when developing the structural maintenance program.
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2.4.3.1 Damage Sources

The assessment of structure for the selection of maintenance tasks should consider

the following damage sources:

a. Accidental Damage (AD), which is characterized by the occurrence of a

random discrete event which may reduce the inherent level of residual

strength. Sources of such damage include ground and cargo handling

equipment, foreign objects, erosion from rain, hail, lightning, runway

debris, spillage, freezing, thawing, etc., and those resulting from human

error during aircraft manufacture, operation or maintenance that are not

included in other damage sources.

 The same sources of accidental damage as those considered for metallic

materials are to be considered for non-metallic material such as

composites. The consequence of a damage may not be readily apparent

and may include internal damage, e.g., disbonding or delamination.

 Large size accidental damage, such as that caused by engine disintegration,

bird strike or major collision with ground equipment, will be readily

detectable and no maintenance task assessment is required.

b. Environmental Deterioration (ED), which is characterized by structural

deterioration as a result of a chemical interaction with its climate or

environment. Assessments are required to cover corrosion, including

stress corrosion, and deterioration of non-metallic materials. Corrosion

may or may not be time/usage dependent. For example, deterioration

resulting from a breakdown in surface protection is more probable as the

calendar age increases; conversely, corrosion due to galley spillage is a

randomly occurring discrete event.

 Stress corrosion cracking in a given environment is directly dependent

upon the level of sustained tensile stress which may result from heat

treatment, forming, fit-up, or misalignment.

 In contrast to the environmental deterioration process of metallic

structures, non-metallic structures such as composites are not normally

susceptible to degradation due to the environment. However, the effect of

long-term aging in an operating environment has to be taken into

consideration when developing the structural maintenance program.

c. Fatigue Damage (FD) which is characterized by the initiation of a crack or

cracks due to cyclic loading and subsequent propagation. It is a

cumulative process with respect to aircraft usage (flight cycles or flight

hours).

2.4.3.2 Inspection Requirements

Inspection requirements in relation to the damage sources are as follows:

a. Accidental Damage (AD), stress corrosion and most other forms of

corrosion are random in nature and can occur any time during the aircraft

service life. In such cases, inspection requirements apply to all aircraft in

the fleet throughout their operational lives.

b. Most forms of corrosion are time/usage dependent and more likely to

occur as the fleet ages. In such cases, operator and manufacturer

experience on similar structure can be used to establish appropriate

maintenance tasks (including CPCP tasks) for the control of environmental

deterioration.
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2.4.3.2 Inspection Requirements (cont’d.)

 The deterioration of non-metallic structures such as composites has to be

taken into consideration when establishing maintenance tasks. Appropriate

inspection levels and frequencies should be based on existing relevant

service experience and manufacturer's recommendations.

c. Detectable size fatigue cracking is not normally anticipated in primary

airframe structure until the fleet has matured. Thereafter, structural

maintenance programs may require revision.

For most transport aircraft structure, aircraft with the highest number of

flight cycles are more susceptible to initial fatigue cracking in the fleet and

are suitable candidates for a fatigue related sampling inspection program,

should this be applicable and effective.

2.4.4 Structural Maintenance Program Development

The structural maintenance program is based on an assessment of structural design

information, fatigue and damage tolerance evaluations, service experience with similar

structure and pertinent test results.

The assessment of structure for selection of maintenance tasks should include the following:

a. The sources of structural deterioration:

1) Accidental Damage

2) Environmental Deterioration

3) Fatigue Damage

b. The susceptibility of the structure to each source of deterioration.

c. The consequences of structural deterioration to continuing airworthiness

1) Effect on aircraft (e.g. loss of function or reduction of residual strength).

2) Multiple site or multiple element fatigue damage.

3) The effect on aircraft flight or response characteristics caused by the

interaction of structural damage or failure with systems or powerplant items.

4) Inflight loss of structural items.

d. The applicability and effectiveness of various methods of preventing, controlling or

detecting structural deterioration, taking into account inspection thresholds and repeat

intervals.

2.4.4.1 Procedure

The procedure for developing a structural maintenance program is shown in the

logic diagram (see Figure 2) and described by a series of process steps (P1, P2, P3,

etc.) and decision steps (D1, D2, D3, etc.) as follows:

a. The structural maintenance program includes all aircraft structure which

is divided into zones or areas (P1) and structural items (P2) by the

manufacturer.

b. The manufacturer categorizes each item (D1) as structurally significant

(SSI) (P3) or Other Structure (P4), on the basis of the consequences to

aircraft safety of item failure or malfunction.

c. The same procedure is repeated until all structural items have been

categorized.
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2.4.4.1 Procedure (cont’d.)

d. Items categorized as Other Structure (P4) are compared to similar items

on existing aircraft (d2). Maintenance recommendations are developed

by the Structures Working Group (SWG) for items which are similar and

by the manufacturer for those which are not, e. g., new materials or design

concepts (P5). All tasks selected by the SWG (P6) are included in the

structural maintenance program (P15).

e. Inspection requirements for timely detection of Accidental Damage (AD)

and Environmental Deterioration (ED) are determined for all SSIs (P7).

These can be determined for individual SSIs or groups of SSIs which are

suitable for comparative assessments on the basis of their location,

boundaries, inspection access, analysis breakdown, etc. The

manufacturer's rating systems (see 2.4.5) are used to determine these

requirements.

f. The process (P7) is repeated until all SSIs are examined.

g. For each SSI, the maintenance requirements are determined (P8) such that

the program expectations of the CPCP (2.4.1.5) are fulfilled.

h. The inspection requirement of the ED analysis is compared with the

requirement of the CPCP (D9). If they are similar or identical, the ED

task will cover the CPCP requirement. If the CPCP task requirement is

not met, the ED task has to be reviewed and/or additional and separate

CPCP tasks have to be determined (P9).

i. The process (P7, P8, D9) is repeated until all SSIs are examined.

j. All tasks, selected by the SWG, are included in the structural maintenance

program (P15).

k. The manufacturer categorizes each SSI as damage tolerant or safe-life

(D3).

l. For each item categorized as safe-life (P10), the manufacturer determines

the safe-life limit which is included in the aircraft Airworthiness

Limitations (P14). No fatigue related inspection program is required to

assure continuing airworthiness.

m. All remaining SSIs are damage tolerant (P11) and the manufacturer

determines if timely detection of fatigue damage is dependent on

scheduled inspections. A scheduled fatigue related inspection program

may not be required for SSIs designed to carry the required load with

damage that will be readily detectable during routine operation of the

aircraft or indicated by safe malfunction (D4).

n. Visual inspections during appropriate scheduled maintenance checks are

used, where applicable and effective, to provide the necessary fatigue

damage detection opportunities (D5).

o. Applicable nondestructive inspection (NDI) methods, during appropriate

scheduled maintenance checks, are used to provide necessary fatigue

damage detection opportunities when visual inspections are inadequate

(D6).

p. Details of the fatigue related inspection requirements are presented to the

SWG who determine if they are feasible (D7). Improved inspection

access and/or redesign of the SSI may be required if no practical and

effective visual and/or nondestructive inspections are available (D8,P12).

If this is not feasible for the manufacturer, the SSI must be categorized as

safe-life (P10).

q. Fatigue related inspection requirements selected by the SWG are included

in the preliminary Structural Maintenance Program (P15).
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2.4.4.1 Procedure (cont’d.)

r. To support Type Certification, selected SSIs (P13, P14) that will

eventually be included in the fatigue related inspection program should be

listed in the Airworthiness Limitations document.

s. The FD analysis procedure is repeated for all damage tolerant SSIs.

t. Tasks from AD, ED, FD, and other structure analyses are listed in the

Structural Maintenance Program (P15).

u. The resulting maintenance requirements for all structure are submitted to

the ISC for approval and inclusion in the MRB report proposal.

v. The structural maintenance portion of the Airworthiness Limitations

should be included in a separate document and submitted to the

appropriate Regulatory Authority (certification) for approval.
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FIGURE 2: STRUCTURAL LOGIC DIAGRAM
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FIGURE 2A: OTHER STRUCTURE LOGIC DIAGRAM
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FIGURE 2B:

ACCIDENTAL DAMAGE and ENVIRONMENTAL DETERIORATION

LOGIC DIAGRAM
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FIGURE 2C: SAFELIFE LIMIT ANALYSIS LOGIC DIAGRAM

 FROM D3

 P10

CATEGORIZE AND LIST AS SAFE LIFE:

MANUFACTURER DETERMINES SAFE LIFE AND

INCLUDES WITH SSI DESCRIPTION IN AIRWORTHINESS

LIMITATIONS

 TO P14
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

Page 34

September 12, 1993

FIGURE 2D: FD ANALYSIS LOGIC DIAGRAM
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2.4.5 Rating Systems for Structural Significant Items

As part of the structural maintenance program development, it is necessary to rate each

Structural Significant Item in terms of susceptibility (likelihood of damage) and

detectability (timely detection of damage). This section provides guidelines to assist

manufacturers in the development of suitable rating systems. The rating system should

account for the susceptibility of the SSI to the likely source of damage and the likely type of

deterioration of the SSI due to the damage source. Differences between metallic and nonmetallic portions of the SSI's must be taken into account.

The structural maintenance program is developed on the basis of requirements to assure

timely detection of Accidental Damage, Environmental Deterioration, and Fatigue Damage.

Rating systems for AD and ED should be compatible to allow comparative assessments for

each group of SSIs. Emphasis is placed on rating each SSI in relation to other SSIs in the

same inspection area, leading to increased inspection emphasis for the most critical SSIs.

Manufacturer and operator experience is a key ingredient for these evaluations.

Rating systems for FD should incorporate results from the manufacturer's residual strength

and crack growth evaluations. The applicability and effectiveness of various inspection

methods, detectable damage sizes and access requirements are key ingredients for these

evaluations.

2.4.5.1 Rating Accidental Damage

Accidental damage rating systems should include evaluations of the following:

a. Susceptibility to minor (not obvious) accidental damage based on

frequency of exposure to and the location of damage from one or more

sources, including:

1) Ground handling equipment

2) Cargo handling equipment

3) Those resulting from human error during manufacture,

maintenance, and/or operation of the aircraft, that are not

included in other damage sources.

4) Rain, hail, etc.

5) Runway debris

6) Lightning strike

7) Water entrapment

b. Residual strength after accidental damage, normally based on the likely

size of damage relative to the critical damage size for the SSI.

c. Timely detection of damage, based on the relative rate of growth after

damage is sustained and visibility of the SSI for inspection.

Assessments should take into account damage growth associated with

non-chemical interaction with an environment, such as disbond or

delamination growth associated with a freeze/thaw cycle.

Rating values should be assigned to groups of SSIs in the same inspection area on

the basis of comparative assessments within the group.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2.4.5.2 Rating Environmental Deterioration

Environmental deterioration rating systems should allow for evaluations of

susceptibility to and timely detection of corrosion and stress corrosion.

Susceptibility to corrosion is assessed on the basis of probable exposure to an

adverse environment and adequacy of the protective system. For example:

a. Exposure to a deteriorating environment such as cabin condensation,

galley spillage, toilet spillage, cleaning fluids, etc.

b. Contact between dissimilar materials (potential for galvanic activity).

c. Breakdown of surface protection systems; for example, deterioration of

paint, primer, bonding, sealant, corrosion inhibiting compounds and

cladding systems with the resulting corrosion of metallic materials or

fluid incursion into permeable non-metallic materials, etc.

Material characteristics, coupled with the likelihood of sustained tensile stress, are

used to assess susceptibility to stress corrosion.

Timely detection is determined by sensitivity to relative size of damage and

visibility of the SSI for inspection.

Note: Rating system evaluations should be made taking into account the

requirement for each operator to control the aircraft structure at corrosion Level 1

or better.

2.4.5.3 Rating Fatigue Damage

The rating system must lead to an inspection program that provides a high

probability of detecting fatigue damage in the fleet before such damage reduces any

aircraft's residual strength below allowable levels. To achieve this, the rating

system should consider the following:

a. Residual strength, including the effects of multiple site fatigue damage,

where appropriate.

b. Crack growth rate, including effects of multiple site or multiple element

fatigue damage, where appropriate.

c. Damage detection period which corresponds to the interval for the fatigue

damage to grow from the threshold of detection (detectable) to the

limiting size defined by "a" (critical). This period will vary according to

the inspection method used, and may be influenced by structural parts or

processes, e.g., sealant obscuring parts of the damage.

d. Detection standards for applicable inspection methods.

 Note: Estimated detectable crack lengths can be used for the fatigue

damage detection evaluations required as part of aircraft type

certification.

e. Applicable inspection levels and methods (e.g., visual, NDI), directions

(e.g., external, internal) and repeat intervals (e.g., C, 2C, 4C).
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

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September 12, 1993

2.5 ZONAL INSPECTION PROGRAM

The zonal inspection program requires a summary review of each zone on the aircraft. This

normally occurs as the MSG-3 analyses of structures, systems, and powerplants are being concluded.

In top down analyses conducted under MSG-3, many support items such as plumbing, ducting, Other

Structure, wiring, etc., may be evaluated for possible contribution to functional failure. In cases

where a general visual inspection is required to assess degradation, the zonal inspection program is

an appropriate method.

2.5.1 Procedure

The following procedures may be used to develop a zonal inspection program:

a. Divide the aircraft externally and internally into zones as defined in ATA

specification 100.

b. Prepare a task listing work sheet for each zone including location, description,

access notes, etc.

c. During analyses of systems, powerplants and structures, list any general visual

inspections which could be conducted as part of the zonal inspection program.

d. Include the interval from the original analyses on the zone work sheet.

e. As the analysis covering items in a zone are completed, the zone should be

reviewed to consolidate inspection requirements and assign accomplishment

intervals. Document in the work sheets any System/Powerplant or Structural

general visual inspections replaced by the zonal inspection task.

2.5.2 Zonal Task Intervals

Accomplishment intervals are based on hardware susceptibility to damage, the amount of

activity in the zone, and operator and manufacturer experience with similar systems,

powerplants and structures. When possible, intervals should correspond to those selected

for targeted scheduled maintenance checks.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

GLOSSARY - APPENDIX A

Page A-1

September 12, 1993

ACCIDENTAL DAMAGE (AD):

Physical deterioration of an item caused by contact or impact with an object or influence which is not a part of

the aircraft, or by human error during manufacturing, operation of the aircraft, or maintenance practices.

AGE EXPLORATION:

A systematic evaluation of an item based on analysis of collected information from in-service experience. It

verifies the item’s resistance to a deterioration process with respect to increasing age.

AIRWORTHINESS LIMITATIONS:

A section of the Instructions for Continued Airworthiness that contains each mandatory replacement time,

structural inspection interval, and related structural inspection task. This section may also be used to define a

threshold for the fatigue related inspections and the need to control corrosion to Level 1 or better. The

information contained in the Airworthiness Limitations section may be changed to reflect service and/or test

experience or new analysis methods.

CORROSION LEVEL 1:

Corrosion damage that does not require structural reinforcement or replacement.

or

Corrosion occurring between successive inspections exceeds allowable limit but is local and can be attributed to

an event not typical of operator usage of other aircraft in the same fleet (e.g. Mercury spill).

CORROSION PREVENTION AND CONTROL PROGRAM (CPCP):

A program of maintenance tasks implemented at a threshold designed to control an aircraft structure to Corrosion

Level 1 or better.

DAMAGE TOLERANT:

A qualification standard for aircraft structure. An item is judged to be damage tolerant if it can sustain damage

and the remaining structure can withstand reasonable loads without structural failure or excessive structural

deformation until the damage is detected.

DELAMINATION/DISBOND:

Structural separation or cracking that occurs at or in the bond plane of a structural element, within a structural

assembly, caused by in service accidental damage, environmental effects and/or cyclic loading.

DISCARD:

The removal from service of an item at a specified life limit.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

GLOSSARY - APPENDIX A

Page A-2

September 12, 1993

DIRECT ADVERSE EFFECT ON OPERATING SAFETY:

Direct:

To be direct, the functional failure or resulting secondary damage must achieve its effect by itself, not

in combination with other functional failures (no redundancy exists and it is a primary dispatch item).

Adverse Effect on Safety:

This implies that the consequences are extremely serious or possibly catastrophic and might cause the

loss of aircraft or injury to occupants.

Operating:

This is defined as the time interval during which passengers and crew are on board for the purpose of

flight.

ECONOMIC EFFECTS:

Failure effects which do not prevent aircraft operation, but are economically undesirable due to added labor an

material cost for aircraft or shop repair.

ENVIRONMENTAL DETERIORATION (ED):

Physical deterioration of an item’s strength or resistance to failure as a result of chemical interaction with its

climate or environment.

FAILURE:

The inability of an item to perform within previously specified limits.

FAILURE CAUSE:

Why the functional failure occurs.

FAILURE EFFECT:

What is the result of a functional failure.

FATIGUE DAMAGE (FD):

The initiation of a crack or cracks due to cyclic loading an subsequent propagation.

FATIGUE RELATED SAMPLING INSPECTION PROGRAM:

Inspections on specific aircraft selected from those which have the highest operating age/usage in order to

identify the first evidence of deterioration in their condition caused by fatigue damage.

FUNCTION:

The normal characteristic actions of an item.

FUNCTIONAL CHECK:

A quantitative check to determine if one or more functions of an item performs within specified limits.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

GLOSSARY - APPENDIX A

Page A-3

September 12, 1993

FUNCTIONAL FAILURE:

How an item failed to perform its function.

HIDDEN FUNCTION:

1. A function which is normally active and whose cessation will not be evident to the operating crew

during performance of normal duties.

2. A function which is normally inactive and whose readiness to perform, prior to it being needed, will

not be evident to the operating crew during performance of normal duties.

INHERENT LEVEL OF RELIABILITY AND SAFETY:

That level which is built into the unit and, therefore, inherent in its design. This is the highest level or reliability

and safety that can be expected from a unit, system, or aircraft if it receives effective maintenance. To achieve

higher levels of reliability generally requires modification or redesign.

INSPECTION - DETAILED:

An intensive visual examination of a specific structural area, system, installation or assembly to detect damage,

failure or irregularity. Available lighting is normally supplemented with a direct source of good lighting at an

intensity deemed appropriate by the inspector. Inspection aids such as mirrors, magnifying lenses, etc. may be

used. Surface cleaning and elaborate access procedures may be required.

INSPECTION - GENERAL VISUAL (SURVEILLANCE):

A visual examination of an interior or exterior area, installation or assembly to detect obvious damage, failure or

irregularity. This level of inspection is made under normally available lighting conditions such as daylight,

hangar lighting, flashlight or drop-light and may require removal or opening of access panels or doors. Stands,

ladders or platforms may be required to gain proximity to the area being checked.

INSPECTION - SPECIAL DETAILED:

An intensive examination of a specific item(s), installation, or assembly to detect damage, failure or irregularity.

The examination is likely to make extensive use of specialized Inspection Techniques and/or equipment.

Intricate cleaning and substantial access or disassembly procedure may be required.

INSPECTION - ZONAL:

A general visual inspection of each aircraft zone, defined by access and area, to check system and powerplant

installations and structure for security and general condition.

ITEM:

Any level of hardware assembly (i.e., system, sub-system, module, accessory, component, unit, part, etc.).

LUBRICATION AND SERVICING:

Any act of lubricating or servicing for the purpose of maintaining inherent design capabilities.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

GLOSSARY - APPENDIX A

Page A-4

September 12, 1993

MAINTENANCE SIGNIFICANT ITEM - (MSI):

Items identified by the manufacturer whose failure:

a. could affect safety (on ground or in flight), and/or

b. is undetectable during operations, and/or

c. could have significant operational impact, and/or

d. could have significant economic impact

MULTIPLE ELEMENT FATIGUE DAMAGE:

The simultaneous cracking of multiple load path discrete elements working at similar stress levels.

MULTIPLE SITE FATIGUE DAMAGE:

The presence of a number of adjacent, small cracks that might coalesce to form a single long crack.

NON-METALLICS:

Any structural material made from fibrous or laminated components bonded together by a medium. Materials

such as graphite epoxy, boron epoxy, fiber glass, kevlar epoxy, acrylics and the like are non-metallics. Nonmetallics include adhesives used to join other metallic or non-metallic structural materials.

OPERATING CREW NORMAL DUTIES:

Operating Crew Qualified cockpit and cabin attendant personnel who are on duty.

Normal Duties Those duties associated with the routine operation of the aircraft, on a daily

basis, to include the following:

a. Procedures and checks performed during aircraft operation;

b. Recognition of abnormalities or failures by the operating crew through

the use of normal physical senses (e.g., odor, noise, vibration,

temperature, visual observation of damage or failure, changes in physical

input force requirements, etc.).

OPERATIONAL CHECK:

An operational check is a task to determine that an item is fulfilling its intended purpose. Does not require

quantitative tolerances. This is a failure finding task.

OPERATIONAL EFFECTS:

Failure effects which interfere with the completion of the aircraft mission. These failures cause delays,

cancellations, ground or flight interruptions, high drag coefficients, altitude restrictions, etc.

OTHER STRUCTURE:

Structure which is judged not to be a Structural Significant Item.

“Other Structure” is defined both externally and internally within zonal boundaries.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

GLOSSARY - APPENDIX A

Page A-5

September 12, 1993

REPEAT INTERVAL:

The interval expressed in flight cycles, flight hours and/or calendar time, between successive accomplishments of

a specific maintenance task.

RESIDUAL STRENGTH:

The strength of a damaged structure.

RESTORATION:

That work necessary to return the item to a specific standard. Restoration may vary from cleaning or

replacement of single parts up to a complete overhaul.

SAFE LIFE STRUCTURE:

Structure which is not practical to design or qualify as damage tolerant. Its reliability is protected by discard

limits which remove items from service before fatigue cracking is expected.

SCHEDULED MAINTENANCE CHECK:

Any of the maintenance opportunities which are prepackaged and are accomplished on a regular basis.

STRUCTURAL SIGNIFICANT ITEM - (SSI):

Any detail, element or assembly, which contributes significantly to carrying flight, ground, pressure or control

loads and whose failure could affect the structural integrity necessary for the safety of the aircraft.

STRUCTURAL ASSEMBLY:

One or more structural elements which together provide a basic structural function.

STRUCTURAL DETAIL:

The lowest functional level in an aircraft structure. A discrete region or area of a structural element, or a

boundary intersection of two or more elements.

STRUCTURAL ELEMENT:

Two or more structural details which together form an identified manufacturer’s assembly part.

STRUCTURAL FUNCTION:

The mode of action of aircraft structure. It includes acceptance and transfer of specified loads in items

(details/elements/assemblies) and provides consistently adequate aircraft response and flight characteristics.

TASKS - MAINTENANCE:

An action or set of actions required to achieve a desired outcome which restores an item to or maintains an item

in serviceable condition, including inspection and determination of condition.
MSG-3 MAINTENANCE PROGRAM DEVELOPMENT

GLOSSARY - APPENDIX A

Page A-6

September 12, 1993

THRESHOLD:

The initial accomplishment of a specific maintenance task expressed in flight cycle, flight hours, and/or calendar

time.

VISUAL CHECK:

A visual check is an observation to determine that an item is fulfilling its intended purpose. Does not require

quantitative tolerances. This is a failure finding task.