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.
\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.

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.
\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.

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 DTbased 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.''
\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.

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.
\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.

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:

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.

\8\ 71 FR 20574.

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.
\9\ AASFR: 70 FR 5518; February 2, 2005. See also 70 FR 23935; May 6, 2005: Aging Airplane Safety; Correcting Amendment.

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.

\10\ 69 FR 45936; July 30, 2004.

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.

\11\ 67 FR 72726; December 6, 2002.

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, SafetyA Shared
ResponsibilityNew Direction for Addressing Airworthiness Issues for Transport Airplanes,\13\ that explains our criteria for adopting DAH requirements like those described in this final rule.
\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 (PSANM1107122005).

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 120XX \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.
\14\ 71 FR 20750.
\15\ Issued as AC 12093.
\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).

On July 7, 2006, we published a notice \17\ that granted industry a 90day extension to comment on the NPRM; and on February 27, 2007, we published a NOA \18\ and request for comments on revised AC 120XX,\19\ which includes guidance from the AAWG on both repairs and alterations. \17\ 71 FR 38541.
\18\ 72 FR 8834.
\19\ Issued as AC 12093.
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.
\20\ 72 FR 63364; November 8, 2007.

\21\ Certification Procedures for Products and Parts.

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.
\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
Part 26 final rules Proposed part 25 Subpart EAging Airplane SafetyDamage Subpart IContinued
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 certificatesRepairs. type certificatesRepairs. 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 certificateAlterations and repairs to alterations. [[Page 70489]]
Sec. 26.49 Compliance Plan.............. Sec. 25.1829 Compliance Plan.

2. New Subparts for Airworthiness Operational Rules

We discussed in the preamble to the proposed rule that we would establish new subparts for airworthinessrelated 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 DTrelated requirements.
Prior sections Redesignated sections Sec. 121.370a........................... Sec. 121.1109 Sec. 129.16............................. Sec. 129.109
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 DTbased 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.
\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.

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) 120XX,\25\ which describes an acceptable means of compliance with the DAH DT Data final rule.
\25\ Issued as AC 12093.

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.
\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.

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 WFDrelated comments in a separate action, so we will not address them here.

\27\ 71 FR 19928; April 18, 2006.

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 2545 \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.

\28\ 43 FR 46242; October 5, 1978.

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 2545 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 DTbased 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.
\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.

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:

2. Guidance Material

The FAA has issued Advisory Circular (AC) 12093, 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 phasedin 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 L300, deHavilland DHC7, 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 stateoftheart 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 DTjustified 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 modelspecific 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 2554 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 2554. To accomplish this, it said, the FAA should revise proposed Sec. 25.1823(a) to limit the applicability to airplanes type certified to preAmendment 2554 requirements.

As discussed in the NPRM, the FAA has identified several airplane models certified to Amendment 2545 or later (including airplane models certified to Amendment 2554) 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 2545 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 25.15291 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 25.15291) and could constitute compliance with the proposed rule.

We agree that TC holders and others designing repairs and alterations for airplanes certificated to Amendment 2545 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 12093 instead of AC 25.15291 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 passengercarrying 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 costeffective 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 DHC5 Buffalo and DHC7. Viking Air Limited said there are about 23 DHC5s in confirmed operation, and the DHC7 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 DHC5 aircraft are presently registered in the U.S. Therefore, Viking proposed that the DHC5 be added as an exception under proposed Sec. 25.1823(h). Viking Air Limited also said that for the DHC7, there presently are the following safety measures in place: Canadian Airworthiness Directive CF9419R1 that mandates a Supplemental Inspection Program; CF200536 that imposes a Structural Life Limit; and CF9803 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 DHC5 does not have a type certificate issued by the U.S. Therefore, there is no need for an exception for the DHC5 Buffalo. Furthermore, we have determined that there are no DHC7 airplanes currently operated under part 121 or U.S.registered DHC7 airplanes operated under part 129. For the reasons discussed earlier in this preamble, we added the DHC7, as well as the Lockheed L300 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 multipleload 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 2545 in 1978. Furthermore, AC 255711C, 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 atrisk 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
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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 25.5711C.

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 FAAapproved 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
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structure that require a reevaluation 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 reevaluation 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 (nondestructive 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 2545 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 repair data as instructions for accomplishing repairs, which are published for general use in SRMs and service bulletins (or equivalent types of documents). As discussed below, we made minor revisions to the proposed definition.

Boeing requested that we revise the definition of ``published repair data'' to make it clearer. It recommended the following revised version of the proposed definition:

Published repair data means applicable instructions for accomplishing repairs, which are published for general use in structural repair manuals and service bulletins (or equivalent types of documents).

The FAA agrees with the recommended revision to the definition of ``published repair data,'' and we have revised the definition, accordingly, with a minor change in wording.

Bombardier said a list of Structural Significant Items (primary structure) is provided in the SRMs for Bombardier Regional Aircraft. It urged the FAA to consider rulemaking to require the SRM to be an approved document. The SRM, Bombardier commented, can then incorporate all of the instructions for continuing airworthiness required by the NPRM and described previously in AC 25.1529. It said this approach has been used by Bombardier and Transport Canada for SRMs and component maintenance manuals (CMMs) applicable to aircraft and components certified as damage tolerant to Sec. 25.571 (Amendment 2545) and later.

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As explained in the NPRM, SRMs, while not required documents, are FAA approved. Their purpose is to provide operators with readily available sources of approved repair data. Because the operational rules require that major repairs be accomplished according to FAA approved data, an SRM that has not been FAA approved would not serve operators' needs. The SRM, if assessed for damage tolerance under Sec. 25.571 (Amendment 2545 or later Amendment), should include the necessary instructions to ensure a particular repair meets the criteria in AC 25.1529.

2. Effects of Multiple Repairs

Mr. Glenn Davis commented that DT data should address the effects of multiple repairs in close proximity on older aircraft, and future inspections should be based on a ``worst case scenario of the `combination effect' of the multiple repairs.'' He said the FAA might consider requiring a time limit for individual or multiple repairs when the repaired structure would have to be replaced, unless the applicant or operator can confirm through a rational fatigue analysis, using an acceptable fatigue model, that the repaired structure does not need to be replaced. Mr. Davis said such a requirement could be applied to high stress areas in older aircraft such as pressure bulkheads, door apertures, attach fitting support structure for wings, and stabilizers.

The FAA agrees with Mr. Davis's comment that the DT data, specifically the DTE, should take into account the close proximity of repairs. AC 25.5711C provides guidance on determining the effects of multiple repairs that are in close proximity. In addition, the repair assessment guideline (RAG) documents developed in support of Sec. 121.370 (redesignated as Sec. 121.1107)\33\ address the effects of these types of repairs on the pressure vessel. The FAA believes that existing guidance in AC 255711C, along with guidance developed in AC 12093, as part of this final rule, adequately addresses this issue. \33\ Repair Assessment for Pressurized Fuselages final rule (65 FR 24108; April 25, 2000).

G. Repair Evaluation Guidelines (REGs)

This final rule requires TC holders to develop REGs that include processes operators could use to support compliance with Sec. Sec. 121.1109 and 129.109 for repairs that affect FCBS. The guidelines must include

TC holders must submit the REGs to the FAA Oversight Office for review and approval and then make them available to affected operators.

As discussed below, we made several minor revisions to the proposed REG requirements.

In Sec. 25.1823(f)(1)(iii) and (f)(4) (adopted as Sec. 26.43(e)(1)(iii) and (e)(4)), we removed the term ``DT data'' from the phrase ``DT data implementation schedule.'' We made this change because the term ``DT data implementation schedule'' may be misunderstood to mean the actual timing of DT inspections (thresholds and inspection intervals). It was only intended to refer to the timing of major process related events (i.e., survey, development of DTIs, and incorporation of the DTI into the maintenance program).

We revised proposed Sec. 25.1823(f)(1)(iii) (adopted as Sec. 26.43(e)(1)(iii)) to make it clear that the implementation schedule must identify the times when actions must be taken as specific numbers of flight cycles, flight hours, or both. In developing its recommendation regarding implementation schedules, the AAWG proposed an approach that would have referenced the design service goal (DSG) for determining the timing of various actions and would have allowed for variability in DSGs for different airplanes of the same model, depending upon actual flight lengths and other factors.

We agree with the AAWG that it is appropriate to allow reference to DSGs in the implementation schedule to allow for industry resources to be allocated for compliance when they are needed. For example, the AAWG recommended that certain actions be taken when an airplane reaches \3/ 4\ DSG, before which fatigue cracking is less likely to have occurred. However, allowing variability in DSG for different airplanes of the same model would introduce a level of complexity and uncertainty to the requirements of the operational rules that would jeopardize their enforceability. Therefore, this rule requires that DSGs be stated as ``hard numbers.''

We revised Sec. 25.1823(f)(3) (adopted as Sec. 26.43(e)(3)) to remove the requirement that TC holders must make REGs available to STC holders. As adopted, this paragraph only requir

FOR FURTHER INFORMATION CONTACT If you have technical questions about this action, contact Greg Schneider, ANM115, Airframe and Cabin Safety, Federal Aviation Administration, 1601 Lind Avenue, SW., Renton, Washington 980573356, telephone: (4252272116); facsimile (425227 1232); email greg.schneider@faa.gov. Direct any legal questions to Doug Anderson, ANM7, Office of Regional Counsel, Federal Aviation Administration, 1601 Lind Avenue, SW., Renton, WA 980573356; telephone (425) 2272166; facsimile (425) 2271007; email
Douglas.Anderson@faa.gov.