Advisory 

Circular U.S. Department 

of Transportation 

Federal Aviation

Administration

 

Subject: CONTINUING 

STRUCTURAL INTEGRITY 

PROGRAM FOR AIRPLANES 

Date: 3/7/2008

Initiated by: ANM-115 

AC No: 91-56B

 

1. PURPOSE. This advisory circular (AC) provides guidance material to design 

approval holders and operators for use in developing a damage tolerance based 

supplemental structural inspection program (SSIP) for older airplanes. This AC also 

references and briefly describes other elements of a continuing structural integrity 

program that support the safe operation of transport category airplanes throughout their 

operational lives. 

2. APPLICABILITY. 

 a. This AC applies to design approval holders and operators of transport category 

airplanes. This AC may also be used by design approval holders and operators of 

normal, acrobatic, utility and commuter category airplanes. This guidance may be useful 

for design approval holders that choose to certificate a small airplane according to the 

damage tolerance requirements of Title 14, Code of Federal Regulations (CFR) part 23. 

This guidance may be useful for small airplane design approval holders and operators 

that choose to develop a structural integrity program as a non-mandatory operational 

safeguard against the effects of structural aging. 

b. This material is neither mandatory nor regulatory in nature and does not 

constitute a regulation. It describes acceptable means, but not the only means, for 

showing compliance with the applicable regulations. The Federal Aviation 

Administration (FAA) will consider other methods of demonstrating compliance that an 

applicant may elect to present. While these guidelines are not mandatory, they are 

derived from extensive FAA and industry experience in determining compliance with the 

relevant regulations. On the other hand, if we become aware of circumstances that 

convince us that following this AC would not result in compliance with the applicable 

regulations, we will not be bound by the terms of this AC, and we may require additional 

substantiation or design changes as a basis for finding compliance. 

c. This material does not change, create any additional, authorize changes in, or 

permit deviations from, regulatory requirements. 
AC 91-56B 3/7/2008

 

3. CANCELLATION. This AC cancels AC 91-56A, Continuing Structural Integrity 

Program for Large Transport Category Airplanes, dated April 29, 1998. 

4. RELATED REGULATIONS AND DOCUMENTS. 

a. Title 14, Code of Federal Regulations (14 CFR). You may get a copy of 14 CFR, 

parts 23, 25, 43, 91, 121, and 129 on-line at http://www.gpoaccess.gov/cfr. You may 

order a paper copy from the U.S. Superintendent of Documents, U.S. Government 

Printing Office, Washington D.C., 20402-001; or by calling telephone number (202) 512-

1800; or by facsimile (202) 512-2250. 

(1) Part 23, §§ 23.571, 23.572, 23.573, 23.574, Small airplane requirements for 

fatigue, fail-safe, and damage-tolerance evaluations. 

(2) Part 25, § 25.571, Damage-tolerance and fatigue evaluation. 

 

(3) Part 43, § 43.16, Airworthiness Limitations. 

(4) Part 91, § 91.403, General. 

(5) Part 121, § 121.1109, Supplemental inspections. 

(6) Part 129, § 129.109, Supplemental inspections for U.S.-registered aircraft. 

b. Advisory Circulars: You may download an electronic copy of the latest version 

of the following ACs from the Internet at http://rgl.faa.gov. 

(1) AC 25.571-1C, Damage Tolerance and Fatigue Evaluation of Structure, 

dated April 29, 1998. 

(2) AC 120-73, Damage Tolerance Assessment of Repairs to Pressurized 

Fuselages, dated December 14, 2000. 

(3) AC 120-93, Damage Tolerance Inspections for Repairs, dated

November 20, 2007. 

5. BACKGROUND. 

a. Service experience has demonstrated that there is a need to have continuing 

updated knowledge concerning the structural integrity of airplanes, especially as they 

became older. The structural integrity of these airplanes is of concern, since such factors 

as fatigue cracking and corrosion are time dependent and knowledge concerning them

can best be assessed on the basis of real time operational experience and the use of the 

most modern tools of analysis and testing. 

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3/7/2008 AC 91-56B

b. The FAA, manufacturers, and operators have continually worked to maintain the 

structural integrity of older airplanes. Traditionally, this has been accomplished through 

an exchange of field service information and subsequent changes to inspection programs, 

and by the development and installation of modifications on particular aircraft. However, 

increased utilization, longer operational lives, and the high safety demands imposed on 

airplanes indicate the need for a program to ensure a high level of structural integrity. 

Accordingly, the inspection and evaluation programs outlined in this AC are intended to 

ensure a continuing structural integrity assessment by each airplane manufacturer and the 

incorporation of the results of each assessment into the maintenance program of each 

operator. 

c. The previous version of this AC (AC 91-56A) provided guidance to support the 

development of a damage tolerance based supplemental structural inspection program

(SSIP) for large transport airplanes certified under the fail-safe and fatigue requirements 

of Civil Air Regulations (CAR) 4b or part 25, prior to Amendment 25-45. That guidance 

was traditionally applied to the following eleven large transport airplane models: Airbus 

Model A300; British Aerospace Model BAC 1-11; Boeing Models B-707/720, B-727, 

B-737, B-747; McDonnell Douglas Model DC-8, DC-9/MD-80, DC-10; Fokker Model 

F28; and Lockheed Model L-1011 series airplanes. For each of these models, 

airworthiness directives (ADs) were issued to mandate the implementation of damagetolerance based SSIPs. These airplanes have a maximum takeoff gross weight greater 

than 75,000 pounds. 

d. In addition to these eleven airplane models, the guidance in the previous version 

of this AC (which is included in this revision) has been successfully used to develop a 

SSIP for airplanes with a maximum takeoff gross weight less than 75,000 pounds. As 

this guidance was determined to be applicable to smaller airplane models, the term

“Large Transport Category” was removed from the title of this revision to the AC. 

e. The Aging Airplane Safety Final Rule (AASFR) (70 FR 5518, February 2, 2005) 

expanded the requirement for damage tolerance based SSIPs beyond the eleven models 

mentioned above. The AASFR is applicable to all U.S.-registered transport category, 

turbine-powered airplanes, operated under Subpart D of part 121 and part 129, having a 

type certificate issued after January 1, 1958, that as a result of original type certification 

or later increase in capacity, have: 

 (1) a maximum type certificated passenger seating capacity of 30 or more; or 

 (2) a maximum payload capacity of 7,500 pounds or more. 

f. An FAA-approved SSIP developed using the guidance in this AC is an acceptable 

means of compliance with the AASFR for those areas addressed by the SSIP. SSIPs 

typically apply to the baseline structure, which is defined as that originally designed by 

the original equipment manufacturer (OEM). 

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AC 91-56B 3/7/2008

g. (1) In addition to SSIPs, this AC discusses the following additional elements of

a continuing structural integrity program:

• Repairs, Alterations, and Modifications 

• Mandatory Modification Program

• Corrosion Prevention and Control Program (CPCP) 

• Repair Assessment Program

 (2) Additional background information can be found in Appendix 4 of 

AC 120-93, Damage Tolerance Inspections for Repairs and Alterations. 

6. SUPPLEMENTAL STRUCTURAL INSPECTION PROGRAMS. The 

manufacturer, in conjunction with operators, is expected to initiate development of a 

SSIP for each airplane model. Such a program should be implemented before analyses, 

tests, and/or service experiences indicate that a significant increase in inspection and/or 

modification is necessary to maintain structural integrity of the airplane. In the absence 

of other data as a guideline, the program should be initiated no later than the time when 

the high-time or high-cycle airplane in the fleet reaches one half its design service goal. 

This should ensure that an acceptable program is available to the operators when needed. 

The program should include procedures for obtaining service information, and 

assessment of service information, available test data, and new analysis and test data. 

The operator should develop a supplemental structural inspection document (SSID) from

this body of data, as outlined in the appendix to this AC. 

 a. The recommended SSIP, along with the criteria used and the basis for the criteria, 

should be submitted to the cognizant FAA Aircraft Certification Office for review and

approval. The supplemental program should be adequately defined in the SSID and 

presented in a manner that is effective. The SSID should include the type of damage 

being considered, and likely sites; inspection access, threshold, interval, method and 

procedures; applicable modification status and/or life limitation; and types of operations 

for which the SSID is valid. 

 b. The FAA review of the SSID will include both engineering and maintenance 

aspects of the proposal. Since the SSID is applicable to all operators and addresses a 

safety concern for older airplanes, it will be made mandatory under the existing 

Airworthiness Directive (AD) system if the FAA deems that an unsafe condition exists. 

In addition, any service bulletin or other service information publications found to be 

essential for safety during the initial SSID assessment process should be implemented by 

AD action. Service bulletins or other service information publications revised or issued 

as a result of in service findings resulting from implementation of the SSID should be 

added to the SSID or implemented by separate AD action, as appropriate. 

 c. In the event an acceptable SSID cannot be obtained on a timely basis, the FAA 

may impose service life, operational, or inspection limitations to assure structural

integrity. 

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3/7/2008 AC 91-56B

 d. The design approval holder should revise the SSID whenever additional 

information shows a need. The original SSID will normally be based on predictions or 

assumptions (from analyses, tests and/or service experience) of failure modes, time to 

detectable fatigue cracking, frequency of damage, typically detectable damage, and the 

damage growth period. Consequently, a relieving change in these factors sufficient to 

justify a revision would have to be substantiated by test data or additional service 

information. Any revision to SSID criteria and the basis for these revisions should be 

submitted to the FAA for review and approval of both engineering and maintenance 

aspects. 

7. REPAIRS, ALTERATIONS, AND MODIFICATIONS. 

 a. The AASFR requires operators of affected airplanes to develop damage tolerance 

(DT) - based inspections for all fatigue critical baseline structure. The AASFR also 

requires that the DT-based inspections address the adverse effects that repairs, alterations, 

and modifications may have on the fatigue critical baseline structure. While this AC 

provides guidance on developing an SSIP for fatigue critical baseline structure, 

AC 120-93, Damage Tolerance Inspections for Repairs, provides guidance to type 

certificate holders and operators on addressing repairs, alterations, and modifications as 

required by the AASFR. 

b. Certain ADs that mandated SSIPs on older airplanes have addressed repairs, 

alterations, and modifications that affect principal structural elements. In addition, the 

Repair Assessment for Pressurized Fuselages rule (65 FR 24108, April 25, 2000) 

addresses repairs to the fuselage pressure boundary. The same data used for compliance 

with the SSIP ADs or the Repair Assessment for Pressurized Fuselages rule can be used 

to comply with the AASFR for the repairs, alterations, and modifications to the structure 

addressed by those data. 

8. MANDATORY MODIFICATION PROGRAM. 

 a. The mandatory modification program was based on the premise that to ensure the 

structural integrity of older airplanes, there should be less reliance on repetitive 

inspections when certain criteria exist. These criteria included: 

• The likelihood that known structural cracking problems exist and are not just 

theoretical or predicted. 

• The consequences of failing to correct the problem must be catastrophic. This 

means that the structural element involved must be a principle structural element 

or other primary structure. 

• The cracks must be difficult to detect during regular maintenance. 

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AC 91-56B 3/7/2008

• Other considerations are: the areas to inspect are difficult to access;

nondestructive test methods are un-suitable; or human factors of inspection are so 

adverse that crack detection may not be sufficiently dependable to assure safety. 

 b. The structural modification programs were mandated on the original eleven 

models (Airbus Model A300; British Aerospace Model BAC 1-11; Boeing Models 

B-707/720, B-727, B-737, B-747; McDonnell Douglas Model DC-8, DC-9/MD-80, 

DC-10; Fokker Model F28; and Lockheed Model L-1011 series airplanes) by ADs. Each 

of the type certificate holders reviewed its service bulletins with the FAA to determine 

which areas of structure needed terminating modifications to inspections. The revised 

service bulletins that included those terminating modifications were then grouped in a 

document and mandated. However, some service bulletins were individually mandated. 

 c. The AASFR requires that all modifications affecting fatigue critical baseline 

structure be assessed, including any new fatigue critical structure created by such a 

modification. Guidelines for addressing modifications are in AC 120-93, Damage 

Tolerance Inspection for Repairs. 

9. CORROSION PREVENTION AND CONTROL PROGRAM. A corrosion 

prevention and control program (CPCP) is a systematic approach to controlling corrosion 

in the airplane’s primary structure. A CPCP consists of a basic corrosion inspection task, 

task areas, defined corrosion levels, and compliance times. The objective of a CPCP is to 

limit the material loss due to corrosion to a level necessary to maintain airworthiness. 

 a. The CPCPs were mandated by ADs for certain large transport category airplanes 

(Airbus Model A300; British Aerospace Model BAC 1-11; Boeing Models B-707/720, 

B-727, B-737, B-747; McDonnell Douglas Model DC-8, DC-9/MD-80, DC-10; Fokker 

Model F28; and Lockheed Model L-1011 series airplanes) and many other transport 

category airplanes. The type certificate holders for these airplanes developed the CPCP 

document that was mandated by AD. These corrosion programs supplemented each 

operator’s maintenance program. 

 b. The corrosion programs were developed based on the premise that operators 

could adjust them when unacceptable corrosion levels were found. These maintenance 

program adjustments should preclude recurrence of unacceptable corrosion findings. 

Adjustments may include actions such as reduced repetitive task intervals, improved 

corrosion treatments, or multiple corrosion inhibitor applications. 

c. The FAA has withdrawn notice of proposed rulemaking (NPRM) Notice 

No. 02-16 (67 FR 62142, October 3, 2002) Corrosion Prevention and Control Program.

That NPRM was withdrawn because the FAA’s safety objectives are currently being met. 

Before issuing the CPCP proposal, the FAA issued ADs to address corrosion concerns on 

various airplane models. Also, during the rulemaking process, airplane manufacturers 

came to better understand the effects of corrosion and voluntarily developed CPCPs for 

their new airplane models using the Manufacturing Steering Group (MSG)-3 process. If 

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3/7/2008 AC 91-56B

an unsafe condition is identified on any airplane model not covered by a CPCP because 

of the withdrawal of the NPRM, the FAA will issue an AD to address that safety issue. 

10. REPAIR ASSESSMENT PROGRAM. The FAA tasked the aviation industry to 

develop a method for airlines to evaluate airplane repairs to determine whether they are 

acceptable permanent repairs incorporating damage tolerance. This program ensures that 

existing repairs do not deteriorate due to accidental, fatigue, or environmental damage 

beyond FAA-approved levels for the remaining usage life of the airplane. 

 a. On January 2, 1998, an NPRM, Repair Assessment for Pressurized Fuselages

(63 FR 126; Notice No. 97-16), was published in the Federal Register. The proposed 

rule would restrict the operation of certain large transport category airplanes (Airbus 

Model A300; British Aerospace Model BAC 1-11; Boeing Models B-707/720, B-727, 

B-737, B-747; McDonnell Douglas Model DC-8, DC-9/MD-80, DC-10; Fokker Model 

F28; and Lockheed Model L-1011 series airplanes) operated under parts 91, 121, 125, 

and 129 beyond a specified compliance time, unless the operator of those airplanes had 

incorporated FAA-approved repair assessment guidelines applicable to the fuselage 

pressure boundary (fuselage skin, door skin, and bulkhead webs) in its operation 

specification(s) or approved inspection program, as applicable. This rulemaking ensures 

that a comprehensive damage tolerance repair assessment be completed for fuselage 

pressure boundary repairs. 

 b. The final rule was published in the Federal Register on April 25, 2000 

(65 FR 24108; Amendment Nos. 91-264, 121-275, 125-33, and 129-28) and became

effective May 25, 2000. As a result of this final rule, the new operating rules are part 91, 

§ 91.410; part 121, § 121.370; part 125, § 125.248; and part 129, § 129.32. AC 120-73, 

Damage Tolerance Assessment of Repairs to Pressurized Fuselages, provides an 

acceptable means of compliance with the regulations that require incorporating 

FAA-approved repair assessment guidelines into an operator’s FAA-approved 

maintenance or inspection program. 

11. Widespread Fatigue Damage. The FAA plans to address widespread fatigue 

damage through the rulemaking process. 

Signed by Ali Bahrami 

Ali Bahrami 

Manager, Transport Airplane Directorate 

Aircraft Certification Service 

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3/7/2008 AC 91-56B 

 Appendix 1 

APPENDIX 1 

GUIDELINES FOR DEVELOPING 

A SUPPLEMENTAL STRUCTURAL INSPECTION DOCUMENT 

1. GENERAL. 

 a. The airplanes subject to this appendix were not certified to a damage tolerance 

requirement. However, the structure to be evaluated, the type of damage considered 

(fatigue, corrosion, service, and production damage), and the inspection and/or 

modification criteria should, to the extent practicable, be in accordance with the damage 

tolerance principles of Title 14 of the Code of Federal Regulations (14 CFR) part 25, 

§ 25.571. An acceptable means of compliance can be found in the current version of 

AC 25.571-1, Damage Tolerance and Fatigue Evaluation of Structure. 

 b. It is essential to identify the structural parts and components that contribute 

significantly to carrying flight, ground, pressure, or control loads, and whose failure 

could affect the structural integrity necessary for the continued safe operation of the 

airplane. The damage tolerance or safe-life characteristics of these parts and components 

must be established or confirmed. 

 c. Analyses made with respect to the continuing assessment of structural integrity 

should be based on supporting evidence, including test and service data. This supporting 

evidence should include consideration of the operating loading spectra, structural loading 

distributions, and material behavior. An appropriate allowance should be made for the 

scatter in life to crack initiation and rate of crack propagation in establishing the 

inspection threshold, inspection frequency, and, where appropriate, retirement life. 

Alternatively, an inspection threshold may be based solely on a statistical assessment of 

fleet experience, provided that it can be shown that equal confidence can be placed in 

such an approach. 

 d. An effective method of evaluating the structural condition of older airplanes is 

selective inspection with intensive use of nondestructive techniques and the inspection of 

individual airplanes, involving partial or complete dismantling (“tear-down”) of available 

structure. 

 e. The effect of major repairs, alterations, and modifications approved by the design 

approval holder should be considered. In addition, it will be necessary to consider the 

effect of all major repairs and operator-approved alterations and modifications on 

individual airplanes. The operator has the responsibility for consideration of any such 

aspects. For airplanes affected by the Aging Airplane Safety Final Rule, AC 120-93, 

Damage Tolerance Inspections for Repairs and Alterations, provides guidance to type 

certificate holders and operators for addressing repairs, alterations, and modifications. 

 1 
AC 91-56B 3/7/2008 

Appendix 1 

2. DAMAGE-TOLERANT STRUCTURES.

 a. The damage tolerance assessment of the airplane structure should be based on the 

best information available. The assessment should include a review of analysis, test data, 

operational experience, and any special inspections related to the type design. A 

determination should then be made of the site or sites within each structural part or

component considered likely to crack, and the time or number of flights at which this 

might occur. 

 b. The growth characteristics of damage and interactive effects on adjacent parts in 

promoting more rapid or extensive damage should be determined. This study should 

include those sites that may be subject to the possibility of crack initiation due to fatigue, 

corrosion, stress corrosion, disbonding, accidental damage, or manufacturing defects in

those areas shown to be vulnerable by service experience or design judgment. 

 c. The minimum size of damage that it is practical to detect and the proposed 

method of inspection should be determined. This determination should take into account 

the number of flights required for the crack to grow from detectable to the allowable 

limit, such that the structure has a residual strength corresponding to the conditions stated 

under § 25.571. 

NOTE: In determining the proposed method of inspection, 

consideration should be given to visual inspection, nondestructive 

testing, and analysis of data from built-in load and defect monitoring 

devices. 

 d. The continuing assessment of structural integrity may involve more extensive 

damage than might have been considered in the original fail-safe evaluation of the 

airplane, such as: 

(1) A number of small adjacent cracks, each of which may be less than the 

typically detectable length, developing suddenly into a long crack; 

(2) Failures or partial failures in other locations following an initial failure due to 

redistribution of loading causing a more rapid spread of fatigue; and 

(3) Concurrent failure or partial failure of multiple load path elements (e.g., lugs, 

planks, or crack arrest features) working at similar stress levels. 

3. INFORMATION TO BE INCLUDED IN THE ASSESSMENT. 

 a. The continuing assessment of structural integrity for the particular airplane type

should be based on the principles outlined in paragraph 2 of this appendix. The following 

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3/7/2008 AC 91-56B

 Appendix 1 

information should be included in the assessment. This information should be kept by 

the manufacturer in a form available for reference: 

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

(2) The typical operational mission, or missions assumed in the assessment; 

(3) The structural loading conditions from the chosen missions; and 

(4) The supporting test evidence and relevant service experience. 

 b. In addition to the information specified in paragraph 3a of this appendix, the 

following should be included for each critical part or component: 

(1) The basis employed for evaluating the damage tolerance characteristics of the 

part or component; 

(2) The site or sites within the part or component where damage could affect the 

structural integrity of the airplane; 

(3) The recommended inspection methods for the area; 

(4) For damage tolerant structures, the maximum damage size at which the 

residual strength capability can be demonstrated and the critical design loading case for 

the latter; and 

(5) For damage tolerant structures, at each damage site the inspection threshold 

and the damage growth interval between detectable and critical, including any likely 

interaction effects from other damage sites. 

NOTE: If an inspection procedure is not reliable or practicable, 

then replacement or modification of the structure may need to be 

defined. 

4. INSPECTION PROGRAM. The purpose of a continuing airworthiness assessment 

is to adjust the current maintenance inspection program, as required, to assure continued 

operational safety of the airplane. 

 a. In accordance with paragraphs 1 and 2 of this appendix, an allowable limit of the 

size of damage should be determined for each site such that the structure has a residual 

strength for the load conditions specified in § 25.571, as defined in paragraph 2c of this 

appendix. The size of damage that it is practical to detect by the proposed method of 

inspection should be determined, along with the number of flights required for the crack 

to grow from detectable to the allowable limit. 

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AC 91-56B 3/7/2008 

Appendix 1 

 b. The recommended inspection program should be determined from the data 

described in paragraph 4a, above, giving due consideration to the following: 

(1) Fleet experience, including all of the scheduled maintenance checks; 

(2) Confidence in the proposed inspection technique; and 

(3) The joint probability of reaching the load levels described above and the final 

size of damage in those instances where probabilistic methods can be used with 

acceptable confidence. 

 c. Inspection thresholds for supplemental inspections should be established. These 

inspections would be supplemental to the normal inspections, including the detailed 

internal inspections. 

(1) For structure with reported cracking, the threshold for inspection should be 

determined by analysis of the service data and available test data for each individual case. 

(2) For structure with no reported cracking, it may be acceptable, provided 

sufficient fleet experience is available, to determine the inspection threshold on the basis 

of analysis of existing fleet data alone. This threshold should be set such as to include the 

inspection of a sufficient number of high-time airplanes to develop added confidence in 

the integrity of the structure (see paragraph 1c of this appendix). Thereafter, if no cracks 

are found, the inspection threshold may be increased progressively by successive 

inspection intervals until cracks are found. In the latter event, the criteria of 

paragraph 4c(1) applies. 

5. THE SUPPLEMENTAL STRUCTURAL INSPECTION DOCUMENT. 

 a. The supplemental structural inspection document (SSID) should contain the 

recommendations for the inspection procedures and replacement or modification of parts 

or components necessary for the continued safe operation of the airplane. The document 

should be prefaced by the following information: 

(1) Identification of the variants of the basic airplane type to which the document 

relates;

(2) A summary of the operational statistics of the fleet in terms of hours and 

flights, as well as a description of the typical mission, or missions; 

(3) Reference to documents giving any existing inspections or modifications of 

parts or components; 

(4) The types of operations for which the inspection program is considered valid; 

and 

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3/7/2008 AC 91-56B

 Appendix 1 

(5) A list of service bulletins (or other service information publication) revised as 

a result of the structural reassessment undertaken to develop the SSID, including a 

statement that the operator must account for these service bulletins. 

 b. The document should contain at least the following information for each critical 

part or component: 

(1) A description of the part or component and any relevant adjacent structure, 

including means of access to the part; 

(2) The type of damage which is being considered (i.e., fatigue, corrosion, 

accidental damage); 

(3) Relevant service experience; 

(4) Likely site(s) of damage; 

(5) Recommended inspection method and procedure and alternatives; 

(6) Minimum-size of damage considered detectable by the method(s) of 

inspection; 

(7) Service bulletins (or other service information publication) revised or issued 

as a result of in-service findings resulting from implementation of the SSID (added as 

revision to the initial SSID); 

(8) Guidance to the operator on which inspection findings should be reported to 

the manufacturer; 

(9) Recommended initial inspection threshold; 

(10) Recommended repeat inspection interval; 

(11) Reference to any optional modification or replacement of part or component 

as terminating action to inspection; and 

(12) Information related to any variations found necessary to “safe lives” already 

declared. 

 c. The design approval holder should check the SSID from time to time against 

current service experience. Any unexpected defect occurring should be assessed as part 

of the continuing assessment of structural integrity to determine the need for revision of 

the document. Future structural service bulletins should state their effect on the SSID. 

5