Damage Tolerance Data for Repairs and Alterations, 70486-70508 [07-6016]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 72, Number 238 (Wednesday, December 12, 2007)]
[Rules and Regulations]
[Pages 70486-70508]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 07-6016]


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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 26, 121, and 129

[Docket No. FAA-2005-21693; Amendment Nos. 26-1, 121-337, 129-44]
RIN 2120-AI32


Damage Tolerance Data for Repairs and Alterations

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: This final rule requires holders of design approvals to make 
available to operators damage tolerance data for repairs and 
alterations to fatigue critical airplane structure. This rule will 
support operator compliance with the Aging Airplane Safety final rule 
with respect to the requirement to incorporate into the maintenance 
program, a means for addressing the adverse effects repairs and 
alterations may have on fatigue critical structure. The intent of this 
final rule is to ensure the continued airworthiness of fatigue critical 
airplane structure by requiring design approval holders to support 
operator compliance with specified damage tolerance requirements.

DATES: These amendments become effective January 11, 2008.

FOR FURTHER INFORMATION CONTACT: If you have technical questions about 
this action, contact Greg Schneider, ANM-115, Airframe and Cabin 
Safety, Federal Aviation Administration, 1601 Lind Avenue, SW., Renton, 
Washington 98057-3356, telephone: (425-227-2116); facsimile (425-227-
1232); e-mail greg.schneider@faa.gov. Direct any legal questions to 
Doug Anderson, ANM-7, Office of Regional Counsel, Federal Aviation 
Administration, 1601 Lind Avenue, SW., Renton, WA 98057-3356; telephone 
(425) 227-2166; facsimile (425) 227-1007; e-mail 
Douglas.Anderson@faa.gov.

SUPPLEMENTARY INFORMATION:

Authority for This Rulemaking

    The FAA's authority to issue rules regarding aviation safety is 
found in Title 49 of the United States Code. Subtitle I, Section 106 
describes the

[[Page 70487]]

authority of the FAA Administrator. Subtitle VII, Aviation Programs, 
describes in more detail the scope of the agency's authority.
    This rulemaking is promulgated under the authority described in 
Subtitle VII, Part A, Subpart III, Section 44701, ``General 
requirements.'' Under that section, the FAA is charged with promoting 
safe flight of civil aircraft in air commerce by prescribing minimum 
standards required in the interest of safety for the design and 
performance of aircraft; regulations and minimum standards in the 
interest of safety for inspecting, servicing, and overhauling aircraft; 
and regulations for other practices, methods, and procedures the 
Administrator finds necessary for safety in air commerce. This 
regulation is within the scope of that authority because it 
prescribes--
     New safety standards for the design of transport category 
airplanes, and
     New requirements necessary for safety for the design, 
production, operation, and maintenance of those airplanes, and for 
other practices, methods, and procedures relating to those airplanes.

Table of Contents

I. Executive Summary
II. Background
    A. Summary of the NPRM
    1. The Proposed Rule
    2. Related Activities
    B. Differences Between the NPRM and the Final Rule
    1. New Part 26 for Design Approval Holders' Airworthiness 
Requirements
    2. New Subparts for Airworthiness Operational Rules
    3. Minor Conforming Changes to the Aging Airplane Safety Final 
Rule
    4. Other Miscellaneous Changes
    C. Summary of Comments
III. Discussion of the Final Rule
    A. Overview
    1. Final Rule
    2. Guidance Material
    B. Airplane Applicability and Exceptions
    1. Airplane Certification Amendment Level
    2. Parts 91, 125, and 135 Operations
    3. Exception of Airplanes Not Operating in the U.S. Under Part 
121 or 129
    C. Fatigue Critical Structure (FCS)
    D. Damage Tolerance Evaluation (DTE)
    E. Damage Tolerance Inspections (DTIs)
    F. DT Data for Repairs
    1. Published Repair Data
    2. Effects of Multiple Repairs
    G. Repair Evaluation Guidelines (REGs)
    H. DT Data for Alterations
    I. Required Documentation
    J. Proprietary Data
    K. Compliance Plan
    1. Process for Continuous Assessment of Service Information
    2. Timing of FAA Approval
    L. Harmonization
    1. Foreign Authority Approval of Required Data
    M. Enforcement
    N. Industry and FAA Resources
    O. Compliance Dates
    P. Costs and Benefits
IV. Final Regulatory Evaluation, Regulatory Flexibility 
Determination, International Trade Impact Assessment, and Unfunded 
Mandates Assessment
V. The Amendments

I. Executive Summary

    Fatigue cracking has been a major aviation safety concern for many 
years. Unless detected and repaired, fatigue cracks can grow to the 
point of catastrophic failure. Since 1978 the FAA has required new 
types of airplanes to meet damage tolerance \1\ (DT) requirements to 
ensure their continued airworthiness. Industry has also used this 
method successfully to develop inspection programs for older airplanes. 
Since the 1980s, the FAA has mandated that operators of most large 
transport airplanes carry out these programs.
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    \1\ Damage tolerance (DT) is a method used to evaluate the crack 
growth and residual strength characteristics of structure. Based on 
the results, inspections or other procedures are established as 
necessary to prevent catastrophic failures due to fatigue. Most 
commonly, the maintenance actions developed are directed inspections 
for fatigue cracking.
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    While these programs have been largely effective, industry has not 
carried out DT methods comprehensively. In particular, while these 
programs apply to the airplane ``baseline'' structure (the airplane 
structure as originally manufactured), they often do not apply to 
repairs and alterations.\2\ This omission is important because 
airplanes are subject to many repairs and alterations throughout their 
operational lives. If fatigue cracking occurs in a repaired or altered 
area, the results can be just as catastrophic as if it had occurred in 
the baseline structure.
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    \2\ Various segments of industry use the term ``modification'' 
to define a design change. We consider this term to be synonymous 
with the term ``alteration.'' We use both terms in this rule to mean 
a design change that is made to an airplane.
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    The FAA adopted the Aging Airplane Safety final rule (AASFR) \3\ in 
early 2005. Among other things, the AASFR requires airline operators of 
certain large transport category airplanes \4\ to implement DT-based 
inspection programs for airplane structure; that is, structure 
susceptible to fatigue cracking that could contribute to a catastrophic 
failure. In this final rule, we refer to this structure as ``fatigue 
critical structure.'' Most importantly for this rule, the AASFR 
requires these inspection programs to ``take into account the adverse 
effects repairs, alterations, and modifications may have on fatigue 
cracking and the inspection of this airplane structure.''
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    \3\ 70 FR 5518; February 2, 2005.
    \4\ The rule applies to turbine powered airplane models with a 
maximum type certificated passenger seating capacity of 30 or more, 
or a maximum payload capacity of 7,500 pounds or more.
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    With the AASFR, we now have in place the regulatory means to 
provide for comprehensive implementation of DT methods on all large 
transport airplanes used by air carriers. To carry out these 
requirements fully, however, it is necessary to place corresponding 
requirements on the holders of FAA design approvals for these 
airplanes. Otherwise, the operators may not be able to obtain the data 
and documents they need to comply with the AASFR. As the owners of the 
data for these airplanes, the design approval holders \5\ (DAHs) are in 
the best position to identify the fatigue critical structure and the 
methods and frequency of inspections that may be needed. Therefore, 
this final rule requires DAHs to develop and make available to 
operators the data and documents they need to support compliance with 
the DT requirements of the AASFR.
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    \5\ For purposes of this rule, design approval holders (DAHs) 
are holders of type certificates (TCs) or supplemental type 
certificates (STCs) issued under 14 CFR part 21.
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    Specifically, this final rule requires DAHs to develop and make 
available the following four types of documents to operators:
    (1) Lists of fatigue critical structure (to aid operators in 
identifying repairs and alterations that need to be addressed for DT).
    (2) Damage tolerance inspections to provide operators with the 
necessary inspection times and methods for the following:
     Repair data published by type certificate (TC) holders.\6\
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    \6\ Published repair data are instructions for accomplishing 
repairs, which are published for general use in structural repair 
manuals (SRMs) and service bulletins. These data are approved for 
general application to a particular airplane model or airplane 
configuration.
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     TC holder's future repair data not published for general 
use.\7\
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    \7\ This includes repairs that are developed for individual 
airplanes at the request of an operator. These repairs are often 
complex or unique to a particular airplane or group of airplanes 
experiencing similar damage conditions.
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     Repair data developed by supplemental type certificate 
(STC) holders.
     Alteration data developed by TC and STC holders.
    (3) Damage tolerance evaluation guidelines for all other repairs 
(to enable operators to obtain the necessary damage tolerance 
inspections).
    (4) Implementation schedules (to define the necessary timing for 
performing damage tolerance

[[Page 70488]]

evaluations and developing damage tolerance inspections and for 
incorporating the DT data into the operator's maintenance program).
    This final rule transfers the responsibility for developing DT-
based data from operators to DAHs and, therefore, has minimal to no 
societal costs. The aviation industry as a whole would also benefit 
because DAHs could amortize their development costs for DT data over a 
larger fleet.

II. Background

A. Summary of the NPRM

1. The Proposed Rule
    On April 21, 2006, the FAA published in the Federal Register the 
Notice of proposed rulemaking (NPRM) entitled, Damage Tolerance Data 
for Repairs and Alterations (DAH DT Data NPRM),\8\ which is the basis 
of this final rule.
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    \8\ 71 FR 20574.
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    In the DAH DT Data NPRM, the FAA proposed to require DAHs to 
develop and make available to operators certain damage tolerance (DT) 
data that address the adverse effects repairs, alterations, and 
modifications may have on fatigue critical structure. These data are 
necessary to support operator compliance with the Aging Airplane Safety 
Final Rule (AASFR).\9\ Specifically, we proposed to require DAHs to 
develop and make available to operators the following: (1) Lists of 
fatigue critical structure for baseline and alteration structure; (2) 
Damage tolerance inspections (DTIs) for existing published repair and 
alteration data; (3) DTIs for future repair and alteration data; (4) 
Repair evaluation guidelines (REGs) that include a process for 
conducting airplane surveys, a process for establishing DT Data, and 
implementation schedules for the above actions. In addition, we 
proposed to require DAHs to develop a compliance plan for meeting these 
four requirements and to obtain FAA approval of the plan.
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    \9\ AASFR: 70 FR 5518; February 2, 2005. See also 70 FR 23935; 
May 6, 2005: Aging Airplane Safety; Correcting Amendment.
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    The NPRM contains the background and rationale for this rulemaking 
and, except where we have made revisions in this final rule, you should 
refer to it for that information.
2. Related Activities
    In July 2004, we published the Fuel Tank Safety Compliance 
Extension (Final Rule) and Aging Airplane Program Update (Request for 
Comments),\10\ where we informed the public of our intent to propose 
DAH airworthiness requirements to support certain operational rules. We 
requested comments on our proposal.
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    \10\ 69 FR 45936; July 30, 2004.
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    In December 2002, we published the Aging Airplane Safety Interim 
final rule; request for comments.\11\ In February 2005, we adopted the 
AASFR in which we responded to the comments from the interim rule and 
made some changes to that rule. The February 2005 AASFR requires 
affected operators to include certain damage tolerance inspections and 
procedures in their maintenance programs by December 20, 2010. Today's 
final rule is directly related to the AASFR in that it provides a means 
for operators to get the data and documents they need to comply with 
the AASFR.
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    \11\ 67 FR 72726; December 6, 2002.
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    In July 2005, we published a disposition of comments document,\12\ 
in which we responded to comments to the July 2004 action. Also in July 
2005, we published a policy statement, Safety--A Shared 
Responsibility--New Direction for Addressing Airworthiness Issues for 
Transport Airplanes,\13\ that explains our criteria for adopting DAH 
requirements like those described in this final rule.
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    \12\ 70 FR 40168; July 12, 2005: Fuel Tank Safety Compliance 
Extension (Final Rule) and Aging Airplane Program Update (Request 
for Comments).
    \13\ 70 FR 40166, July 12, 2005 (PS-ANM110-7-12-2005).
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    On April 21, 2006,\14\ along with the NPRM for this rulemaking, we 
published a Notice of Availability (NOA) and request for comments on 
draft AC 120-XX \15\ (Damage Tolerance Inspections for Repairs). This 
AC included guidance related to repairs, which the Aviation Rulemaking 
Advisory Committee's (ARAC) Airworthiness Assurance Working Group 
(AAWG) \16\ developed.
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    \14\ 71 FR 20750.
    \15\ Issued as AC 120-93.
    \16\ AAWG Member Organizations: Boeing Commercial Airplanes, 
Federal Express (FedEx), Airbus, Air Transport Association (ATA), 
American Airlines, British Airways, Continental Airlines, Japan 
Airlines, Northwest Airlines, United Airlines, United Parcel Service 
(UPS), Airborne Express, U.S. Airways, Federal Aviation 
Administration (FAA), and European Aviation Safety Agency (EASA).
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    On July 7, 2006, we published a notice \17\ that granted industry a 
90-day extension to comment on the NPRM; and on February 27, 2007, we 
published a NOA \18\ and request for comments on revised AC 120-XX,\19\ 
which includes guidance from the AAWG on both repairs and alterations.
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    \17\ 71 FR 38541.
    \18\ 72 FR 8834.
    \19\ Issued as AC 120-93.
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B. Differences Between the NPRM and the Final Rule

1. New Part 26 for Design Approval Holders' Airworthiness Requirements
    In the NPRM (and other Aging Airplane Program rules), we placed the 
DAH airworthiness requirements in part 25, subpart I. As we explained 
in the recently adopted Enhanced Airworthiness Program for Airplane 
Systems/Fuel Tank Safety final rule (EAPAS/FTS),\20\ we have placed 
these requirements in new part 26, and we have moved the enabling 
regulations into part 21.\21\ We determined that this was the best 
course of action because it keeps part 25 as strictly airworthiness 
standards for transport category airplanes, thus maintaining 
harmonization and compatibility among the United States, Canada, and 
the European Union regulatory systems. Providing references to part 26 
in part 21 clarifies how the part 26 requirements will address existing 
and future design approvals.
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    \20\ 72 FR 63364; November 8, 2007.
    \21\ Certification Procedures for Products and Parts.
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    In creating new part 26, we renumbered the proposed sections of 
part 25, subpart I and we incorporated the changes discussed in this 
preamble. A table of this renumbering is shown below.
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    \22\ This section, which includes an applicability table for 
part 26, was adopted as part of the EAPAS final rule.
    \23\ These definitions were proposed in Sec.  25.1823(b).

  Table 1.--Relationship of Proposed Part 25 Subpart I to Part 26 Final
                                  Rules
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            Part 26 final rules                   Proposed part 25
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Subpart E--Aging Airplane Safety--Damage    Subpart I--Continued
 Tolerance Data for Repairs and              Airworthiness.
 Alterations.
Sec.   26.5 Applicability table...........  New.\22\
Sec.   26.41 Definitions..................  New.\23\
Sec.   26.43 Holders of and applicants for  Sec.   25.1823 Holders of
 type certificates--Repairs.                 type certificates--Repairs.
Sec.   26.45 Holders of type certificates-- Sec.   25.1825 Holders of
 Alterations and repairs to alterations.     type certificates--
                                             Alterations and repairs to
                                             alterations.
Sec.   26.47 Holders of and applicants for  Sec.   25.1827 Holders of
 a supplemental type certificate--           and applicants for a
 Alterations and repairs to alterations.     supplemental type
                                             certificate--Alterations
                                             and repairs to alterations.

[[Page 70489]]

 
Sec.   26.49 Compliance Plan..............  Sec.   25.1829 Compliance
                                             Plan.
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2. New Subparts for Airworthiness Operational Rules
    We discussed in the preamble to the proposed rule that we would 
establish new subparts for airworthiness-related operational rules. 
Since there were several other aging airplane proposals (e.g., EAPAS) 
published around the same time, each proposal contained language that 
established the new subparts and redesignated certain sections of those 
rules. We said when any one of those proposals became a final rule, we 
would remove the duplicative provisions that established the new 
subparts and redesignated sections from the other aging airplane rules. 
In the DAH DT Data proposal, we included regulatory text to add 
subparts AA and B (Continued Airworthiness and Safety Improvements) to 
include the airworthiness requirements from parts 121 and 129, 
respectively. We also included regulatory language to redesignate the 
section numbers in parts 121 and 129 that were moved to the new 
subparts. However, since the EAPAS final rule was the first to be 
codified, that final rule adopted subparts AA and B and redesignated 
appropriate sections of parts 121 and 129. Therefore, we have removed 
the duplicative regulatory text from this final rule.
    To aid understanding of our discussion about the DAH DT Data rule 
as it relates to the AASFR, we have indicated below the prior and 
redesignated sections of parts 121 and 129 of the AASFR that include 
DT-related requirements.

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              Prior sections                    Redesignated sections
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Sec.   121.370a...........................  Sec.   121.1109
Sec.   129.16.............................  Sec.   129.109
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3. Minor Conforming Changes to the Aging Airplane Safety Final Rule
    During the rulemaking process for the DAH DT Data rule, the FAA 
determined that minor changes to the AASFR were needed to ensure 
clarity of the two rules. The original wording in Sec. Sec.  121.370a 
and 129.16 (redesignated as Sec. Sec.  121.1109 and 129.109, 
respectively) required that changes to the certificate holder's 
maintenance program (i.e., inclusion of DT-based inspections and 
procedures and any revisions to them) be approved by the Aircraft 
Certification Office (ACO) \24\ or office of the Transport Airplane 
Directorate with oversight responsibility for the relevant type 
certificate or supplemental type certificate, as determined by the 
Administrator.
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    \24\ The regulatory text in this rule refers to the ACO or 
office of the Transport Airplane Directorate with oversight 
responsibility for the relevant type certificate or supplemental 
type certificate as the FAA Oversight Office.
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    Although the ACO will approve the documentation that the DAH DT 
Data final rule requires DAHs to submit to the FAA, the DT inspections 
and procedures resulting from this documentation, which certificate 
holders must incorporate into their maintenance programs, should be 
approved by their Principal Maintenance Inspector (PMI). Therefore, we 
revised Sec. Sec.  121.1109 and 129.109 to state that it is the PMI's 
responsibility to review and approve changes to a certificate holder's 
maintenance program.
    Also, we believe the requirements in current Sec. Sec.  
121.1109(c)(1) and 129.109(b)(1) that address DT relative to baseline 
structure and repairs, alterations, and modifications would be clearer 
if they were in separate paragraphs. Therefore, we revised Sec. Sec.  
121.1109 and 129.109 to include requirements related to baseline 
structure in Sec.  121.1109(c)(1) and Sec.  129.109(b)(1) and those 
related to repairs, alterations, and modifications in Sec.  
121.1109(c)(2) and Sec.  129.109(b)(2). We also made minor wording 
changes for clarity and consistency with the new part 26 requirements 
and Advisory Circular (AC) 120-XX,\25\ which describes an acceptable 
means of compliance with the DAH DT Data final rule.
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    \25\ Issued as AC 120-93.
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4. Other Miscellaneous Changes
    Based on comments to the proposed rule, we have revised the final 
rule as summarized below and discussed in more detail under the 
Discussion of the Final Rule heading.
    We extended the compliance times for DAHs to develop the required 
lists of fatigue critical structure. For TC holders, we extended the 
compliance date for them to submit their lists of fatigue critical 
baseline structure to the FAA Oversight Office for review and approval 
from 90 to 180 days after the effective date of the final rule. We also 
added a provision that makes it clear to future TC holders that the 
lists of fatigue critical baseline structure must be submitted as part 
of the type certification process.
    In the NPRM, we proposed TC holders submit their lists of fatigue 
critical alteration structure to the FAA Oversight Office for review 
and approval 90 days after the effective date of the final rule. We 
proposed 270 days for STC holders. In the final rule, we extended the 
compliance date to 360 days after the effective date of the final rule 
for both TC holders and STC holders to submit these lists.
    The NPRM included a requirement for TC and STC holders to develop a 
process to enable operators to ``establish'' damage tolerance 
inspections (DTIs) for repairs and alterations to fatigue critical 
baseline structure (FCBS). This final rule replaces the term 
``establish'' with ``obtain.'' We made this change because the term 
``obtain'' better reflects the intent of the rule and is meant to be 
all inclusive. That is, the operator may ``obtain'' a DTI by 
establishing it themselves, or by receiving the DTI directly from a TC 
holder, STC holder, or a third party.
    Section 25.1823(f)(1)(iii) (adopted as Sec.  26.43(e)(1)(iii)) 
proposed an implementation schedule for repairs covered by the repair 
evaluation guidelines (REGs). To clarify this proposed requirement, we 
revised it in the final rule to specify that the implementation 
schedule must identify the times when actions must be taken as specific 
numbers of flight cycles, flight hours, or both.
    We revised proposed Sec.  25.1823(f)(3) (adopted as Sec.  
26.43(e)(3)) to remove the reference to Sec.  25.1827. That reference 
would have required TC holders to make their REGs available to STC 
holders. We made this change because TC holders do not need to provide 
REGs to STC holders. However, they must provide their lists of fatigue 
critical structure (FCS) to STC holders.
    As discussed in more detail later in this preamble, based on 
comments submitted to other DAH airworthiness rules, we removed some 
provisions of the compliance plan in proposed Sec.  25.1829 (adopted as 
Sec.  26.49). Specifically, we removed the proposed requirements in 
Sec.  25.1829(a)(3) for DAHs to identify the intended means of 
compliance that differ from those described in FAA advisory materials. 
Similarly, we removed the requirement in proposed Sec.  25.1829(c) that 
would have authorized the FAA Oversight Office to identify deficiencies 
in a compliance plan or the DAH's implementation of the plan and to 
require specified corrective actions to remedy those deficiencies. We 
do not

[[Page 70490]]

believe removal of these requirements will adversely affect our ability 
to facilitate DAH compliance.
    In Sec.  25.1829(5), we proposed a requirement for including in the 
compliance plan a process for continuous assessment of service 
information related to structural fatigue damage. As discussed later in 
this preamble, we have determined that existing regulations should 
enable us to determine whether the objectives of this DAH DT Data final 
rule are being met. Therefore, we have removed this provision from this 
final rule.
    In addition to the changes discussed above, we made minor changes 
to clarify the definitions of damage tolerance inspections and 
published repair data in proposed Sec.  25.1823 (the definitions are 
now in Sec.  26.41). We also made other minor changes to clarify the 
requirements in proposed Sec. Sec.  25.1823 (adopted as Sec.  26.43), 
25.1825 (adopted as Sec.  26.45), 25.1827 (adopted as Sec.  26.47), and 
25.1829 (adopted as Sec.  26.49).

C. Summary of Comments

    The FAA received multiple comments from 17 commenters, including 
the Air Transport Association (ATA) and a collective group of certain 
industry representatives who are members of the AAWG.\26\ In the 
following discussion of the comments received to the proposed rule, we 
will refer to the comments received from those industry representatives 
of the AAWG as the ``AAWG industry representatives.'' Also, several of 
the AAWG and the ATA member organizations sent separate comments on 
behalf of their organizations, with some specifically expressing 
support for the comments submitted by the AAWG industry representatives 
and the ATA. The comments to the proposed rule covered an array of 
topics and contained a range of responses, which we discuss more fully 
below under the Discussion of the Final Rule heading. In general, 
commenters supported the intent of the rule and the guidance material. 
They also requested some changes and clarifications.
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    \26\ AAWG industry representatives (a collective group of 
commenters who are members of the AAWG): Boeing Commercial 
Airplanes, Federal Express (FedEx), Airbus, American Airlines, 
British Airways, Continental Airlines, Japan Airlines, Northwest 
Airlines, United Airlines, United Parcel Service (UPS), Airborne 
Express, US Airways.
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    Many of the comments to the proposed rule concerned issues specific 
to the Widespread Fatigue Damage (WFD) \27\ proposal. The FAA intends 
to address the WFD-related comments in a separate action, so we will 
not address them here.
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    \27\ 71 FR 19928; April 18, 2006.
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    We also received several comments about the DAH airworthiness 
requirements. We addressed many of the same or similar comments and 
issues in the July 2005 disposition of comments document to the Fuel 
Tank Safety Compliance Extension (Final Rule) and Aging Airplane 
Program Update (Request for Comments). In addition, we explained in 
detail the need for these requirements in our July 2005 policy 
statement. As a result, we will not revisit those comments and issues 
here.

III. Discussion of the Final Rule

A. Overview

1. Final Rule
    Fatigue cracking has been a major aviation safety concern for many 
years. Unless detected and repaired, fatigue cracks can grow to the 
point of catastrophic failure. Since the adoption of Amendment 25-45 
\28\ in 1978, the FAA has required new types of airplanes to meet 
damage tolerance (DT) requirements to ensure their continued 
airworthiness. Industry has also used this method successfully to 
develop inspection programs for older airplanes, such as Supplemental 
Structural Inspection Programs (SSIP). Since the 1980s, the FAA has 
mandated that operators of most large transport airplanes carry out 
these programs.
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    \28\ 43 FR 46242; October 5, 1978.
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    Although these programs have been effective for baseline structure 
(the airplane structure as originally manufactured), industry has not 
comprehensively implemented DT methods for repairs and alterations. For 
airplanes certified to Amendment 25-45 and later, repairs and 
alterations were not always evaluated for damage tolerance. This 
omission is important because airplanes are subject to many repairs and 
alterations throughout their operational lives. If fatigue cracking 
occurs in a repaired or altered area, the results can be just as 
catastrophic as if it had occurred in the baseline structure.
    The AASFR requires airline operators of certain large transport 
category airplanes \29\ to implement DT-based inspection programs for 
airplane structure; that is, structure susceptible to fatigue cracking 
that could contribute to a catastrophic failure. In today's DAH DT Data 
final rule, we refer to this structure as ``fatigue critical 
structure.'' Most importantly for today's DAH DT Data final rule, the 
AASFR requires the maintenance program for the airplane include a means 
to address the adverse effects repairs and alterations may have on 
airplane structure.
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    \29\ The rule applies to turbine powered airplane models with a 
maximum type certificated passenger seating capacity of 30 more, or 
a maximum payload capacity of 7,500 pounds or more.
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    With the AASFR, we now have in place the regulatory means to 
provide for comprehensive implementation of DT methods on all large 
transport category airplanes used by air carriers operating under 14 
CFR parts 121 and 129. To carry out these requirements fully, however, 
we must place corresponding requirements on the holders of FAA design 
approvals for these airplanes. Otherwise, the operators may not be able 
to obtain the data and documents they need to comply with the AASFR. As 
the owner of the design data for these airplanes, the DAH is in the 
best position to identify the fatigue critical structure and the 
methods and frequency of inspections that may be needed.
    As indicated in our July 2005 policy statement about the shared 
responsibility for addressing airworthiness issues, in cases where 
operators must rely on data or documents from DAHs to comply with 
operational rules, we will require DAHs to develop that information by 
a specified date. This final rule includes such requirements.
    Specifically, 14 CFR 26.43, 26.45, and 26.47 require that the TC 
holders and STC holders develop certain information that will provide a 
means for operators to address the adverse effects of repairs and 
alterations. The information required by this final rule includes the 
following:
     List of Fatigue Critical Structure (baseline and 
alteration).
     Damage tolerance inspections (DTIs) for existing published 
repair data and all future repair data.
     DTIs for all existing and future alteration data.
     Repair evaluation guidelines (REGs), which include--

--Instructions for conducting airplane surveys;
--Instructions an operator uses to obtain DTIs; and
--An implementation schedule that provides timing for the above 
actions.
2. Guidance Material
    The FAA has issued Advisory Circular (AC) 120-93, Damage Tolerance 
of Repairs and Alterations, concurrently with this rule. The AC 
provides TC and STC holders with an acceptable method of compliance 
with this final rule. The AC, which was developed through a 
collaborative effort between the FAA and the Aviation Rulemaking 
Advisory Committee

[[Page 70491]]

(ARAC), supports operator compliance with the AASFR with respect to 
repairs and alterations.
    As amended by this final rule, Sec.  121.1109(c)(2) of the AASFR 
requires operators to incorporate into their maintenance program a 
``means'' for addressing the adverse effects that repairs and 
alterations may have on fatigue critical structure. This AC provides 
guidance that TC holders, STC holders, and operators can use in 
developing a means for addressing repairs and alterations.
    To facilitate operators' timely compliance with the AASFR for 
repairs, the guidance material in this AC includes implementation 
schedules that specify acceptable time frames for when operators can 
incorporate required DT data into their maintenance programs. The 
implementation schedules allow for a phased-in program where existing 
repairs on the older and higher utilization airplanes are assessed 
first, and the newer airplanes assessed as they approach their Design 
Service Goal (DSG). This approach ensures that DTIs will be available 
when needed for both older and newer airplanes.

B. Airplane Applicability and Exceptions

    This rule applies to transport category, turbine powered airplane 
models with an original TC issued after January 1, 1958. With certain 
exceptions, this rule applies to those airplanes that, as a result of 
the original certification or later increase in capacity, have a 
maximum type certificated passenger seating capacity of 30 or more or a 
maximum payload capacity of 7,500 pounds or more. The final rule 
differs from the proposal in that we revised the list of excepted 
airplanes to include the Lockheed L-300, deHavilland DHC-7, and Boeing 
707/720 airplanes. We included these airplanes on the excepted list 
because they are not currently being operated in commercial service in 
the U.S., and we do not expect they will be in the future.
1. Airplane Certification Amendment Level
    Airbus and United Parcel Service (UPS) expressed concern that the 
requirements of this rule duplicate certain requirements of current 
regulations.
    Airbus said because newer airplanes like the A330/A340 and A380 
have a state-of-the-art damage tolerance assessment for all activities 
related to baseline structure, repairs, and alterations, the TC 
holder's activities under proposed Sec. Sec.  25.1823(d) and (e) and 
25.1825(c) and (d) would be ``senseless.'' It said applying the 
proposed requirements to its newer model airplanes would offer no 
additional safety benefit because they are already inherent in the 
consistent application of the damage tolerance requirements in Sec.  
25.571. It also said the proposed activities for these airplane models 
would create an unnecessary administrative burden and would require re-
approval of already DT-justified modifications and repairs. Airbus 
asked the FAA to reconsider applying proposed Sec. Sec.  25.1823 and 
25.1825 to TC holders as they relate to airplane models A330/A340/A380 
and future Airbus models. It suggested addressing this issue under 
proposed Sec.  25.1829 in the model-specific compliance plans.
    UPS said if the proposed rule is adopted, it would force operators 
to survey every airplane in their fleet to find repairs and then 
evaluate them based on guidelines produced by TC holders. UPS believes 
airplanes certified to comply with Amendment 25-54 or later already 
have DT data developed for fatigue critical structure, which includes 
certain baseline structure, as well as all repairs and alterations. UPS 
suggested the FAA make the proposed surveys applicable only to 
airplanes certified prior to Amendment 25-54. To accomplish this, it 
said, the FAA should revise proposed Sec.  25.1823(a) to limit the 
applicability to airplanes type certified to pre-Amendment 25-54 
requirements.
    As discussed in the NPRM, the FAA has identified several airplane 
models certified to Amendment 25-45 or later (including airplane models 
certified to Amendment 25-54) for which published repair data have not 
been evaluated for DT. Therefore, unless accomplished previously, a 
damage tolerance evaluation (DTE) needs to be accomplished for all 
airplanes, regardless of the certification level. For those airplanes 
certified to Amendment 25-45 or later that have had a DTE completed for 
all published repair and alteration data, the compliance plan required 
by Sec.  26.49 (proposed as Sec.  25.1829) should contain a statement 
to that effect, and the TC holder will need to substantiate this 
statement with previously approved data from their certification effort 
to show compliance with this rule. TC holders who have already 
substantiated compliance with DT requirements should not find 
compliance with this rule burdensome.
    Regarding UPS's comment, if the TC holder can substantiate 
compliance for its repairs and alterations, it is still likely that 
operators have installed repairs and alterations that were not designed 
by the TC holder on many airplanes. It is also likely that many of 
these repairs and alterations were not assessed for damage tolerance. 
Therefore, a survey will still be necessary to identify those repairs 
and alterations and to determine if DT data are available to support 
operator compliance with the AASFR.
    Bombardier noted that the proposed rule would apply only to DAHs 
for airplanes currently operated under parts 121 or 129. It said this 
would not change the requirement to maintain damage tolerance for all 
airplanes originally certified as damage tolerant under Sec.  25.571 
(Amendment 45 or later). It said it presumes these airplanes will 
continue to be regulated under Sec.  25.1529, using AC 25.1529-1 as 
guidance (and under Canadian Air Regulations & Airworthiness Manual 
511.34 for Canadian DAHs). Bombardier asserted that the four DAH 
deliverables required by proposed Sec.  25.1823 (lists of fatigue 
critical baseline structure, damage tolerance inspections, damage 
tolerance evaluation guidelines, and implementation schedules) are 
already required under Sec.  25.1529 (with guidance provided in AC 
25.1529-1) and could constitute compliance with the proposed rule.
    We agree that TC holders and others designing repairs and 
alterations for airplanes certificated to Amendment 25-45 or later 
amendments will continue to be required to comply with Sec.  25.1529, 
regardless of the types of operations conducted. For airplanes subject 
to this DAH DT Data rule, DAHs and operators should use the guidance in 
AC 120-93 instead of AC 25.1529-1 for repairs. Because this rule is 
entirely consistent with Sec. Sec.  25.571 and 25.1529, DTIs that 
comply with this rule will also comply with those sections. To the 
extent such data have been developed previously, their compliance will 
be simplified.
2. Parts 91, 125, and 135 Operations
    Transport Canada and Mr. Thomas A. Knott expressed concern that the 
proposed rule only applies to airplanes operated under parts 121 and 
129. Mr. Knott also stated that it leaves out airplanes operated under 
parts 91, 125, and 135. Transport Canada expressed concern that the DAH 
DT Data proposal and the AASFR do not apply to airplanes operated under 
part 125 and would allow airplanes such as the B727 and B747 to operate 
as passenger-carrying airplanes under part 125 without having to meet 
DT or the aging airplane safety requirements.
    As we discussed earlier in this preamble, the purpose of this rule 
is to support parts 121 and 129 operators'

[[Page 70492]]

compliance with the AASFR. For the reasons discussed in the preamble to 
the AASFR, we limited applicability of the DT requirements 
(supplemental inspections) in that rule to certain large transport 
airplanes that are typically operated under parts 121 or 129. For the 
affected airplanes that are operated under parts 91, 125, or 135, their 
utilization is much lower and the risks associated with fatigue damage 
that the AASFR is intended to address is, therefore, also much lower. 
Because of this, we determined it would not be cost-effective to impose 
the AASFR's supplemental inspection requirements on parts 91, 125, or 
135 operators.
3. Exception of Airplanes Not Operating in the U.S. Under Part 121 or 
129
    Viking Air Limited said it owns seven de Havilland heritage 
aircraft, including the DHC-5 Buffalo and DHC-7. Viking Air Limited 
said there are about 23 DHC-5s in confirmed operation, and the DHC-7 
has about 66 in confirmed operation. Many of those in confirmed 
operation are used in military operations and are not subject to part 
121 or 129. According to the FAA Registry, no DHC-5 aircraft are 
presently registered in the U.S. Therefore, Viking proposed that the 
DHC-5 be added as an exception under proposed Sec.  25.1823(h). Viking 
Air Limited also said that for the DHC-7, there presently are the 
following safety measures in place: Canadian Airworthiness Directive 
CF-94-19R1 that mandates a Supplemental Inspection Program; CF-2005-36 
that imposes a Structural Life Limit; and CF-98-03 that mandates the 
Corrosion Prevention and Control Program. With these actions, the DHC-
7, the commenter stated, has already met the intentions of aging 
aircraft initiative for structures.
    The FAA researched its data bases and found that the DHC-5 does not 
have a type certificate issued by the U.S. Therefore, there is no need 
for an exception for the DHC-5 Buffalo. Furthermore, we have determined 
that there are no DHC-7 airplanes currently operated under part 121 or 
U.S.-registered DHC-7 airplanes operated under part 129. For the 
reasons discussed earlier in this preamble, we added the DHC-7, as well 
as the Lockheed L-300 and the Boeing 707/720, to the list of excepted 
airplanes in Sec.  26.43(g) of this final rule.

C. Fatigue Critical Structure (FCS)

    This final rule requires TC and STC holders to evaluate their 
designs for baseline and alteration structure to identify FCS. They 
must also develop lists of FCS and make the lists available to 
operators.
    This final rule defines fatigue critical structure as airplane 
structure that is susceptible to fatigue cracking that could contribute 
to a catastrophic failure, as determined under Sec.  25.571. This is 
structure that may need special maintenance actions to manage the 
threat of fatigue. This would be the case for structure that has the 
potential to develop fatigue cracks that, without intervention, could 
lead to a catastrophic failure. The fatigue evaluations are performed 
to determine if special actions are needed and if so, to provide the 
data needed to define the maintenance action requirements. Fatigue 
critical structure may be part of the baseline structure or part of an 
alteration to the baseline structure. As explained in the NPRM,\30\ by 
referencing Sec.  25.571 in the sentence noted below, we intended to 
rely on the many precedents established in finding compliance with this 
section.

    \30\ 71 FR 20583.
---------------------------------------------------------------------------

    Because of industry's extensive experience in showing compliance 
with the damage tolerance requirements of Sec.  25.571, these key 
terms [e.g., fatigue critical structure] should be readily 
understood and applied.

    To clarify how the criteria of Sec.  25.571 apply within the 
context of this rule, we revised the definition of ``fatigue critical 
structure'' by adding the following language: ``Fatigue critical 
structure includes structure, which, if repaired or altered, could be 
susceptible to fatigue cracking and contribute to a catastrophic 
failure.''
    Airbus, the ATA, and UPS, asked for a more detailed definition of 
fatigue critical structure. They expressed concern that, as proposed, 
the definition is open to varying interpretations, so it may not be 
applied consistently across industry or across different airplane 
models. UPS added that some STC holders do not have experience in 
complying with Sec.  25.571. It asserted, the definition must be clear 
so that it can be interpreted and applied in the same manner across the 
industry.
    The ATA and UPS said the methodology for identifying fatigue 
critical structure should include quantitative criteria for assessing 
the criticality of structural elements, based on a comparison of their 
operational loads to their design limit loads or ultimate loads; and it 
should account for load type and single- and multiple-load paths. Also, 
the ATA said, the methodology should define what ``could contribute'' 
means as stated in the definition of fatigue critical structure. It 
recommended possibly using criteria similar to that in Sec.  25.1309 to 
clarify the definition.
    The term ``fatigue critical structure,'' as explained in the 
proposed rule, is intended to identify the same kind of structure for 
which applicants must perform fatigue evaluations to comply with Sec.  
25.571.\31\ These evaluations have been required for new type 
certificates since the adoption of Amendment 25-45 in 1978. 
Furthermore, AC 25-571-1C, published in 1998, provides many examples of 
the types of structural elements that should be evaluated. Therefore, 
we believe there is little, if any, room for differing interpretations 
of this term.
---------------------------------------------------------------------------

    \31\ Sec.  25.571(a): ``An evaluation of the strength, detail 
design, and fabrication must show that catastrophic failure due to 
fatigue, * * * will be avoided throughout the operational life of 
the airplane. This evaluation must be conducted * * * for each part 
of the structure which could contribute to a catastrophic failure 
(such as wing, empennage, control surfaces, fuselage, engine mounts, 
and their related primary attachments) * * *.''
---------------------------------------------------------------------------

    We believe many of the commenters' concerns result from differences 
in the way industry has used the term ``principal structural elements'' 
(PSEs). This term, as used in Sec.  25.571 and AC 25.571, is synonymous 
with the term ``fatigue critical structure.'' That is, a PSE is 
structure that needs to be evaluated to determine if special 
maintenance actions are needed to manage fatigue. And if such actions 
are needed, they must be defined. The meaning of PSE in Sec.  25.571 
contrasts significantly with its usage in certain industry practices 
that have evolved over the years.
    For some TC and STC holders, a PSE is considered to be a specific, 
localized area within fatigue critical structure where special, 
directed inspections are required by an Airworthiness Directive (AD) or 
airworthiness limitations. For example, all longitudinal skin splices 
in a pressurized fuselage should be considered fatigue critical 
structure if they are not immune to fatigue cracking which could lead 
to a catastrophic failure. However, it may be reasonable to manage 
fatigue in these splices by only performing a special directed 
inspection on the most highly stressed area, which may only constitute 
a small percentage of the at-risk structure.
    Some TC and STC holders have identified the PSE as being limited to 
this localized area. While this narrow usage of the term might be 
acceptable within the context of specific supplemental inspection 
documents (SID) or Airworthiness Limitations Sections (ALS), it could 
and has led to confusion and inappropriate actions when taken out of 
context. For this

[[Page 70493]]

reason, we have chosen not to use the term ``principal structural 
element'' in this rule.
    The purpose of requiring identification and listing of fatigue 
critical structure under this rule is to provide operators with a tool 
that will help in the evaluation of existing and future repairs and 
alterations. In this context, fatigue critical structure (FCS) is any 
structure that, if repaired or altered, could be susceptible to fatigue 
cracking and contribute to a catastrophic failure.
    In the case of the longitudinal skin splices discussed above, we 
would expect that the FCS listed by the TC holder would include much 
more structure than just, for example, the localized area that is being 
inspected to gauge the fatigue state of all the splices. A hypothetical 
repair applied to even the lowest stress area of the splices could 
potentially make it more critical than the highest stressed area 
without a repair by increasing and redistributing structural loads. The 
result would be a repair needing its own special directed inspection to 
prevent potentially catastrophic failure. The only way to cover this 
contingency would be to perform a DTE.
    As discussed above, we revised the proposed definition of FCS to 
clarify how the criteria of Sec.  25.571 apply in the context of this 
rule. As we stated in the NPRM, \32\ we intend for this rule to apply 
to future type certificate holders, as well as current holders. Because 
the list of FCS required by this rule may be more extensive than the 
structure identified as airworthiness limitations items currently 
developed by TC applicants, we added provisions to Sec.  26.43 
paragraphs (a) and (e) to make it clear that the list of FCS must be 
submitted as part of the type certification process. This requirement 
will help ensure that, new TC holders are properly addressing DT 
requirements in developing structural repair manuals (SRMs) and other 
service documents for use by operators. It will also assist operators 
in ensuring that a DTE is performed for all repairs and alterations to 
structure identified as FCS, as required by the AASFR, from the 
beginning of an airplane's operational life.
---------------------------------------------------------------------------

    \32\ 71 FR at 20583.
---------------------------------------------------------------------------

    Regarding the concern that STC holders may not have experience in 
complying with Sec.  25.571, current and earlier versions of AC 25.571-
1C provide guidance on identifying PSEs that is also applicable to 
identification of FCS under this rule. Also, one reason this rule 
requires a compliance plan is to ensure that TC holders, STC holders, 
and the FAA have a common understanding of the rule's requirements, 
including acceptable compliance methods.
    Regarding suggestions to use quantitative methods or methodologies 
used to comply with Sec.  25.1309, our intent is to use the same method 
to identify FCS that is required by Sec.  25.571. Paragraph (a) of 
Sec.  25.571 states that an evaluation must be conducted for ``each 
part of structure that could contribute to a catastrophic failure.'' 
Therefore, the applicant must determine which parts of structure could 
contribute to a catastrophic failure when damaged as a result of 
fatigue cracking. Applying a probabilistic approach to determine if and 
when a part will contribute to a catastrophic failure has not been 
industry practice in complying with Sec.  25.571. TC holders are 
required under Sec.  25.571 to perform a damage tolerance evaluation on 
structure to determine when fatigue cracking may occur. At that point 
an inspection is performed to determine if cracking has occurred. A 
probabilistic approach would raise many implementation questions 
because fatigue cracking in metallic structure is a certainty and 
detection is imperative in order to prevent catastrophic failure of 
airplane structure. Probabilistic approaches would not be consistent 
with our objective of facilitating timely compliance.

D. Damage Tolerance Evaluation (DTE)

    This rule requires TC holders and STC holders to review their 
repair and alteration data and determine if a DTE is needed. Unless 
previously accomplished, a DTE must be performed on all repairs and 
alterations that affect fatigue critical structure. A DTE is a process 
that leads to a determination of maintenance actions necessary to 
detect or preclude fatigue cracking that could contribute to a 
catastrophic failure. As applied to repairs and alterations, a DTE 
includes the evaluation of the repair or alteration and the fatigue 
critical baseline structure affected by the repair or alteration. 
Acceptable methods for performing DTEs are described in AC 25.571-1C.
    The maintenance actions developed as a result of a DTE may include 
inspections, time limits for removal and replacement of repairs, 
modification of the repair, alteration to improve its fatigue 
characteristics, or in some cases modification of the affected FCS. The 
type of maintenance action that is appropriate depends upon the type of 
structure affected and the type of fatigue anticipated. For example, 
for fatigue cracks that grow at a predictable rate and that can be 
detected by inspections, a repetitive inspection program would be 
acceptable. For cracks in locations that cannot be inspected and for 
cracking that may grow too rapidly to be detected reliably, replacement 
or modification may be necessary.
    Section 26.43(c) requires TC holders to perform a DTE of those 
repairs specified in their published repair data that affect fatigue 
critical structure. Similarly, Sec. Sec.  26.45(c) and 26.47(c) require 
TC and STC holders to perform a DTE on their FAA-approved alteration 
data. In addition to the published repair and alteration data, this 
final rule requires that all future repair and alteration data receive 
a DTE to determine if inspections or other actions are necessary to 
ensure the airworthiness of the repair or alteration. This rule also 
requires TC holders to develop Repair Evaluation Guidelines (REGs) that 
will enable operators to survey their airplanes to identify repairs 
that affect fatigue critical baseline structure (FCBS) and to obtain 
any necessary damage tolerance inspections (DTI) for those repairs. If 
the REG directs the operator to obtain assistance from the TC holder 
for developing the DTI, the TC holder must make such assistance 
available.
    As discussed below, based on comments to the NPRM, we revised the 
proposed requirements in Sec. Sec.  25.1825(c) and 25.1827(c) (adopted 
as Sec. Sec.  26.45(c)(1) and 26.47(c)(1), respectively)) to clarify 
that a DTE must be performed and the DTI developed for the alteration 
and the FCBS that is affected by the alteration.
    Boeing and AAWG industry representatives asked that the regulatory 
text in proposed Sec. Sec.  25.1825 and 25.1827 be revised to clarify 
that both alteration and baseline structure need to be assessed. They 
state that the description of the work proposed in these sections of 
the NPRM may be interpreted to mean that DTIs only need to be developed 
for the alteration that happens to affect FCBS. However, AAWG industry 
representatives do not believe this is the interpretation the FAA 
intends. AAWG industry representatives recommended that the language in 
both Sec. Sec.  25.1825 and 25.1827 be changed to clearly say that the 
following three components must be addressed for alterations:

    1. Identification of alterations that affect baseline fatigue 
critical structure.
    2. Identification of the structural design details of the 
alteration that require DTE.
    3. Identification of the affected design details of the baseline 
fatigue critical

[[Page 70494]]

structure that require a re-evaluation of their DTE.

    The commenters are correct in that we did not intend for the 
development of DTIs to be limited to the alteration structure. When a 
DTE is performed for an alteration, the DTE must be applied to both the 
alteration and the FCBS that is affected by the alteration. Therefore, 
the DTI developed (as determined by the DTE) for an alteration would 
apply to the alteration structure and to the FCBS that is affected by 
the alteration. As stated above, we revised Sec. Sec.  25.1825(c) and 
25.1827(c) (adopted as Sec. Sec.  26.45(c)(1) and 26.47(c)(1), 
respectively) to clarify that for the alteration and the FCBS that is 
affected by the alteration a DTE must be performed and the DTI 
developed.
    The FAA does not believe that Sec. Sec.  25.1825(c) and 25.1827(c) 
(adopted as Sec. Sec.  26.45(c) and 26.47(c), respectively) need to be 
revised to clarify that alterations that affect FCBS need to be 
assessed, or to provide clarification on which structural design 
details of an alteration would require a DTE. Sections 25.1825(c)(1) 
(adopted as Sec.  26.45(c)(1)) and 25.1827(c)(1) (adopted as Sec.  
26.47(c)(1)) already specify that a DTE must be performed for 
alterations that affect FCBS. In addition, the structure of the 
alteration that requires development of a DTI will be identified as 
part of a DTE performed on the alteration. The DTI may need to be 
developed for fatigue critical alteration structure or for other 
alteration structure that may affect the FCBS. We expect that this 
identification would be part of the DTE of the alteration.
    Regarding the commenters' position that the proposed rule needs to 
be revised to clarify the design details of the affected FCBS that will 
need a re-evaluation of their DTE, the DTE of an alteration will 
include an evaluation of the FCS that is affected by the alteration. 
Therefore, in performing the evaluation of the affected FCBS, it must 
be determined if new or revised DTIs need to be developed for this 
structure. Such a determination is made as part of a DTE.
    Mr. Thomas A. Knott, P.E., said the proposed rule ``is fine,'' 
except it does not address repairs and modifications done under part 
43. He said there are many alterations and repairs that were not 
approved under an STC or developed by TC holders.
    The FAA acknowledges that there are existing repairs and 
alterations that were developed and installed under 14 CFR part 43 
without involvement by DAHs. This final rule takes into account these 
types of repairs. The guidelines the DAHs are required to develop will 
describe procedures for operators to follow in developing DTIs for 
repairs. For alterations affecting FCS for which no DAH is responsible, 
the AASFR requires operators either to develop the DT data themselves 
or contract for their development. Because there is no DAH for these 
alterations, they may be especially problematic if the installers 
failed to consider the fatigue characteristics of the alterations or 
their effects on the baseline structure. Both repairs and alterations 
will be identified and assessed as part of surveys conducted to support 
compliance with the AASFR.

E. Damage Tolerance Inspections (DTIs)

    A DTI is defined in this final rule as inspections developed as a 
result of a DTE. The DTI includes the location of the airplane 
structure to be inspected, the inspection method, inspection procedures 
that include acceptance and rejection criteria, and the thresholds and 
intervals associated with those inspections. The DTI may also specify a 
time limit when the repair or alteration needs to be replaced. As 
discussed below, this definition reflects minor changes from the one in 
the proposed rule.
    Boeing asked that the FAA revise the definition of DTI. It said the 
phrase ``and corrective maintenance actions'' could be confused with a 
requirement to provide repair instructions or other corrective measures 
for a condition found during an inspection. It said, historically, the 
only instructions provided are how to accomplish the inspection 
contained in the DTI and what action should be taken if the inspection 
could not be accomplished. Therefore, Boeing requested that the phrase 
``and corrective maintenance actions'' be removed from the definition 
and replaced with the phrase, ``or a time limit when the repair needs 
to be replaced, or both.''
    We agree and have revised the definition in the final rule as 
requested. The purpose of this rule is to support operators' 
implementation of damage tolerance inspection programs, as required by 
the AASFR. Operators already have access to information on corrective 
actions in the form of SRMs and other documents that may be necessary 
if the inspections reveal fatigue cracks. Therefore, it is not 
necessary to include the phrase ``and corrective maintenance actions'' 
in the definition of DTI.
    Bombardier asked, with respect to inspections of repairs, that we 
clarify the phrase ``the location of the airplane structure to be 
inspected'' used in the DTI definition. Bombardier said it understands 
this phrase to mean that the DTI should clearly define which regions of 
the repair and underlying structure should be inspected and the NDT 
(non-destructive testing) method to be used in carrying out the 
inspection. It said the DTI should be clearly linked to the repair 
data, which will of itself define the repair location.
    The FAA agrees that the DTI should clearly define the areas of the 
repair and underlying structure that should be inspected and the 
inspection method to be applied. The DTI will be applicable to specific 
repair data that will define the repair location. This approach is the 
same as that currently used by TC holders in developing SRMs to comply 
with Sec.  25.571, Amendment 25-45 and later.

F. DT Data for Repairs

1. Published Repair Data
    This final rule requires TC holders to review their published 
repair data and determine if DT data exist for the repairs or if the DT 
data need to be developed. This final rule defines published re