[Federal Register Volume 75, Number 219 (Monday, November 15, 2010)]
[Rules and Regulations]
[Pages 69746-69789]
[FR Doc No: 2010-28363]

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Part II

Department of Transportation

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Federal Aviation Administration

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14 CFR Parts 25, 26, 121, et al.

Aging Airplane Program: Widespread Fatigue Damage; Final Rule

Federal Register / Vol. 75 , No. 219 / Monday, November 15, 2010 /
Rules and Regulations

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 25, 26, 121, and 129

[Docket No. FAA-2006-24281; Amendment Nos. 25-132, 26-5, 121-351, 129-
48]
RIN 2120-AI05

Aging Airplane Program: Widespread Fatigue Damage

AGENCY: Federal Aviation Administration (FAA), DOT.

ACTION: Final rule.

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SUMMARY: This final rule amends FAA regulations pertaining to
certification and operation of transport category airplanes to prevent
widespread fatigue damage in those airplanes. For certain existing
airplanes, the rule requires design approval holders to evaluate their
airplanes to establish a limit of validity of the engineering data that
supports the structural maintenance program (LOV). For future
airplanes, the rule requires all applicants for type certificates,
after the affective date of the rule, to establish an LOV. Design
approval holders and applicants must demonstrate that the airplane will
be free from widespread fatigue damage up to the LOV. The rule requires
that operators of any affected airplane incorporate the LOV into the
maintenance program for that airplane. Operators may not fly an
airplane beyond its LOV unless an extended LOV is approved.

DATES: These amendments become effective January 14, 2011.

FOR FURTHER INFORMATION CONTACT: If you have technical questions
concerning this rule, contact Walter Sippel, ANM-115, Airframe/Cabin
Safety Branch, Federal Aviation Administration, 1601 Lind Avenue SW.,
Renton, WA 98057-3356; telephone<PHONE_NUMBER>; facsimile (425) 227-
1232; e-mail<EMAIL_ADDRESS>If you have legal questions,
contact Doug Anderson, Office of Regional Counsel, Federal Aviation
Administration, 1601 Lind Avenue SW., Renton, WA 98057-3356; telephone
(425) 227-2166; facsimile<PHONE_NUMBER>; e-mail
<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

Authority for This Rulemaking

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

Contents

I. Executive Summary
II. Background
    A. Summary of the NPRM
    B. Related Activities
    C. Differences between NPRM and Final Rule
    1. Substantive changes
    2. Regulatory Evaluation changes
    3. New part 26 for design approval holders' airworthiness
requirements
    4. New subparts for airworthiness operational rules
    D. Summary of Comments
III. Discussion of the Final Rule
    A. Overview
    1. Widespread fatigue damage
    2. Final rule
    B. Requests for Deferral or Withdrawal of Rule
    1. Safety benefits don't justify rule
    2. Existing programs serve purpose of rule
    3. Divide rule into two
    C. Concept of Operational Limits
    1. Requests for requiring maintenance programs instead
    2. Single retirement point for a model
    3. Potentially adverse effect on safety
    D. Change in Terminology (Initial Operational Limit to LOV)
    1. Rationale for the term LOV
    2. Refer to the structural maintenance program
    E. Repairs, Alterations, and Modifications
    1. Whether repairs, alterations, and modifications pose WFD
risks
    2. Relationship to damage tolerance requirements (Sec.  25.571)
    a. Pre-Amendment 25-96 airplanes
    b. Airplanes certified to Amendment 25-96 or later
    3. Guidelines for repairs, alterations, and modifications
    4. Rely on the Changed Product Rule
    F. LOVs for Existing Airplanes
    1. NPRM compliance date
    2. When to set LOVs for existing airplanes
    a. Pre-Amendment 25-45 airplanes
    b. Airplanes certified to Amendment 25-45 or later
    3. Varying implementation strategies
    4. FAA review and approval time
    G. LOVs for Future Airplanes: Revisions to Sec.  25.571 and
Appendix H
    1. Opposition to changes to Sec.  25.571
    2. Change to Appendix H
    3. When to set LOVs for future airplanes
    H. How to Set LOVs
    I. How to Extend LOVs
    1. Change the procedure for extending LOVs
    2. Evaluation of repairs, alterations, and modifications for LOV
extensions
    3. Alternate means of compliance (AMOCs)
    4. Extension procedure doesn't allow public comment
    J. Applicability for Existing Airplanes
    1. Type certificates issued after January 1, 1958
    2. Original type certification
    3. Airplane configuration
    4. Weight cutoff
    5. Default LOVs and excluded airplanes
    a. Table 1--Default LOVs
    b. Table 2--Airplanes excluded from Sec.  26.21
    6. Bombardier airplanes
    7. Intrastate operations in Alaska
    8. Composite structures
    K. Harmonization
    L. Regulatory Evaluation
    1. Benefits of proposed rule
    2. Costs of proposed rule
    a. Need to know LOVs to determine cost
    b. Need to know maintenance actions to determine cost
    c. Costs to manufacturers
    d. Cost of failing to harmonize rule
    e. Cost to replace an airplane
    f. Residual value of airplanes
    3. ``Rotable'' parts
    4. Use of LOVs for financial evaluations
IV. Regulatory Notices and Analyses

I. Executive Summary

    This final rule requires certain actions to prevent catastrophic
failure due to widespread fatigue damage (WFD) throughout the
operational life of certain existing transport category airplanes and
all those to be certificated in the future. Existing airplanes subject
to the rule are turbine-powered airplanes with a type certificate
issued after January 1, 1958, which have a maximum takeoff gross weight
greater than 75,000 pounds and are operated under part 121 or 129. The
rule applies to all transport category airplanes to be certificated in
the future, regardless of maximum takeoff gross weight or how they are
operated. The benefits of this rule are estimated at a present value of
$4.8 million. The cost is estimated at a present value of $3.6 million.

               Figure 1--WFD Final Rule Benefits and Costs
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                                                            7% Present
                                           Nominal value     value ($
                                           ($ millions)      millions)
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Benefits................................             9.8             4.8
Costs...................................             3.8             3.6
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    Fatigue damage to a metallic structure occurs when the structure is
subjected to repeated loads, such as the pressurization and
depressurization that occurs with every flight of an airplane. Over
time this fatigue damage results in cracks in the structure, and the
cracks may begin to grow together. Widespread fatigue damage is the
simultaneous presence of fatigue cracks at multiple structural
locations that are of sufficient size and density that the structure
will no longer meet the residual strength requirements of Sec.
25.571(b).\1\ Structural fatigue characteristics of airplanes are
understood only up to the point where analyses and testing of the
structure are valid. There is concern about operating an airplane
beyond that point for several reasons. One reason is that WFD is
increasingly likely as the airplane ages, and is certain if the
airplane is operated long enough. Another is that existing inspection
methods do not reliably detect WFD because cracks are initially so
small and may then link up and grow so rapidly that the affected
structure fails before an inspection can be performed to detect the
cracks.
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    \1\ After sustaining a certain level of damage, the remaining
structure must be able to withstand certain static loads without
failure. In the context of WFD, the damage is a result of the
simultaneous presence of fatigue cracks at multiple locations in the
same structural element (i.e., multiple site damage) or the
simultaneous presence of fatigue cracks in similar adjacent
structural elements (i.e., multiple element damage).
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    To preclude WFD related incidents in existing transport category
airplanes, this final rule requires holders of design approvals for
those airplanes subject to the rule to perform the following actions:
    1. Establish a limit of validity of the engineering data that
supports the structural maintenance program (LOV);
    2. Demonstrate that WFD will not occur in the airplane prior to
reaching the LOV; and
    3. Establish or revise the Airworthiness Limitations section in the
Instructions for Continued Airworthiness to include the LOV.
    As used in this preamble, the term ``design approval holders''
includes holders of type certificates, supplemental type certificates,
or amended type certificates, and applicants for such approvals. In the
context of this final rule, the design approval holder is generally the
type certificate holder. Requiring design approval holders to perform
the actions listed above is intended to support compliance by operators
with today's amendments to parts 121 and 129. This final rule amends
those parts to require that operators incorporate the LOV as
airworthiness limitations into their maintenance program for each
affected model that they operate.
    The amendments to the operating rules have the effect of
prohibiting operation of an airplane beyond its LOV. However, today's
rule provides an option for any person to extend the LOV for an
airplane and to develop the maintenance actions which support the
extended limit. Thereafter, to operate an airplane beyond the existing
LOV, an operator must incorporate the extended LOV and associated
maintenance actions into its maintenance program. The airplane may not
be operated beyond the extended LOV.
    In response to comments on the notice of proposed rulemaking, the
FAA has made a number of substantive changes which significantly reduce
the costs presented in the proposal. The FAA has--
     Eliminated the requirement to evaluate WFD associated with
most repairs, alterations, and modifications of the baseline \2\
airplane structure.
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    \2\ Baseline structure means structure that is designed under
the original type certificate or amended type certificate for that
airplane model.
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     Simplified how an LOV may be extended.
     Extended the compliance dates by which design approval
holders must establish an LOV for existing airplanes.
     Extended the time for operators to incorporate LOVs into
their maintenance programs.
     Limited the applicability of the final rule to ``transport
category, turbine-powered airplanes with a type certificate issued
after January 1, 1958.''
    Today's rule requires that design approval holders take the
necessary steps to preclude WFD in the future by requiring that they
establish LOVs. Although the rule allows design approval holders to
establish LOVs without relying on maintenance actions, the FAA expects
most current design approval holders to adopt LOVs that will rely on
such actions. Since WFD is by definition a condition in which structure
will no longer meet the residual strength requirements of Sec.
25.571(b), it could lead to a catastrophic failure. Thus the FAA would
mandate those maintenance actions by airworthiness directive. The
agency expects these actions to greatly reduce the number of
unanticipated inspections and repairs resulting from emergency
airworthiness directives the FAA issues when WFD is discovered in
service. The FAA estimates the value of managing WFD with maintenance
actions developed under this final rule versus the current practice of
issuing airworthiness directives as WFD is found is worth $4.8 million
in present value. There are other benefits of this rule that were not
included in the final benefit assessment. They include prevention of
accidents and a longer economic life for the airplane. The FAA
estimates that this rule will cause one airplane to be retired because
of its reaching the anticipated LOV in the 20-year analysis period. The
retirement of this one airplane will result in costs of approximately
$3.8 million, with a present value of approximately $3.6 million. This
operator's cost is the only cost attributed to the final rule, since
manufacturer costs were found to be minimal.
    Thus, as noted earlier, this final rule's estimated present value
benefits of $4.8 million exceed the estimated present value costs of
approximately $3.6 million.

II. Background

A. Summary of the NPRM

    On April 18, 2006, the FAA published a notice of proposed
rulemaking (NPRM), entitled Aging Aircraft Program: Widespread Fatigue
Damage.\3\ That proposal was based on a recommendation from the
Aviation Rulemaking Advisory Committee (ARAC). The NPRM contained
extensive requirements for setting and supporting an initial
operational limit for an airplane model. The FAA proposed that the rule
apply to transport category airplanes with a maximum gross takeoff
weight of greater than 75,000 pounds. The due date for comments was
July 17, 2006.
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    \3\ 71 FR 19928
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    The FAA proposed that design approval holders for those airplanes
be required to take actions to preclude WFD. For new airplanes, the FAA
proposed to amend Sec.  25.571 and Appendix H to part 25 to require
that applicants for a new type certificate establish an initial
operational limit and include that limit in the Airworthiness
Limitations section of the Instructions for Continued Airworthiness for
the airplane. The agency also proposed that applicants develop
guidelines for evaluating repairs, alterations, and modifications for
WFD.

    Section 25.1807 proposed that holders of design approvals for
existing airplanes or applicants for such approvals be required to do
the following:
    1. Establish an initial operational limit; and
    2. Establish a new Airworthiness Limitations section or revise an
existing Airworthiness Limitations section to include the initial
operational limit.
    Section 25.1807(g) proposed that holders of design approvals for
existing airplanes or applicants for such approvals be required to
prepare the following:
    1. A list of repairs and modifications developed and documented by
the design approval holder;
    2. Service information for maintenance actions necessary to
preclude WFD from occurring before the initial operational limit; and
    3. Guidelines for identifying, evaluating, and preparing service
information for repairs, alterations, and modifications for which no
service information exists.
    For existing airplanes for which an initial operational limit is
established, Sec.  25.1809 proposed that design changes be evaluated
for susceptibility to WFD and, if a change were susceptible, that the
design approval holder identify when WFD is likely to occur and whether
maintenance actions would be required. Section 25.1811 provided that
any person could apply to extend an operational limit, using a process
similar to that for establishing the initial operational limit. Under
Sec.  25.1813, certain repairs, alterations, and modifications proposed
for installation on airplanes with an extended operational limit would
also be evaluated.
    The FAA proposed to amend the operating requirements of parts 121
and 129 to require that no operator could operate an airplane unless
the initial operational limit or extended operational limit for the
airplane had been incorporated into the operator's maintenance program.
    The NPRM contains the background and rationale for this rulemaking
and, except where the FAA has made revisions in this final rule, should
be referred to for that information.

B. Related Activities

    In July 2004, the FAA published the notice entitled ``Fuel Tank
Safety Compliance Extension (Final Rule) and Aging Airplane Program
Update (Request for Comments)'' \4\ to propose airworthiness
requirements for design approval holders to support certain operational
rules. The FAA requested comments on the agency's proposal.
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    \4\ 69 FR 45936, July 30, 2004.
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    In July 2005, the FAA published a disposition of comments received
in response to our request.\5\ Also in July 2005, the agency published
a policy statement, ``Safety-A Shared Responsibility-New Direction for
Addressing Airworthiness Issues for Transport Airplanes,'' \6\ that
explains our reasons for adopting requirements for design approval
holders.
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    \5\ 70 FR 40168, July 12, 2005: Fuel Tank Safety Compliance
Extension (final rule) and Aging Airplane Program Update (Request
for comments).
    \6\ 70 FR 40166, July 12, 2005 (PS-ANM110-7-12-2005).
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    On May 22, 2006, the FAA published a Notice of Availability and
request for comments on proposed Advisory Circular (AC) 120-YY,
Widespread Fatigue Damage on Metallic Structure. The notice stated that
the proposed AC could be found on the Internet at http://www.faa.gov/aircraft/draft_docs. This proposed advisory circular provides guidance
to design approval holders on establishing initial and extended
operational limits to preclude WFD for certain transport category
airplanes and evaluating repairs, alterations, and modifications to the
airplanes. The advisory circular also provides guidance to operators on
incorporating the initial or extended operational limit and any related
airworthiness limitation items into their maintenance programs. The
notice specified that comments on the proposed advisory circular were
to be received by July 17, 2006.
    On July 7, 2006, at the request of a number of commenters, the FAA
published a notice \7\ extending the comment period on both the NPRM
and proposed AC 120-YY to September 18, 2006. On August 18, 2006, the
agency posted proposed AC 25.571-1X, Damage Tolerance and Fatigue
Evaluation of Structure, on the Internet at http://www.faa.gov/aircraft/draft_docs. Comments on this document, which proposed
revision of existing AC 25.571-1C, were due by October 21, 2006.
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    \7\ 71 FR 38540.
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    On November 26, 2006, the FAA held a public meeting with the ARAC
Transport Airplane and Engine Issues Group. Under ARAC, the
Airworthiness Assurance Working Group (AAWG) had previously provided
recommendations to the FAA on how to address widespread fatigue damage.
Because the FAA had received several comments concerning differences
between the AAWG's recommendations and the NPRM, the meeting was held
to discuss the reasons for these differences. The FAA's presentation at
the meeting has been placed in the docket for this rulemaking. Except
as discussed in the context of specific issues affecting this final
rule, the FAA will not revisit those differences here.
    On December 11, 2008, at the request of the Acting Administrator,
the FAA held a public meeting to allow comments on the changes that had
occurred to the rule since it had been proposed in the NPRM. A
Technical Document describing those changes was posted in the docket,
and the announcement of the meeting and opening of the comment period
for the Technical Document was published in the Federal Register on
Nov. 7, 2008 (73 FR 66205). The public was invited to submit comments
on the Technical Document either in person at the meeting or by sending
them to the docket. Seventy-one people attended the meeting and Boeing,
the Air Transport Association of America (ATA), and FedEx made
presentations, along with the FAA. Many attendees commented or asked
questions. In addition, 12 commenters submitted comments about the
Technical Document to the docket. The comment period closed on December
22, 2008.
    While some of the comments received during the comment period for
the Technical Document were new, many were restatements of comments
made after publication of the NPRM. We address all of the comments,
from both comment periods, in the section below. Comments received
during both comment periods are posted to the docket. A transcript of
the public meeting, including presentations given and comments
delivered there, may also be found in the docket.

C. Differences Between NPRM and Final Rule

1. Substantive Changes
    The FAA has eliminated the requirement to evaluate WFD associated
with most repairs, alterations, and modifications of the baseline
airplane structure.\8\ The agency has also made a change in
terminology. This final rule uses the term ``limit of validity of the
engineering data that supports the maintenance program'' (LOV) rather
than the term ``initial operational limit.'' The FAA finds that the
term ``limit of validity'' is more appropriate than the term ``initial
operational limit'' in defining the point to which an airplane

may be safely operated. The requirements in this final rule for
establishing the LOV under Sec.  26.21 are that it be supported by test
evidence and analysis at a minimum and, if available, by service
experience or service experience and teardown inspection results for
those airplanes of similar structural design with the highest total
accumulation of flight cycles or flight hours (commonly referred to as
high-time airplanes). This criterion is similar to the criterion used
in Sec.  25.571(b). This final rule also clarifies how the LOV may be
extended, using the same type of evaluation as that required for
setting the LOV under Sec.  26.21.
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    \8\ The final rule requires that design approval holders
evaluate airplane configurations that include modifications mandated
by airworthiness directive.
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    In response to requests for more time, the FAA has extended the
compliance dates by which design approval holders must establish an LOV
for existing airplanes. Those dates vary according to the age of the
airplanes, from 18 months after the effective date for the oldest
airplanes to 60 months after the effective date for the newest ones.
Additionally, the agency has extended the time for operators to
incorporate LOVs into their maintenance programs. These dates vary with
the age of the airplanes as well, and are 12 months later than the
related design approval compliance dates, thus giving operators 12
months to incorporate the LOV into their maintenance programs. Operator
compliance dates range from 30 to 72 months after the effective date.
The FAA has also changed the proposed operational rules to correct an
inadvertent ambiguity in the NPRM regarding obligations of operators of
airplanes for which the type certificate holder might fail to establish
an LOV as required.
    Another change involves applicability to existing transport
category airplanes. This final rule applies to ``transport category,
turbine-powered airplanes with a type certificate issued after January
1, 1958.'' This limitation was added to make applicability of today's
rule consistent with that of the other aging airplane rules. The FAA
also added airplanes to the list of those excluded from the LOV
requirements of Sec.  26.21 because the airplanes are not operated
under parts 121 or 129. Either they are being operated under different
parts of the Code of Federal Regulations (CFR) or they are not in
service at this time. The number of these airplanes still operating is
very small, and the probability of their retirement in the near future
is high.
2. Regulatory Evaluation Changes
    The FAA has substantially revised the Regulatory Evaluation for
several reasons. One concerns differences between the rule as proposed
and the final rule. For example, the requirement to evaluate WFD
associated with repairs, alterations, and modifications of the baseline
airplane structure, except for those mandated by airworthiness
directives, has been eliminated from this final rule. Another reason
concerns information received during the rulemaking process which
indicated that some of the initial assumptions about benefits and costs
of the rule were not valid. For example, initially, the FAA assumed
that design approval holders would set the LOV for a specific airplane
model at the design service goal for that model. However, subsequently,
some design approval holders indicated that they planned to set the LOV
33% to 180% higher. The net effect of these changes has been to
dramatically reduce the costs estimated for compliance with the rule.
    Our revised Regulatory Evaluation lists three potential sources of
benefits of the rule, namely (1) prevention of accidents; (2) extension
of the economic life of the airplane with corresponding revenues from
that additional economic life; and (3) near elimination of emergency
airworthiness directives.
    Preventing a WFD accident is estimated to have benefits ranging
from $20 million to $680 million. There are multiple factors, however,
that make it difficult to forecast that this rule absolutely would
prevent accidents. Among them are earlier FAA rulemaking actions to
prevent known fatigue problems from reoccurring.
    Similarly, although specific maintenance actions designed to extend
the life of airplane structure have added years of service to the DC-9
fleet, quantification of such values for other models is unnecessary,
given that benefits already exceed the nearly minimal costs.
    As a result, the quantified benefit of this final rule is based
solely on the near elimination of emergency ADs pertaining to WFD. The
analysis assumes the rule will prevent 1.5 days of down time associated
with emergency ADs.
3. New Part 26 for Design Approval Holders' Airworthiness Requirements
    In the WFD proposed rule, and in proposals for other Aging Airplane
Program rules, the FAA placed the airworthiness requirements for design
approval holders in part 25, subpart I. As explained in the Enhanced
Airworthiness Program for Airplane Systems/Fuel Tank Safety final rule
(EAPAS/FTS),\9\ the FAA decided after further review and input from
industry and foreign aviation authorities to place these requirements
in a new part 26 and move the enabling regulations into part 21.\10\
The FAA determined that this was the best course of action because it
keeps part 25 applicable only to airworthiness standards for transport
category airplanes. This is important because it maintains
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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    \9\ 72 FR 63363, November 8, 2007.
    \10\ Certification Procedures for Products and Parts.
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    In creating part 26, the FAA renumbered the proposed sections of
part 25, subpart I, and incorporated the changes discussed in this
preamble. A table of this renumbering is shown below.
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    \11\ This section, which includes an applicability table for
part 26, was adopted as part of the EAPAS final rule.

                                Figure 2--Table Showing Relationship of Proposed Part 25 Subpart I to Part 26 Final Rule
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                 Part 26 final rule                                                            Proposed part 25
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SUBPART C--Aging Airplane Safety--Widespread         Subpart I--Continued Airworthiness
 Fatigue Damage.
Sec.   26.5 Applicability table....................  New \11\
Sec.   26.21 Limit of validity (LOV)...............  Sec.   25.1807 Initial operational limit: Widespread Fatigue Damage (WFD).
                                                     Sec.   25.1809 Changes to type certificates: Widespread Fatigue Damage (WFD).
Sec.   26.23 Extended limit of validity (LOV)......  Sec.   25.1811 Extended operational limit: Widespread Fatigue Damage (WFD)

                                                     Sec.   25.1813 Repairs, alterations, and modifications: Widespread Fatigue Damage (WFD).
--------------------------------------------------------------------------------------------------------------------------------------------------------

4. New Subparts for Airworthiness Operational Rules
    The WFD NPRM was among several Aging Airplane Program rulemaking
initiatives that proposed new subparts (subparts AA and B in parts 121
and 129, respectively) for airworthiness requirements, and redesignated
certain sections of parts 121 and 129. Since the EAPAS/FTS final rule
was the first of these rulemaking initiatives to be codified, the new
subparts and redesignated sections were adopted in that rule.
Therefore, the FAA has removed the regulatory language and related
discussion about these changes from this final rule. This final rule
adds new sections that include WFD-related requirements: Sec. Sec.
121.1115 and 129.115.

D. Summary of Comments

    The FAA received comments about the NPRM from 40 commenters,
including airplane manufacturers, operators, aviation associations, and
others. The comments covered an array of topics and contained a range
of responses. There was much support from airplane manufacturers,
operators, and associations for the concept of precluding WFD in aging
airplanes. There were also a number of recommendations for changes and
requests for clarification. As previously discussed, at the December
11, 2008 public meeting, Boeing, FedEx, and ATA gave presentations of
their responses to the Technical Document.
    In addition, the FAA received comments about airworthiness
requirements for design approval holders. We addressed many of the same
or similar comments 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). We also explained in
detail the need for these requirements in our July 2005 policy
statement. As a result, the FAA will not revisit those comments here.

III. Discussion of the Final Rule

A. Overview

1. Widespread Fatigue Damage
    Widespread fatigue damage is the simultaneous presence of cracks at
multiple structural locations that are of sufficient size and density
that the structure will no longer meet the residual strength
requirements of 14 CFR 25.571(b). This may result in catastrophic
structural failure and loss of the airplane.
    Fatigue is the gradual deterioration of a material subjected to
repeated structural loads. When it occurs in more than one location,
cracks manifest themselves as multiple site damage or multiple element
damage. Multiple site damage is the simultaneous presence of fatigue
cracks at multiple locations that grow together in the same structural
element, such as a large skin panel or lap joint. Multiple element
damage is the simultaneous presence of fatigue cracks in similar
adjacent structural elements, such as frames or stringers. Some
structural elements are susceptible to both types of damage, and both
types may occur at the same time.
    Cracks associated with multiple site damage and multiple element
damage are initially so small that they cannot be reliably detected
with existing inspection methods. Widespread fatigue damage is
especially hazardous because these small, undetectable cracks in
metallic structure can ``link up'' and grow very rapidly to bring about
catastrophic failure of the structure. Although operators perform
routine structural inspections to detect fatigue damage, fatigue cracks
related to WFD grow so rapidly that operators cannot inspect
susceptible structures often enough to detect the cracks before they
cause structural failure. As a result, many of the findings of these
types of cracks have been fortuitous: mechanics and others have
observed fatigue cracks while doing other work. For example, cracks
have been found by workers while stripping and painting an airplane.
Cracks have also been found by mechanics conducting unrelated
inspections of skin anomalies on the external fuselage; further
investigation revealed multiple cracks in stringers and circumferential
joints.
    In other cases, undetected multiple site damage in wing or fuselage
structure has eventually led to catastrophic failure of the structure
in flight. For example, wing failures have resulted in losses of C-130
and P4Y-2 airplanes. Failures of aft pressure bulkheads have caused
decompression of B-747, DC-9, and L-1011 airplanes.
    Concern about WFD was brought to the forefront of public attention
in April 1988, when an 18-foot-long section of the upper fuselage of a
Boeing Model 737 airplane separated from the airplane during flight.
The airplane, operated by Aloha Airlines, was en route from Hilo to
Honolulu, Hawaii, at 24,000 feet. Onboard were 89 passengers and 6
crewmembers. A flight attendant died as a result of the accident, and
eight passengers were injured.
    The damage to the airplane consisted of a total separation and loss
of a major portion of the upper crown skin and other structure. The
damaged area extended from the main cabin entrance door aft for about
18 feet. At the time of the accident, the airplane had accumulated
89,680 flight cycles and 35,496 flight hours.
    In the years after the Aloha Airlines accident, WFD was discovered
in the following airplanes:
     Boeing 727: Cracking along a lap joint.
    In 1998, during maintenance, two cracks were found growing out from
underneath the lap joint. Disassembly of the joint revealed a 20-inch
hidden crack from multiple site damage on the lower row of rivet holes
in the inner skin.
     Boeing 737: Cracking along a lap joint.
    In July 2003, a mechanic preparing to paint discovered extensive
multiple site damage with up to 10 inches of local link-up of cracks in
one area.
     Boeing 747: Cracking of the aft pressure bulkhead.
    In 2005, Boeing issued service information to address multiple site
damage of the aft pressure bulkhead radial lap splices. The service
information was based on analysis and fatigue testing of the aft
pressure bulkhead.
     Boeing 767: Cracking of the aft pressure bulkhead.
    On November 5, 2003, cracks were found at multiple sites common to
a single radial lap splice during an inspection of the aft pressure
bulkhead.
     McDonnell Douglas DC-9: Cracking of the aft pressure
bulkhead.
    On June 22, 2003, widespread fatigue damage on a DC-9 airplane led
to rapid decompression at 25,000 feet. Later inspection revealed
multiple site

damage with extensive link-up of cracks.
     Lockheed C-130A: Fatigue cracks in the wing structure.
    On August 13, 1994, while responding to a forest fire in the
Tahachapi Mountains near Pearblossom, California, the airplane
experienced an in-flight separation of the right wing. All 3 flight
crewmembers were killed, and the airplane was completely destroyed.
     Lockheed C-130A: Fatigue cracks in the wing structure.
    On June 17, 2002, while executing a fire retardant drop over a
forest fire near Walker, California, the airplane's wings folded upward
at the center wing-to-fuselage attachment point, and the airplane broke
apart. All three flight crewmembers were killed, and the airplane was
completely destroyed.
     Consolidated-Vultee P4Y-2: Fatigue cracks in the wing
structure.
    On July 18, 2002, the airplane was maneuvering to deliver fire
retardant over a forest fire near Estes Park, Colorado, when its left
wing separated from the airplane. Both flight crewmembers were killed,
and the airplane was destroyed. An examination of other Consolidated-
Vultee P4Y-2 airplanes revealed that the area was difficult to inspect
because of its location relative to fuselage structure.
     Lockheed L-1011: Failure in-flight of the aft pressure
bulkhead stringer attach fittings.
    In August 1995, an L-1011 airplane experienced a rapid
decompression at 33,000 feet. Twenty stringer end fittings were found
severed and the aft pressure bulkhead was separated from the fuselage
crown by a crack approximately 12 feet long. The flight crew was unable
to maintain cabin pressure control until after rapid descent.
     Boeing 747: Cracking of adjacent fuselage frames.
    In 2005, during an overnight maintenance visit, missing skin
fasteners common to a fuselage frame were discovered in the upper deck
area. Further inspection revealed that the frame was severed.
Substantial cracking was also found in the adjacent left and right
frames.
     Airbus A300: Cracking of adjacent fuselage frames.
    In 2002, investigations conducted as a result of fatigue cracks
found on a test article and later in service revealed that cracking of
certain adjacent fuselage frames could result in multiple element
damage. The determination was based on analysis, service experience,
and fatigue testing.
    Since 1988, the FAA has issued approximately 100 airworthiness
directives to address WFD in airplanes. Approximately 25 percent of
these airworthiness directives were too urgent to allow the public an
opportunity to comment in advance. These airworthiness directives
required inspections, and the FAA later superseded the majority of them
to expand the inspections or require modifications because inspections
were not enough to preclude WFD.
    Shortly after the Aloha Airlines accident, the AAWG \12\ was formed
to identify procedures to ensure continued structural airworthiness of
aging transport category airplanes. Basic approaches defined by the
group and accepted by the FAA included recommending procedures to
preclude WFD in those airplanes. When ARAC was formed in 1991 to
provide advice and recommendations on safety-related matters to the
FAA, the AAWG became a working group under its auspices. In 2003 the
AAWG completed its recommendation on WFD.
---------------------------------------------------------------------------

    \12\ The group was initially known as the Airworthiness
Assurance Task Force.
---------------------------------------------------------------------------

    In 2004, the FAA tasked ARAC to ``provide a written report on part
121 and 129 certificate holders operating airplanes with a maximum
takeoff gross weight of greater than 75,000 pounds to assess the WFD
characteristics of structural repairs, alterations, and modifications
as recommended in a previous tasking of the Aviation Rulemaking
Advisory Committee.'' \13\ During the comment period on the NPRM for
this final rule, the AAWG was working to complete Task 3, to recommend
how an operator would include consideration of WFD for repairs,
alterations, and modifications to airplanes operated under part 121 or
129.
---------------------------------------------------------------------------

    \13\ Task 3.--Widespread Fatigue Damage (WFD) of Repairs,
Alterations, and Modifications. Provide a written report providing
recommendations on how best to enable part 121 and 129 certificate
holders of airplanes with a maximum gross take-off weight of greater
than 75,000 pounds to assess the WFD characteristics of structural
repairs, alterations, and modifications as recommended in a previous
ARAC tasking. The written report will include a proposed action plan
to address and/or accomplish these recommendations including actions
that should be addressed in Task 4 [below]. The report is to be
submitted to the ARAC, Transport Airplane and Engine Issues Group,
for approval. The ARAC, Transport Airplane and Engine Issues Group,
will determine as appropriate the means by which the action plan
will be implemented. The proposed actions and implementation process
approved by the ARAC, Transport Airplane and Engine Issues Group,
will be subject to FAA concurrence. Published in 69 FR 26641, May
13, 2004.
---------------------------------------------------------------------------

    On April 17, 2007, the AAWG presented its final report on Task 3 to
ARAC. Many of the conclusions and recommendations in the final report
are the same as those provided in the comments on the proposed rule
which are discussed in this preamble.
2. Final Rule
    This final rule requires actions to preclude WFD in transport
category airplanes. It applies to both existing transport category
airplanes that have a maximum takeoff gross weight greater than 75,000
pounds and to all transport category airplanes to be certified in the
future, regardless of the maximum takeoff weight.
    Today's rule imposes requirements on those holding design approvals
for existing transport category airplanes that are subject to the rule.
The design approval holders are required to evaluate the structural
configuration of each model for which they hold a type certificate to
determine its susceptibility to WFD and, if it is susceptible, to
determine that WFD would not occur before the proposed LOV. The
evaluation would be based on test evidence and analysis at a minimum
and, if available, service experience or service experience and
teardown inspection results of airplanes with a high number of total
accumulated flight cycles or flight hours or both, which are frequently
referred to as high-time airplanes. The evaluation would be performed
on airplanes of similar structural design, accounting for differences
in operating conditions and procedures. Using the results of the
evaluation, the design approval holder must then establish an LOV.
    Holders of approvals for design changes that increase an airplane's
maximum takeoff gross weight to more than 75,000 pounds, or decrease it
from more than 75,000 pounds to 75,000 pounds or less after the
effective date of the rule, must also evaluate the affected airplanes
for WFD and establish LOVs for those airplanes.
    The final rule amends Appendix H to part 25 to require that the LOV
which is established by the design approval holder be included in the
Airworthiness Limitations section of the Instructions for Continued
Airworthiness. It also amends operating rules in parts 121 and 129 to
require that operators of an affected airplane incorporate into their
maintenance programs an Airworthiness Limitations section that includes
an LOV for that airplane.
    The amendments to parts 121 and 129 have the effect of prohibiting
operation of an airplane beyond its LOV.\14\ For

transport airplane designs developed in the future, the LOV will be
included in the airplane's airworthiness limitations and will apply
regardless of how or by whom the airplane is operated. However, the
final rule allows any person to extend the LOV for an airplane (if the
person can demonstrate that it will be free of WFD up to the extended
LOV) and to develop a maintenance program that supports the extended
limit. Thereafter, the operator must incorporate the extended LOV and
the associated maintenance actions into the Airworthiness Limitations
section of its Instructions for Continued Airworthiness and may not
operate the airplane beyond that limit.
---------------------------------------------------------------------------

    \14\ Under 14 CFR 91.403(c), no person may operate an airplane
unless applicable airworthiness limitations have been complied with.
By requiring operators to incorporate the LOV airworthiness
limitations developed by the design approval holders under this
rule, this final rule makes those LOVs applicable to the affected
airplanes, and Sec.  91.403(c) requires operators to comply with
them.
---------------------------------------------------------------------------

    The remainder of this section of the preamble discusses specific
comments received.

B. Requests for Deferral or Withdrawal of Rule

    The FAA received a number of comments that rulemaking to preclude
WFD was not warranted and that the rule, as proposed, should be
deferred or withdrawn. Commenters included United Parcel Service,
American Airlines, FedEx, Cargo Airline Association (CAA), National Air
Carrier Association (NACA), Lynden Air Cargo, ATA, Northwest Airlines,
Transport Aircraft Technical Services, and Continental Airlines.
1. Safety Benefits Don't Justify Rule
    American Airlines, ATA, and Lynden Air Cargo commented that the
rule was not justified in terms of safety. They pointed out that there
has been no catastrophic accident directly attributable to WFD since
the Aloha Airlines accident in 1988 and that the National
Transportation Safety Board found that WFD was a contributory factor,
but not the sole factor, in that accident.
    In contrast, Boeing commented that issuance of this final rule
would cast a broad safety net on airframe structural performance for
those types of details the industry has determined may be susceptible
to WFD. Boeing said this final rule would provide for the establishment
of safe operational limits and the maintenance actions necessary to
preclude WFD prior to reaching those limits.
    There have been several instances of major structural failure in
flight due to fatigue. Therefore the potential for catastrophic
structural failure is significant. The FAA considers that this
rulemaking is essential to prevent future accidents or incidents. In
the past, industry practice for new airplane design certification has
been to develop some level of understanding of structural fatigue
characteristics up to the design service goal, but not beyond it. A
significant number of airplanes being operated currently have already
accumulated a number of flight cycles or flight hours greater than the
original design service goal. As the existing fleet continues to age,
the number of such airplanes will increase. Structural fatigue
characteristics of airplanes are understood only up to a certain point
consistent with the analyses performed and the amount of testing
accomplished. Operation beyond this point without further engineering
evaluation should not be allowed because, in the absence of
intervention, the likelihood of WFD increases with the airplane's time
in service.
2. Existing Programs Serve Purpose of Rule
    United Parcel Service, American Airlines, the CAA, ATA, Transport
Aircraft Technical Services Company, and Lynden Air Cargo recommended
that the proposed rule be withdrawn because existing programs serve the
same purpose as an inspection program for WFD. These commenters were
referring to existing elements of the Aging Aircraft Program, which
resulted from the Aloha Airlines accident. They include the following:
     Supplemental Structural Inspection Program,
     Mandatory Modification Program,
     Repair Assessment Program,
     Corrosion Prevention and Control Program.
    In addition, the FAA has issued airworthiness directives to address
aging airplane safety concerns. Lynden Air Cargo and Transport Aircraft
Technical Services Company said that the Aloha Airlines accident might
not have happened if proper accomplishment and FAA oversight of the
maintenance program had been performed.
    The FAA recognizes that the four elements of the Aging Aircraft
Program have some inherent ability to detect multiple site damage or
multiple element damage, but existing inspection methods cannot detect
such damage reliably. As acknowledged by some of the commenters, these
four elements were not specifically designed to address WFD; they were
designed as elements of an overall program to address structural
degradation on the pre-Amendment 25-45 airplanes over 75,000 pounds
maximum takeoff gross weight, commonly known as the ``elite eleven.''
\15\ This final rule, which specifically addresses WFD, is intended to
be the last element of the overall Aging Aircraft Program.
---------------------------------------------------------------------------

    \15\ The elite eleven are the original models considered under
the Aging Aircraft Program. These were airplanes over 75,000 pounds,
operating under part 121 or 129, that were at a greater risk for
age-related structural problems because they had high-time airplanes
that were near or over their design service goals. They include the
Airbus A300, Boeing 707/720, Boeing 727, certain Boeing 737s,
certain Boeing 747s, McDonald Douglas DC-8, DC-9/MD-80, and DC-10,
Lockheed L-1011, Fokker F-28, and the BAC 1-11.
---------------------------------------------------------------------------

    The AAWG, of which several of these commenters were members,
recognized the inadequacy of existing programs to address WFD when it
submitted its recommendation for FAA rulemaking on this subject in
2001. The recommendation included the following discussion:

    Regulatory and industry experts agree that, as the transport
airplane fleet continues to age, eventually WFD is inevitable. Long-
term reliance on existing maintenance programs, even those that
incorporate the latest mandatory changes introduced to combat aging,
creates an unacceptable risk of age-related accidents. Even with the
existing aging airplane program for large transports in place, WFD
can and does occur in the fleet. Therefore, the FAA has determined
that, at a certain point of an airplane's life, the existing aging
airplane program is not sufficient to ensure the continued
airworthiness of that fleet of airplanes.

    As discussed previously, the FAA has issued approximately 100
airworthiness directives to address unsafe conditions due to WFD on a
number of airplanes. Airworthiness directives are reactive in the sense
that the agency issues them only after determining that an unsafe
condition exists in one or more airplanes and is likely to exist or to
develop in other airplanes of the same type design. Typically, unsafe
conditions associated with WFD or its precursors have been discovered
largely by chance by people performing unrelated airplane maintenance.
    The FAA concludes that the agency cannot rely on existing
programs--including issuing airworthiness directives if the FAA learns
of an unsafe condition--to detect or address WFD that occurs in aging
airplanes. These programs do not obviate the need for a rule to prevent
catastrophic accidents due to WFD. This final rule specifically
addresses WFD and its precursors by requiring design approval holders
to evaluate their airplanes for WFD to prevent development of unsafe
conditions.
    Although maintenance program oversight can always be improved, the

fact remains that WFD is difficult, if not impossible, to detect. Small
cracks that can lead to WFD often cannot be detected until they
suddenly increase in size and ``link up,'' to cause catastrophic
damage. Dramatic crack growth can occur quite suddenly and quickly,
after being undetectable for long periods of time. That is why
maintenance inspections cannot be relied on to detect and repair such
cracking. Airplane maintenance programs include inspections that are
designed to detect obvious damage and irregularities. WFD, by its
nature, is usually hidden, and not readily detectable. Discovery of WFD
in some airplanes by mechanics has been a purely random occurrence,
where damage detected was the result of WFD that had progressed to the
point of failure of structural members. An example is discovery of WFD
on a Boeing 747, with adjacent frame cracking and separations. It was
detected because of loose rivets on the skin. Mechanics happened upon
the WFD damage by chance, because inspections had not uncovered any
problem. Improving a maintenance program by adding or modifying
inspections would not necessarily have the effect of improving
detection of WFD. In general, the only way to address WFD is by
modifying or replacing structure.
    The National Transportation Safety Board report stated the
following:

    It is probable that numerous small fatigue cracks in the lap
joint along S-10L joined to form a large crack (or cracks) similar
to the crack at S-10L that a passenger saw when boarding the
accident flight. The damage discovered on the accident airplane,
damage on other airplanes in the Aloha Airlines fleet, fatigue
striation growth rates, and the service history of the B-737 lap
joint disbond problem led the Safety Board to conclude that, at the
time of the accident, numerous fatigue cracks in the fuselage skin
lap joint along the S-10L linked up quickly to cause catastrophic
failure of the large section of the fuselage.

    The AAWG worked on various solutions to the safety problems
encountered by aging airplanes and was instrumental in developing the
four programs listed earlier in this document. However, they decided
that additional actions were needed to preclude WFD in airplanes, and
the steps they outlined included:
     Setting limits of validity of the maintenance program.
     Deciding whether WFD can be inspected for, and, if so, for
how long such inspections would be effective.
     Defining when WFD-susceptible structure should be modified
or replaced.
    Lynden Air Cargo stated that it supported an approach that used
airworthiness directives to address WFD-susceptible structural
components instead of an LOV approach for the entire airplane. Lynden
Air Cargo further stated that the unique design of the L-382G allows
for the whole airframe to be renewed by replacing WFD-susceptible
sections (e.g., center wing and outer wing).
    The FAA agrees with Lynden Air Cargo that WFD-susceptible structure
can be replaced when the engineering data determines it should be
replaced to preclude WFD. However, as airplanes age, other areas may
also need to be replaced. The only way to determine that is to evaluate
the engineering data (analyses, tests, service experience) for the
entire airplane. Without the LOV, the operational life of an airplane
is undefined. As a result, the list of areas to inspect, modify,
replace, or any combination of these may be extensive, since the data
would need to substantiate an indefinite life.
3. Divide Rule into Two
    FedEx, Northwest Airlines, Continental Airlines, NACA, and ATA
stated that the proposed draft final rule does not allow the public an
opportunity to comment on the LOVs that design approval holders propose
as compliance to part 26. They suggested the rule be divided into two
rules: one for design approval holders and one for operators. The
commenters noted that this two-step process would provide the public
the opportunity to comment on design approval holders' proposed LOVs.
Deferral of the operator rule would also allow for public comment on
the WFD maintenance actions at the same time LOVs are established. In
support of this approach, FedEx specifically argued that the
incremental costs for the part 26 work to design approval holders is
minimal, as design approval holders have confirmed in their comments to
this docket.
    The FAA has determined that complementary, concurrent requirements
for design approval holders and operators are necessary to achieve the
safety benefits of the proposed rule in a timely manner. Although
design approval holders would be required to develop LOVs for affected
airplanes under part 26, the safety benefit for this rulemaking
initiative is not met until operators incorporate LOVs and only operate
airplanes up to the point in time for which it can be shown that the
airplane will be free from WFD. Until design approval holders actually
comply with part 26, it's not possible to identify the precise LOV for
any particular airplane. However, operators have had adequate general
notice of the objectives of this rulemaking and the proposed methods
for achieving those objectives in the form of the design approval
holders' anticipated LOVs. Since the public meeting, both Boeing and
Airbus have provided revised information about where they anticipate
those LOVs will be set.
    If additional, multiple rulemakings are necessary to require
operators to incorporate LOVs into their maintenance programs, there is
a risk of airplanes exceeding LOVs before those rules become effective.
The FAA concludes that, to achieve our safety objectives, design
approval holders and operators must have a shared responsibility on
certain safety issues affecting the existing fleet. We also conclude,
from reviews such as the Commercial Airplane Certification Process
Study (March 2002), that we need to facilitate more effective
communication of safety information between design approval holders and
operators. As both technology and airworthiness issues become more
complex, certain fleet-wide safety issues require the FAA to implement
complementary requirements for design approval holders and operators,
when appropriate.

C. Concept of Operational Limits

    This final rule requires design approval holders to establish
limits of validity of the engineering data that supports the
maintenance program. The proposed rule would have required that design
approval holders establish initial operational limits beyond which
airplanes may not be operated. The initial operational limit would be
based on the demonstration of freedom from WFD up to that initial
operational limit.
    Several commenters supported the concept of early detection of WFD
for aging airplanes but opposed the requirement to establish initial
operational limits beyond which the airplanes could not be operated.
These commenters equated establishment of such limits with mandatory
retirement of airplanes and suggested that, instead, the FAA enhance
current maintenance programs and practices.
1. Requests for Requiring Maintenance Programs Instead
    An aircraft leasing and trading company named AWAS recommended that
an inspection-based maintenance program become mandatory as airplanes
reach their design service goal or their operational limit. Lynden Air
Cargo stated that there are better, less intrusive

methods to achieve early detection of WFD than the ``application of
onerous initial and extended operational limits.'' According to the
commenter, these methods include proper establishment, accomplishment,
and enforcement of current airplane maintenance programs, such as the
maintenance programs required by parts 121 and 135. Lynden Air Cargo
said it is continuously revising its Continuous Airworthiness
Maintenance Program to include a design approval holder inspection
program of Structural Significant Items and recommended structural
service bulletins.
    These commenters raise some of the same issues as did those who
opposed the rule altogether. They suggest that current programs for
aging airplanes or new maintenance programs to detect WFD--along with
issuance of airworthiness directives when WFD is detected--would
obviate the need for setting operational limits.
    As stated in the NPRM, the structural fatigue characteristics of
airplanes are only understood up to a point in time consistent with the
analyses performed and amount of testing accomplished. Structural
maintenance programs are designed with this in mind. The LOV is defined
as the limit of the engineering data that supports the structural
maintenance program and the current regulatory maintenance requirements
of parts 121 and 129 do not require that WFD be specifically addressed.
    Also as discussed previously, WFD cannot be detected reliably by
existing inspection methods. Therefore, the FAA considers that WFD in
existing airplanes needs to be proactively addressed by requiring
design approval holders to use relevant engineering data to project the
number of flight cycles or flight hours or both which the airplanes can
accumulate without incurring WFD. The engineering data may include the
evaluation and establishment of maintenance actions that address WFD.
2. Single Retirement Point for a Model
    The Modification and Replacement Parts Association (MARPA) opposed
a single, mandatory retirement age for airplanes because of the ``vast
differences possible between aircraft models, missions, and
maintenance.'' In a similar vein, a company named Safair, which is
based in South Africa, commented that the difference in structural
integrity of aging airframes lies in their use and abuse during their
lives and is largely dependent on the specific load factors to which
the airframe is subjected. Safair added that the proposed rule may be
based on inadequate technical evaluation of the actual operational
experience, considering the number of older aircraft that have been
safely operated well beyond the actual cycles listed in the proposed
rule.
    It is true that there may be differences between airplanes of the
same model which reflect differences in use and maintenance by
different operators. When manufacturers design an airplane, they
consider the various ways it may be used, and they develop a ``mission
profile'' to account for the different loads the airplane may be
subjected to that must be addressed in their design. In setting the
LOV, manufacturers will take this information into account, along with
service experience of the particular airplane model and fatigue test
evidence. The LOV must apply to an airplane model, because it is based
on analysis of the service experience of the entire fleet of affected
airplanes.
3. Potentially Adverse Effect on Safety
    Lynden Air Cargo, MARPA, and the airplane leasing and trading
company AWAS also suggested that mandatory retirement of airplanes may
have an adverse effect on safety which has not been considered by the
FAA. Specifically, AWAS envisioned that operators of airplanes
approaching their operational limit may perform minimal maintenance on
airframes to save money. MARPA said that mandatory retirement could
have a negative influence on the degree and timing of safety-related
investment, particularly as the aircraft nears its ``throwaway years.''
The owner and operator may not intend to be unsafe, suggested MARPA,
but the question ``Why invest now?'' will arise. A similar comment from
Lynden Air Cargo anticipated that operators ``are unlikely to apply the
same level of maintenance effort for an airplane 1,000 flight hours
from the scrap heap as one with 20,000 flight hours remaining.''
    Under existing operating rules, operators are responsible for
maintaining their airplanes in an airworthy condition. These
maintenance requirements apply equally to new and old airplanes. Even
without this final rule, operators have always planned to retire
airplanes, and service experience indicates that they generally
continue to maintain them safely up to that point. The purpose of this
final rule is to ensure that airplanes are retired before the point
where they can no longer be safely maintained with respect to WFD.

D. Change in Terminology (Initial Operational Limit to LOV)

1. Rationale for the Term LOV
    The NPRM proposed to establish an initial operational limit,
expressed in flight cycles, flight hours, or both, beyond which an
airplane could not be operated. Several commenters, including industry
representatives on the AAWG and Boeing, objected to this term and
suggested that instead the FAA refer to the ``limit of validity of the
engineering data that supports the maintenance program,'' or LOV. This
final rule uses the term LOV to express the point beyond which an
airplane cannot be operated (unless an extended LOV has been approved).
    In recommending that the FAA refer to the ``limit of validity of
the engineering data that supports the maintenance program,'' or LOV,
industry representatives on the AAWG stated that the term ``initial
operational limit'' implies that the use of an airplane is limited in
operation. According to the commenters, the limitation is actually
based on the engineering knowledge of the structural behavior of the
airplane model and is intended to ensure that required inspections are
sufficient to ensure safe operations until a certain number of flight
cycles or flight hours or both have been reached. The engineering data
that support such inspection requirements change with time due to
knowledge gained from in-service experience and additional testing.
    Boeing defined LOV as the point (usually measured in flight cycles)
in the structural life of an airplane where the engineering basis for
the maintenance actions contained in the Airworthiness Limitations
section of the Instructions for Continued Airworthiness is no longer a
valid predictor of future structural behavior.
    Our intent, as stated in the NPRM, was to ensure that large
transport category airplanes not be operated beyond their initial
operational limit, unless operators had incorporated an extended
operational limit and the service information necessary to support it
into their maintenance programs. Just as the structural fatigue
characteristics of airplanes are understood only up to a point
consistent with analyses performed, testing accomplished, and in-
service experience gained, the engineering data used to develop
inspections and modifications to preclude WFD is valid only to a
certain point.
    For these reasons, the FAA finds the term ``limit of validity''
more appropriate than the term ``initial operational limit''

in defining the point to which an airplane may be safely operated in
relation to WFD. The LOV is substantiated by test evidence and
analysis. This test evidence and analysis may be augmented by service
experience, or by service experience and teardown inspection results,
if available. The service experience and teardown inspection results
must be for high-time airplanes of similar structural design,
accounting for differences in operating conditions and procedures.
Additional engineering data would be necessary to support operation of
an airplane beyond the LOV. The legal effect of the terms initial
operational limit and limit of validity is the same. Therefore, this
final rule uses the term limit of validity instead of the term initial
operational limit.
2. Refer to the Structural Maintenance Program
    Airbus stated that the term limit of validity of the engineering
data that supports the maintenance program should be revised for
clarification. Because WFD is addressed by performing inspections or
modifications or replacements of airframe structure, the phrase
``maintenance program'' should be changed to ``structural maintenance
program.''
    The FAA agrees with Airbus and that change is reflected here.

E. Repairs, Alterations, and Modifications

    This final rule requires design approval holders to establish LOVs
for airplane models subject to this rule. However, it does not include
separate requirements to address WFD for repairs, alterations, and
modifications to those airplanes or to develop guidelines to address
repairs, alterations, or modifications. The proposed rule would have
required evaluation of repairs, alterations, and modifications of the
baseline structure of the airplane. The proposed rule would have also
required development of guidelines for repairs, alterations, and
modifications. Persons repairing or altering airplanes certified to
Sec.  25.571 at Amendment 25-96 or later are already required to show
the repair or alteration to be free from WFD up to the airplane's
design service goal. This requirement has not changed since adoption of
Amendment 25-96 in 1998.\16\
---------------------------------------------------------------------------

    \16\ March 31, 1998, 63 FR 15708.
---------------------------------------------------------------------------

1. Whether Repairs, Alterations, and Modifications Pose WFD Risks
    The Technical Document, discussed earlier, stated that the FAA, in
response to comments, had removed the proposed requirements for
repairs, alterations, and modifications. In response to the Technical
Document, Lynden Air Cargo, Northwest Airlines, ATA, Continental
Airlines, and FedEx stated that they support removal of requirements
for repairs, alterations, and modifications from the draft final rule.
These commenters stated that repairs, alterations, and modifications
present a reduced risk for WFD because they will be surveyed and
assessed under the Aging Airplane Safety Final Rule and the Damage
Tolerance Data for Repairs and Alterations Rule (hereafter referred to
as the Damage Tolerance Data Rule).\17\ Commenters often used the term
``Aging Airplane Safety Rule'' to refer to the Damage Tolerance Data
Rule or the Aging Airplane Safety Final Rule, or both. In instances
where this occurs, to avoid confusion, the name of the specific rule
has been inserted in parentheses.
---------------------------------------------------------------------------

    \17\ 72 FR 70486, December 12, 2007.
---------------------------------------------------------------------------

    These commenters expressed the belief that a new WFD requirement
for repairs, alterations, and modifications is unnecessary because of
these other requirements, which are already in place. Lynden Air Cargo
stated that, although it supports removal of requirements to evaluate
repairs, alterations, and modifications for WFD because the Damage
Tolerance Data Rule already adequately addresses them, it does not
understand how each design approval holder is going to establish the
validity of its maintenance program without validating the repairs and
alterations it has established under that program. Northwest Airlines
said that it supported the conclusion of the AAWG that the costs of
including repairs, alterations, and modifications in the rule
outweighed the benefits that such a requirement would have.
    Boeing, Airbus, and the European Aviation Safety Agency (EASA) said
the FAA should reconsider its decision to remove from the rule the
requirements for evaluating certain repairs, alterations, and
modifications. All three commenters stated that removing those
requirements could affect safety because certain alterations could
affect the LOV and the structural maintenance program that supports the
LOV. An example of an alteration that could affect the LOV and
structural maintenance program, the commenter maintained, is one that
would cause a global loading increase, such as an alteration allowing a
higher cabin differential pressure. Airbus stated that, although the
Changed Product Rule (14 CFR 21.101) may address future alterations and
modifications, it does not cover existing ones.
    Boeing recommended that the FAA revise subpart E of part 26, the
Damage Tolerance Data Rule, for repairs and alterations, and Sec. Sec.
121.1109 and 129.109, the Aging Airplane Safety Final Rule, to include
requirements for evaluating repairs, alterations, and modifications for
WFD. Boeing's recommendation contains two parts. First, it requests
that the FAA extend the compliance date for both rules by 18 months
after the effective date of the WFD rule. Second, it says the FAA
should incorporate the 2007 ARAC recommendations on evaluating repairs,
alterations, and modifications into those rules.
    Boeing, Airbus, EASA, and the Allied Pilots Association (APA)
stated that certain repairs, alterations, and modifications need to be
evaluated for WFD. APA stated that eliminating the requirement to
evaluate WFD associated with most repairs, alterations and
modifications from the final rule is risky, because many high-time
airplanes fall into this category and will not have any current
analysis done on their modified airframes.
    In its final report to ARAC concerning Task No. 3, the AAWG stated
that it has reviewed the accident record and has observed that--while
there is a technical possibility of a WFD-related accident involving a
repair or alteration--there are no recorded accidents attributed to WFD
occurring in properly-installed repairs or alterations. The group added
that a review of certain repairs, alterations, and modifications is
necessary, because some of them have the potential to develop WFD.
    The FAA agrees with the commenters that some repairs, alterations,
and modifications may pose a risk of developing WFD. However, the risk
appears to be less than that for baseline airplane structure because
all adverse service experience to date has been limited to baseline
airplane structure. Type certificate holders design repairs,
alterations, and modifications using the same design philosophies and
load cases as for baseline airplane structure. As they do with the
baseline airplane structure, type certificate holders re-evaluate their
repairs, alterations, and modifications as service experience is
gained. Therefore, these repairs, alterations, and modifications should
be acceptable up to the LOV.
    The repairs, alterations, and modifications developed by persons
other than type certificate holders may present a slightly greater
risk, because those persons typically do not have the

type certificate holder's data or expertise. Although those repairs,
alterations, and modifications may pose a higher risk for developing
WFD, there are no recorded accidents attributed to WFD occurring in
these repairs, alterations, and modifications. Nor have there been a
significant number of findings of multiple site or element damage
associated with them.
    The FAA is funding additional research at the agency's Technical
Center to get a better understanding of these risks and how to address
them.\18\ This research includes conducting a field survey of repairs,
alterations, and modifications on high-time airplanes to document the
existing configurations. The research also includes removing some
repairs, alterations, and modifications to further evaluate their
condition. In some cases, testing of particular structure may be
performed to obtain data for calibration and validation of
methodologies for predicting WFD. If this research demonstrates that
additional actions are needed to address risks for repairs,
alterations, and modifications, the FAA will consider further
rulemaking.
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    \18\ Task Area II, Project I, Survey of Transport Airplane
Structural Repairs and Alterations, Statement of Work 064070723-1,
dated October 23, 2007; FAA William J. Hughes Technical Center,
Atlantic City, New Jersey. The Scope of Work for this research is
available in the docket for this rule.
---------------------------------------------------------------------------

    Based on the above, the FAA has re-evaluated the NPRM and
determined that the proposed requirements to address repairs,
alterations, and modifications should be removed from the final rule.
2. Relationship to Damage Tolerance Requirements (Sec.  25.571)
a. Pre-Amendment 25-96 Airplanes
    The FAA received numerous comments requesting that the proposed
requirements for repairs, alterations, and modifications in the NPRM
and the related proposed requirements of the Damage Tolerance Data Rule
NPRM \19\ be combined and aligned in a single rulemaking. These
commenters included industry representatives who are members of the
AAWG,\20\ the ATA, Boeing, Airbus, Cessna, and American Airlines. They
were concerned that separate requirements for repairs, alterations, and
modifications in the Aging Airplane Safety Rule (the Damage Tolerance
Data Rule) and the NPRM for this rule would require duplicative
efforts.
---------------------------------------------------------------------------

    \19\ 71 FR 20574, April 21, 2006.
    \20\ The companies represented are Boeing, Airbus, American
Airlines, Northwest Airlines, US Airways, United Parcel Service,
FedEx, ABX (previously known as Airborne Express), Continental
Airlines, Japan Air Lines, United Airlines, and British Airways.
Although the comments are not representative of the views of other
members of the AAWG, including national authorities, for simplicity
the source of these comments is identified hereafter as ``industry
representatives on the AAWG.''
---------------------------------------------------------------------------

    Given the proposed timeframes for compliance and the shortage of
qualified industry resources to perform the required analyses, the
commenters suggested that separate requirements are unnecessary and
could not be accomplished within the proposed compliance times. The
industry representatives on the AAWG stated that there are fewer than
50 persons in industry who are qualified to perform damage tolerance
and WFD assessments and most of them are employed by the major design
approval holders.
    The AAWG stated in its final report on Task 3 that existing
alterations and repairs would receive a damage tolerance assessment
under the Aging Airplane Safety Final Rule (developed under the Damage
Tolerance Data Rule).\21\ The report indicated that this should provide
an improved level of safety because repairs, alterations, and
modifications would be surveyed and evaluated. The AAWG recommended
that repairs not be re-reviewed for WFD if they had already been
reviewed for damage tolerance.
---------------------------------------------------------------------------

    \21\ The Damage Tolerance Data Rule is Amendment 26-1 and the
Aging Airplane Safety Final Rule is Amendment 121-337 to the CFR.
---------------------------------------------------------------------------

    Since adoption of Amendment 25-45 in 1978,\22\ the damage tolerance
provisions of Sec.  25.571 have required consideration of damage at
multiple sites, the precursor for WFD. While recent efforts on damage
tolerance have focused on localized cracking, in most cases the design
approval holders have addressed multiple site damage in their design of
both baseline structure and of repairs, alterations, and modifications,
even if indirectly. As a result, the FAA agrees that damage tolerance
assessment of repairs, alterations, and modifications should provide
some degree of mitigation of risk, even though the focus of the
assessments has been on developing inspections, and inspections cannot
reliably detect WFD.
---------------------------------------------------------------------------

    \22\ October 5, 1978, 43 FR 46238.
---------------------------------------------------------------------------

    The FAA recognizes the scarcity of expert resources in the area of
damage tolerance and WFD. By removing requirements to address repairs,
alterations, and modifications from this final rule, the agency is
allowing those resources to be focused on meeting the compliance dates
for the Damage Tolerance Data Rule and addressing WFD in baseline
airplane structure, where the risks are greater. The FAA has recently
been providing training to its designees and to industry members
regarding compliance with Sec.  25.571 and the Damage Tolerance Data
and Aging Airplane Safety Final Rules. In that training, we have
provided additional guidance on performing a damage-tolerance
evaluation to assess damage at multiple sites. Adoption of this final
rule should also result in significant commitments from industry to
develop resources with this expertise.
b. Airplanes Certified to Amendment 25-96 or Later
    The Technical Document described the agency's intent to remove
requirements for evaluating repairs, alterations, and modifications for
WFD. Airbus requested that the FAA clarify that today's final rule will
not negate those requirements for persons making repairs, alterations,
or modifications to their airplanes certified to Amendment 25-96. As
another option, Airbus requested that the WFD rule applicability not
include Amendment 25-96 or later airplanes, because those airplanes are
already certified to WFD requirements.
    The FAA agrees that clarification is necessary for airplanes
certified to Sec.  25.571, Amendment 25-96 or later. Amendment 25-96
revised Sec.  25.571 to require that full-scale fatigue test evidence
\23\ be developed to show freedom from WFD up to an airplane model's
design service goal. Also, any person performing a repair, alteration,
or modification to those airplanes must address WFD for the repair,
alteration, or modification, and show compliance with those
requirements. The newest airplanes, like the Airbus A-380, are
certified to Amendment 25-96, but most other airplanes operating today
are certified to an Amendment level prior to 25-96, and thus would not
be required to comply with those WFD requirements. They would, however,
be required to comply with the requirements of the Damage Tolerance
Data Rule.
---------------------------------------------------------------------------

    \23\ Test evidence comprises full fatigue testing up to at least
two times the proposed design service goal and may include, for
derivative airplanes, analysis, service experience, or service
experience and results of tear-down inspections of high-time
airplanes, if available.
---------------------------------------------------------------------------

    For today's rule, Sec.  25.571 and Appendix H to Part 25 require
that applicants show an airplane model to be free from WFD up to the
LOV instead of to the design service goal. Unlike Amendment 25-96,
which did not require the design service goal to be included in the
Airworthiness Limitations section, this final rule mandates LOV
placement in the Airworthiness Limitations section. The

requirements of today's rule are similar to those of Amendment 25-96.
Any person who repairs, alters, or modifies any airplane certified
under today's rule must show that repair, alteration, or modification
to be free from WFD up to the airplane's LOV.
3. Guidelines for Repairs, Alterations, and Modifications
    Industry representatives on the AAWG and several other commenters
recommended that proposed Sec.  25.1807(g), along with Sec. Sec.
25.1809 and 25.1813, be withheld until the working group completed
relevant taskings from ARAC. In particular, the commenters stated that
the guidelines in Sec.  25.1807(g)(3) could not be technically
accomplished because the design approval holders do not have the data
or knowledge necessary to provide guidance for all possible repair or
alteration configurations.
    Boeing and Airbus commented that they could support WFD guidelines
that are limited in scope. The guidelines should identify structure
prone to development of WFD and provide processes and procedures by
which operators can access valid data for complying with the rule. But
these commenters said that such guidelines should not attempt to
describe methods for determining when WFD is likely to occur or for
developing service information to preclude WFD. The commenters objected
to providing guidelines as defined under proposed Sec.  25.1807(g)(3)
because design approval holders would have no control over how the
guidelines would be used. They further stated that such guidelines
could expose design approval holders to potential liability if they are
applied incorrectly.
    When the FAA issued the NPRM, the agency was relying on the AAWG,
under an ARAC tasking, to identify a means of compliance that would be
practical for both design approval holders and operators. Although ARAC
did not provide detailed recommendations for developing guidelines, it
did provide a general approach.
    Requirements pertaining to repairs, alterations, and modifications
were included in the proposed rule to ensure that they would not
degrade the level of safety provided by the design approval holder's
compliance with the rule. Although the FAA has removed these proposed
requirements from the final rule, the agency is engaged with industry
in a number of activities to address these concerns.
    For repairs, the AAWG recommended in its final report on Task 3
that each design approval holder update its publications (e.g.,
structural repair manuals, service bulletins, and repair assessment
guidelines) to include instructions for inspecting and, if necessary,
modifying structure susceptible to WFD. This update should occur by the
time the design approval holder has established the LOV for an airplane
model. The AAWG recommended that design approval holders update their
service documents for WFD at the same time they are revising these
documents for the Aging Airplane Safety Rule (the Damage Tolerance Data
Rule) if the WFD data are available. The FAA expects that design
approval holders will fulfill this recommendation. To the extent that
design approval holders update their service documents for WFD,
operators, when complying with requirements of the Aging Airplane
Safety Final Rule by using those updated service documents for repairs,
will be addressing the WFD risks for these repairs. In addition, Sec.
25.571 already requires consideration of the potential for WFD for
repairs to airplanes certified to Amendment 25-96 or later.
    For alterations, the AAWG surveyed 642 supplemental type
certificates. Out of the 642, they identified only 14 alterations and
modifications that would require assessment for WFD. Based on this,
they suggested that the FAA review these types of existing alterations
to determine whether any action is necessary. The Task 3 report did not
specifically recommend that design approval holders address their
alterations for WFD. However, recent meetings conducted by certain
design approval holders indicate that they intend to address their own
alterations and modifications for WFD in addition to repairs in the
Task 4 \24\ structures task group activity. The majority of transport
airplanes operating in the U.S. that are subject to this final rule
will be addressed by these design approval holders. We anticipate that
other design approval holders will also review their alterations and
modifications for WFD.
---------------------------------------------------------------------------

    \24\ Task 4.--Model Specific Programs.
    Oversee the Structural Task Group (STG) activities that will be
coordinated for each applicable airplane model by the respective
type certificate holders and part 121 and 129 certificate holders.
These STG activities will involve the development of model specific
approaches for compliance with Sec. Sec.  121.370a and 129.16 under
the guidance material supplied in Task 1. As part of this tasking,
the AAWG will identify those airplane models that do not have an
STG, and will assess the need to form one (based on industry
benefit). For those airplane models that will need to form an STG,
the AAWG will initiate the coordination required to form the STG
with the respective type certificate holder and/or part 121 and 129
certificate holders. In addition, the AAWG will support
implementation of the action plan to address recommendations made in
tasks 2 and 3 as determined necessary by the ARAC, Transport
Airplane and Engine Issues Group, and concurred with by the FAA.
---------------------------------------------------------------------------

    While these activities will not address alterations and
modifications developed by other persons (including supplemental type
certificate holders), as stated earlier, the FAA is conducting research
to get a better understanding of the risks that repairs, alterations,
and modifications may pose for developing WFD and whether they need to
be assessed for WFD. If the FAA determines that the risks are
unacceptable, the FAA will consider further rulemaking to mandate
assessments.
    This research may also assist in refining means of compliance with
Sec.  25.571, at Amendment 25-96 or later, for repairs, alterations,
and modifications. For airplanes certified to Amendment 25-96 or later,
persons who repair or alter the airplane must address WFD. This has
typically been done by showing the repair or alteration to be adequate
up to the airplane's design service goal. With adoption of this final
rule, repairs, alterations, and modifications to airplanes designed in
the future will have to be shown to be free from WFD up to the
airplane's LOV.
4. Rely on the Changed Product Rule
    Northwest Airlines stated that it supports the FAA in removing WFD
requirements for most repairs, alterations, and modifications, but
requested that references to future alterations be removed from the
final rule and addressed by the Changed Product Rule, 14 CFR 21.101.
The Changed Product Rule requires that significant changes to type-
certificated products comply with the latest amendments of the
airworthiness standards unless one of the stated exceptions applies. In
support of its position, Northwest Airlines cited concerns published by
the AAWG about industry not having the resources or sufficient FAA
guidance to accomplish WFD analysis for the expected quantities of
supplemental type certificate alterations.
    Similarly, ATA stated that in view of their coverage under the
Changed Product Rule, the FAA should exclude future supplemental type
certificate applications from the applicability of this rule. Northwest
Airlines and ATA requested that the FAA use the Changed Product Rule to
regulate which future alterations would need to be evaluated for WFD.
    The Changed Product Rule would require applicants for future
alterations and modifications to include the latest

amendment of part 25 for Sec.  25.571 in the certification basis for
the proposed alteration or modification if the change is considered
significant. For the purposes of today's rule, applicants would use the
examples of significant changes identified in AC 21.101-1. For
transport category airplanes, that AC may be used as a starting point
for determining whether alterations or modifications are significant
and must be evaluated to the latest amendment of Sec.  25.571. Examples
of significant changes from AC 21.101-1 that would be required to be
assessed for WFD include passenger-to-cargo conversions, gross weight
increases, and cabin pressure increases. We have revised AC 25.571-1X
to provide additional guidance for identifying whether a change, or
structure affected by the change, requires an assessment for WFD.
Affected structure can be new structure installed by the change or
existing structure modified by a change. Structure may be affected if
it is physically changed or if there is a change or redistribution of
internal loads. The long-term result will be that a changed product
will have a certification basis that provides a similar level of safety
to that provided by the certification basis of a new type certificate
for the same product.

F. Compliance Times for Developing and Implementing LOVs

    For existing airplanes, this final rule uses a phased approach for
establishing LOVs and divides the compliance dates for holders of
design approvals and applicable airplane models into three groups. The
NPRM proposed that design approval holders establish LOVs for all
affected airplanes by one specific date. The proposed rule did not
account for the age of airplanes within a model.
    For this final rule, the compliance dates for the different
airplane groups are identified based on their certification basis
relative to Sec.  25.571 and are as follows:
     Group I: Pre-Amendment 25-45 airplanes (those with a
certification basis dating before 1978). The Boeing 727 and the Airbus
A300 are examples of pre-Amendment 25-45 airplanes.
     Group II: Amendment 25-45 up to but not including
Amendment 25-96 airplanes (those with a certification basis dating from
1978 to 1998). This group of airplanes would include the Boeing 757 and
767 and the Airbus A318.
     Group III: Amendment 25-96 and later airplanes (those with
a certification basis dating from 1998 to the present). The Airbus A380
and the Embraer ERJ 170 and 190 are among the airplanes that have this
certification basis.
    Table 1 in Sec.  26.21 indicates the compliance times for these
various groups of airplanes. They are 18, 48, and 60 months,
respectively. These compliance times apply to all existing versions of
these airplane models.
    For airplane models for which a type certificate is approved as of
the effective date, but which are not specifically named in Table 1 of
Sec.  26.21, an LOV must be established within 60 months after the
effective date of the rule. In Table 1 of Sec.  26.21, those airplanes
would fall under the category of ``All Other Airplane Models Listed on
a Type Certificate as of January 14, 2011.''
    For type certificate or amended type certificate approvals that are
pending as of this final rule's effective date, and for future
amendments to existing or pending type certificates, this final rule
requires the applicants to establish an LOV by the latest of the
following dates:
     Within 60 months after the effective date of the rule,
     The date a certificate is issued, or
     The date specified in the plan approved under Sec.
25.571(b) indicating when the full-scale fatigue testing and evaluation
will be complete.
    This final rule requires operators to incorporate the Airworthiness
Limitations section that includes the LOV into their maintenance
program within 30, 60, or 72 months after the effective date for Groups
I, II, and III, respectively. Table 1 in Sec. Sec.  121.1115 and
129.115 gives the compliance times for operators.
    This final rule also requires operators of affected airplanes whose
applications for type certificates or amended type certificates are
pending as of the effective date, or whose application for a type
certificate or amended type certificate is made after the effective
date of the rule, to incorporate the Airworthiness Limitations section
that includes the LOV into their maintenance program at the latest of
the following compliance times:
     Within 72 months after the effective date of the rule,
     Within 12 months after the LOV is approved, or
     Before operating the airplane.
    In Table 1 of Sec.  121.1115 and Sec.  129.115, those airplanes
would fall under the category of ``All Other Airplane Models (TCs and
Amended TCs) not Listed in Table 2.''
    Amended or supplemental type certificates that change the maximum
takeoff gross weight are grouped separately. Holders of amended type
certificates or supplemental type certificates that increase the
maximum takeoff gross weight to greater than 75,000 pounds, regardless
of whether such change was applied for before or after the effective
date of the rule, must comply within 18 months after the effective date
of the rule. Applicants for this type of design change approval whose
applications are either pending as of the effective date of this final
rule or submitted after the effective date must comply by the latest of
the following dates:
     Within 18 months after the effective date of the rule,
     The date the approval is issued, or
     The date specified in the plan approved under Sec.
25.571(b) indicating when the full-scale fatigue testing and evaluation
will be complete.
    Applicants for amended type certificates or supplemental type
certificates applied for after the effective date of the rule that
decrease the maximum takeoff gross weight to 75,000 pounds or less must
also comply by the latest of the following dates:
     Within 18 months after the effective date of the rule,
     The date the certificate is issued, or
     The date specified in the plan approved under Sec.
25.571(b) indicating when the full-scale fatigue testing and evaluation
will be complete.
    This final rule requires operators of airplanes whose maximum
takeoff gross weight was decreased to 75,000 pounds or below after the
effective date of the rule or increased to greater than 75,000 pounds
at any time by an amended type certificate or supplemental type
certificate to incorporate the Airworthiness Limitations section that
includes the LOV into their maintenance program by the latest of the
following compliance times:
     Within 30 months after the effective date of the rule,
     Within 12 months after the LOV is approved, or
     Before operating the airplane.
    Those airplanes would fall under the category of ``Maximum Takeoff
Gross Weight Changes'' in Table 1 of Sec.  121.1115 and Sec.  129.115.
    Under 14 CFR 91.403(c), no person may operate an airplane unless
that person is in compliance with applicable airworthiness limitations.
By requiring operators to incorporate the Airworthiness Limitations
Section containing the LOV into the maintenance program, this final
rule makes those LOVs applicable to the affected airplanes, and Sec.
91.403(c) requires operators to comply with them.
    Operators of airplanes whose type certificate was pending approval
as of the effective date of the rule will be required to include one of
the following

airworthiness limitations in their maintenance program:
     The LOV that has been specified in the Airworthiness
Limitations section of the Instructions for Continued Airworthiness; or
     If the LOV has not yet been established, a number equal to
\1/2\ the number of cycles accumulated on the fatigue test article if a
type certificate is issued prior to completion of full-scale fatigue
testing.
    Comments received during the NPRM comment period were responding to
the one specific compliance date published in the NPRM. Comments
received during the comment period for the Technical Document, which
described changes that had occurred to the rule since it had been
proposed in the NPRM, were in response to the phased compliance dates
published in the Technical Document, which are the dates cited in
today's rule.
1. NPRM Compliance Date
    Commenters--including industry representatives on the AAWG, Cessna,
Continental Airlines, Embraer, AWAS, the CAA, American Airlines,
Boeing, Airbus, and FedEx--objected to the proposed compliance date of
December 18, 2007, for both technical and practical reasons. Several
commenters stated that hard compliance dates and an expected final rule
issuance in December 2006 would leave design approval holders with less
than 12 months to comply with the subpart I requirements (now part 26).
These commenters requested that the FAA revise the compliance dates to
represent a number of months after the effective date of the rule
rather than a hard date. This approach would prevent the FAA's schedule
for issuing the final rule from affecting compliance by design approval
holders.
    We have revised the compliance dates in this final rule to specify
that persons must comply either by a date determined as a specified
number of months after the effective date of the final rule or (for
applicants) by the date of approval of the related certificate.
2. When to Set LOVs for Existing Airplanes
    Industry representatives on the AAWG, Boeing, Continental Airlines,
Northwest Airlines, ATA, Lynden Air Cargo, and FedEx stated that there
should be a phased approach to setting LOVs, with the oldest airplane
models being addressed first. The industry representatives on the AAWG
suggested that existing airplane models subject to the rule be divided
into two groups: (1) Pre-Amendment 25-45 airplanes and (2) airplanes
certified to Amendment 25-45 or later. The commenters stated that
performing WFD evaluations on airplane models before the high-time
airplane reaches its design service goal, as proposed in Sec.  25.1807
(now Sec.  26.21) and as specified in the Technical Document, would not
significantly increase operational safety. This is because WFD is
typically not a concern until later in an airplane's operational life.
As discussed earlier, these commenters objected to the proposed
compliance date of December 18, 2007. Commenters also objected to the
compliance times identified in the Technical Document--that is, 18
months for pre-Amendment 25-45 airplanes, 48 months for Amendment 25-45
up to but not including Amendment 25-96 airplanes, and 60 months for
Amendment 25-96 airplanes.
    Boeing said that the final rule should provide the greatest amount
of time for design approval holders to develop LOVs, so that LOVs
provide the greatest flexibility for the fleet. Several commenters
argued that requiring compliance prior to or concurrent with the Aging
Airplane Safety Rule (Damage Tolerance Data Rule) would not be
practical because of limited industry and FAA resources. In addition,
Boeing and Northwest Airlines argued that establishing an LOV for an
airplane model before significant service experience had been
accumulated would result in an erroneous LOV.
    We agree that it makes sense to have compliance dates for
establishing LOVs for existing airplanes based on the relative safety
risk (i.e., addressing the oldest airplanes first) and on available
resources. However, the agency does not agree that ``early''
establishment of an LOV will result in an ``erroneous'' LOV. Setting an
LOV without benefit of significant service experience might result in
an LOV that sets the limit at a lower number of flight hours or flight
cycles than one that benefits from significant service experience, but
it would be incorrect to characterize it as ``erroneous.'' This is
because the LOV is a function of the fatigue knowledge base available
at the time it is established.
a. Pre-Amendment 25-45 Airplanes
    Industry representatives on the AAWG, Boeing, Continental Airlines,
Northwest Airlines, ATA, and FedEx pointed out that the first group of
airplanes is collectively at the highest risk because of cumulative
time in service and the limited fatigue test data available for these
models. They recommended that the compliance date for the first group
of airplanes should be by a certain date after the effective date of
the rule. The AAWG's final report \25\ recommends that LOVs be
established for the first group of airplanes by June 2009, or 18 months
prior to the operator's compliance date for the final rule, whichever
occurs later. This would also provide sufficient time for Structures
Task Groups \26\ including operators of affected airplanes, to
participate in establishing the LOVs. A later Boeing comment, however,
requested that the compliance dates for those airplanes be 36 months,
instead of 18 months (as stated in the technical document), from the
effective date of the rule. Boeing stated that this additional time
would allow them to have the FAA review and accept the Boeing
proprietary LOV methodology, prepare LOV fleet proposals, and
coordinate them within Boeing and with operators before submitting them
to the FAA for review and approval.
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    \25\ Aviation Rulemaking Advisory Committee; Transport Airplane
and Engine Issues--New Task, dated April 11, 2007.
    \26\ A Structures Task Group is a model-specific group that
consists of type certificate holders and operators responsible for
the development of aging airplane model-specific programs. It also
includes regulatory authorities which approve and monitor those
programs.
---------------------------------------------------------------------------

    The FAA agrees that pre-Amendment 25-45 airplanes should be
addressed first because they are among the oldest airplanes and at the
highest risk for developing WFD. In fact, most high-time pre-Amendment
25-45 airplanes have exceeded their design service goals. While the FAA
understands that LOVs have been developed for a number of affected
airplanes, the agency also understands that not all design approval
holders have begun or completed this activity on all affected models.
The FAA recognizes the benefits of allowing Structures Task Groups to
participate in setting LOVs. Therefore, the FAA has determined that the
compliance period for the oldest affected airplanes should be increased
to 18 months to allow sufficient time for design approval holders to
show compliance with today's rule. This increases by six months the
amount of time design approval holders have to comply over what was
anticipated in the NPRM. The 2007 AAWG Task 3 Report further supports
the compliance date of 18 months. In its report, the AAWG stated that
most of the work for the pre-Amendment 25-45 airplanes has already been
completed. As a result, we do not concur with the commenter that 36
months is necessary to establish LOVs.

b. Airplanes Certified to Amendment 25-45 or Later
    For the second group of airplanes (certified to Amendment 25-45 or
later), industry representatives on the AAWG, Boeing, Continental
Airlines, Northwest Airlines, ATA, and FedEx recommended setting a
compliance date for design approval holders to establish LOVs that are
tied to both the design service goal and the cumulative time on the
high-time airplanes of that model. Specifically, the industry
representatives on the AAWG proposed that within 180 days of the
effective date of the rule, the type certificate holders provide design
service goals for all affected airplane models to the FAA for approval.
Once approved, these design service goals would be placed in an
appropriate certification document. Other commenters--including Cessna,
Continental Airlines, Embraer, AWAS, the CAA, American Airlines,
Boeing, Airbus, and FedEx--agreed with industry representatives on the
AAWG that the compliance date for setting LOVs should take into account
both the design service goal and the cumulative time on the high-time
airplanes of that model.
    The industry representatives on the AAWG proposed that the design
approval holder prepare a compliance plan with a binding schedule for a
WFD evaluation when the high-time airplane reaches a point five years
from its design service goal. The AAWG industry representatives
suggested that a means of determining this time should be included in
AC 120-YY. FedEx and Lynden Air Cargo suggested that the FAA use the
design service goals that are being developed under the Damage
Tolerance Data Rule to establish compliance dates for establishing LOVs
and associated WFD maintenance actions. The commenters said that if no
design service goal or design service objective exists, the LOV should
be established when the high-time airplane of a particular model
reaches 20 years of age.
    In contrast, United Parcel Service and Technical Data Analysis,
Inc. supported establishing LOVs for all affected airplane models as
soon as possible, because of the uncertainty associated with estimating
future operating costs and the length of time that airplanes can be
operated.
    The WFD risk for these newer airplane models is lower than for the
pre-Amendment 25-45 airplanes because these airplanes are generally
younger and have been certified to damage tolerance requirements.
Therefore, the FAA agrees with the industry representatives on the AAWG
and other commenters that the compliance times can be longer for these
airplanes. On the other hand, the proposal of the AAWG industry
representatives would add a level of complexity and uncertainty to
determining compliance times that the FAA considers unnecessary and
inappropriate and that would make operators' long-term planning
difficult.
    Therefore, as discussed earlier, to accommodate the need for a
longer compliance time for these airplanes, this final rule creates
three groups of airplane models for determining compliance dates.
     Group I--Pre-Amendment 25-45 (1978) airplanes.
     Group II--Airplanes certified to the requirements of Sec.
25.571, Amendment 25-45, up to but not including Amendment 25-96
(1998).
     Group III--Airplanes certified to requirements of Sec.
25.571, Amendment 25-96 or later.
    Group II airplane models were all subjected to full-scale fatigue
test programs. In addition, all the models in this group have been in
service for a period of time. There should, therefore, be a reasonable
knowledge base readily available on which to base an LOV. Today's rule
requires establishment of an LOV for all these models within 48 months
of the effective date of the rule, as indicated in Table 1 of Sec.
26.21. This would allow design approval holders to schedule development
of these LOVs after the more urgent development of LOVs for pre-
Amendment 25-45 airplanes, so project schedules would not conflict. At
the same time, this compliance time would ensure that LOVs are
established long before the high-time airplanes of these models would
reach their anticipated LOVs.
    Design approval holders of those models in Group III have had to
demonstrate or will have to demonstrate with sufficient full-scale test
evidence that WFD will not occur within the design service goal of the
airplane. Therefore, the design service goal would be a valid LOV that
is based on the knowledge base considered. However, because these
airplanes have not accumulated much time in service, there is less
urgency in establishing an LOV. As a result, the final rule provides 60
months after the effective date of the rule to establish an LOV for
these models. (See Table 1 of Sec.  26.21.) This provides time to re-
evaluate the fatigue data and to establish an LOV which may exceed the
design service goal. Extending the compliance date for Group III
airplanes beyond the compliance date for Group II airplanes reduces the
resource concerns about developing LOVs for multiple airplane models at
the same time.
    Table 1 of Sec.  26.21 includes a compliance date for airplanes
that do not appear in the table but may have had a type certificate
approved by the effective date. These have a compliance period of 60
months. Some type certificates are pending and may be approved shortly.
This last row of the table is meant to capture any additional airplanes
that fit the applicability criteria of Sec.  26.21(a).
    Table 1 of Sec.  26.21 is used to call out existing airplanes and
assign compliance dates. Holders of type certificates for these models
must comply with Sec.  26.21(c)(1). The remainder of Sec.  26.21(c)
specifies additional people who must comply.
    Under today's rule, the compliance times specified in Sec.
26.21(c) for when applicants must establish an LOV include the date
specified in the applicant's plan for completion of the full-scale
fatigue testing and analyses of the testing to demonstrate compliance
with Sec.  25.571(b).\27\ All applicants who must comply with Sec.
26.21 may use this date as one option for compliance.
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    \27\ Under Sec.  21.17, these applicants are subject to Sec.
25.571 at Amendment 25-96. In addition to this certification basis,
they are subject to the requirements of this final rule.
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    Applicants who have the same compliance times and the option to use
the date specified in the Sec.  25.571(b) plan are:
     Applicants for type certificates for which the application
is pending as of the effective date.
     Applicants for amendments to type certificates (with the
exception of those that change the weight of the airplane).
    All of these applicants are required to establish LOVs at the
latest of the following dates:
     The date the type certificate or amended type certificate
is issued,
     Within 60 months after the effective date of the rule, or
     The date specified in the plan approved under Sec.
25.571(b) indicating when the full-scale fatigue testing and evaluation
will be complete.
    Among these applicants, WFD is of less immediate concern because
their high-time airplanes will have accumulated relatively few flight
cycles or flight hours by the compliance date. Establishing LOVs early
in the service life of these airplanes will assist operators in their
long-term planning. This approach also serves as a transition to Sec.
25.571 as amended by this final rule, which requires establishing LOVs
as part of initial type certification.

    Maximum takeoff gross weight changes to an airplane are treated
separately in this rule. Holders of either supplemental type
certificates or amendments to type certificates that increase maximum
takeoff gross weights from 75,000 pounds or less to greater than 75,000
pounds must comply no later than 18 months after the effective date.
    Applicants for supplemental type certificates or amended type
certificates that increase the maximum takeoff gross weight to greater
than 75,000 pounds must comply by the latest of the following:
     Within 18 months after the effective date of the rule,
     The date the certificate is issued, or
     The date specified in the plan approved under Sec.
25.571(b) indicating when the full-scale fatigue testing and evaluation
will be complete.
    The option of 18 months after the effective date as a compliance
choice for this group represents a six-month increase in the time to
comply over what was originally proposed. We based these compliance
dates on the length of time given for design approval holders of Group
I airplanes to comply.
    The NPRM did not specify a compliance time for applicants for
design change approvals that, after the effective date of the rule,
decrease the maximum takeoff gross weight to 75,000 pounds or less.
This is because the applicability provision in the NPRM included
airplanes with maximum takeoff gross weights exceeding 75,000 pounds,
as approved during the original type certification. By referencing the
capacity resulting from original type certification, the NPRM required
applicants to establish LOVs for design change approvals that, after
the effective date of the rule, decrease the maximum takeoff gross
weight to 75,000 pounds or less. Although not explicitly stated in the
NPRM, the LOV for those airplanes is required to be established by the
compliance date for the original type certification or, in the case of
applicants, by the date the approval of the design change has been
issued. Because the NPRM was not clear about when those applicants must
comply, the FAA has revised today's rule. Applicants for design change
approvals that decrease the maximum takeoff gross weight to 75,000
pounds or less after the effective date of the rule must comply within
18 months after the effective date of the rule or by the date the
certificate is issued or by the date specified in the plan approved
under Sec.  25.571(b), whichever occurs latest.
    The FAA has also revised the compliance times to require those
applicants who would decrease the gross weight of their airplanes after
the effective date of the rule to submit a compliance plan within 90
days after the date of application.
3. Varying Implementation Strategies
    APA suggested a way to address concerns about the time needed to
develop an LOV. The commenter stated that the initial LOVs under
consideration, as defined in the Technical Document, appear to be
extremely liberal and based on limited data and minimal analysis. APA
assumed that manufacturers would need more time to develop their
analysis procedures, and said that a better approach for establishing
the initial LOV would be to increase the design service goal by 10% to
15% and mandate inspections of high-time airplanes that are over their
design service goal. APA based its suggestion on an assumption that the
design service goals were based on hard test and engineering data. The
commenter suggested halving the interval between maintenance checks for
airplanes over their design service goal. Then, the commenter
suggested, results of these inspections could be given to the
manufacturer for use in substantiating the engineering WFD analysis.
This data could be used to validate future incremental LOV increases.
    Although this commenter maintained that design service goals are
based on hard test and engineering data, that has not always been the
criteria by which design service goals have been set. Amendment 25-96
to Sec.  25.571 introduced requirements that applicants show freedom
from WFD up to the design service goal. Prior to Amendment 25-96,
however, there was no requirement for setting a design approval
holder's design service goal or for validating it. Design approval
holders have always used engineering data to substantiate their
designs. Most design approval holders set design service goals for
their airplanes, even though they were not required to do so. But since
there were no requirements prior to Amendment 25-96 about what criteria
must be used to set the design service goal, they have often been set
for purposes driven more by sales and marketing than by engineering
data.
    Some design approval holders have stated that LOVs may be
established at a point anywhere from 33% to 180% higher than the
airplane's design service goal for certain models. This is because, for
those design approval holders, there is a large body of in-service data
to support these higher LOVs. Other design approval holders have taken
an approach similar to APA's recommendation, in that they have been
incrementally increasing their airplane model's LOV as the data
supports it. Today's rule allows for an implementation strategy that
provides flexibility to design approval holders in determining the
timing of service information development (with FAA approval), while
providing operators with certainty regarding the LOV applicable to
their airplanes. However, no matter how the design approval holder
chooses to manage LOV development, those LOVs must still be
substantiated by engineering data.
4. FAA Review and Approval Time
    Industry representatives on the AAWG, Boeing, Airbus, and CAA
requested that the rule include required time periods for FAA review
and approval activities. These commenters noted that the rules do not
currently limit the amount of time the FAA will take to review and
approve documents and that this will negatively affect their compliance
time. Several commenters also noted that the amount of time the FAA
will take to review and approve design approval holders' LOVs could
reduce operator compliance time significantly.
    We are not including required time periods for FAA review and
approval of the required compliance activities. Instead, expectations
for FAA personnel have been defined in FAA Order 8110.104, which
directs the Aircraft Certification and Flight Standards Services in
their roles and responsibilities for implementing these initiatives.
The order includes expected times (6 weeks) for reviewing and approving
design approval holder compliance plans, plans to correct deficiencies,
and draft and final compliance data and documents. To facilitate
implementation, the FAA will train affected personnel in their roles
and responsibilities and provide in-depth familiarization with
requirements of the regulations and associated guidance. Ultimately,
however, the timing of FAA approvals will be determined by the quality
of the design approval holder submissions and their responsiveness to
issues raised by the FAA.
    We have structured the requirements of the design approval holder
rule and developed complementary guidance to facilitate timely review
and approval of design approval holder submittals (such as compliance
plans). An increase in operator compliance time would help ensure that
operators are not affected by the FAA review and approval process. We
have revised the WFD compliance

date for operators from 6 months to 12 months after the relevant design
approval holder compliance date. This date is measured after the
effective date of the final rule. As previously noted, for Group I, II,
and III airplanes, the operator compliance dates are 30, 60, and 72
months, respectively, after the effective date of the rule.

G. LOVs for Future Airplanes: Sec.  25.571, Appendix H, and Operational
Rules

    This final rule revises Sec.  25.571 to require that--
     An LOV be established that corresponds to the time during
which it is demonstrated that WFD will not occur in the airplane
structure, and
     The LOV be included in the Airworthiness Limitations
section of the Instructions for Continued Airworthiness required by
Sec.  25.1529.
    Except for the change in terminology from initial operational limit
to LOV, these revisions to Sec.  25.571 are as proposed in the NPRM.
    For operators of airplanes type certificated in the future, this
final rule relies on existing operational rules to require operators to
include the airplane's LOV, which is established under Sec.  25.571 of
today's rule, into their maintenance/inspection programs. This
requirement is the same as that which was proposed in the NPRM.
1. Opposition to Changes to Sec.  25.571
    Industry representatives on the AAWG and Airbus commented that no
change is needed to Sec.  25.571 because airplanes certified to
Amendment 25-96 must be free from WFD until they reach the design
service goal, and the design service goal must be declared in the
appropriate certification document.
    We recognize that Sec.  25.571 at Amendment 25-96 requires full-
scale fatigue test evidence to demonstrate freedom from WFD up to the
design service goal. However, the current regulations do not require
that the Airworthiness Limitations section include the design service
goal as an airworthiness limitation, so operators would be permitted to
operate airplanes beyond this goal indefinitely. Therefore, the FAA
finds it necessary to revise Sec.  25.571, as proposed, to require that
full-scale fatigue test evidence be used to demonstrate freedom from
WFD up to the LOV and that the LOV be included in the Airworthiness
Limitations section. These changes are consistent with recommendations
made in 2003 by the General Structures Harmonization Working Group, a
separate working group within ARAC.
2. Change to Appendix H
    Under Sec.  25.571, the FAA may issue a type certificate for an
airplane model prior to completion of full-scale fatigue testing. As
stated in the NPRM, the FAA did not propose to change this provision
because the FAA intends that operators be able to operate these
airplanes while the design approval holder is performing fatigue
testing. Today's rule retains the requirement of Sec.  25.571 that--if
a type certificate is issued prior to completion of full-scale fatigue
testing--the Airworthiness Limitations section must include a number
equal to [frac12] the number of cycles accumulated on the fatigue test
article. As additional cycles on the test article are accumulated, the
number may be adjusted accordingly. This number is an airworthiness
limitation, and no airplane may be operated beyond it until the fatigue
testing is completed and the LOV is established.
    For consistency however, the FAA has revised paragraph (a)(4) of
H25.4 to part 25 (Appendix H) to include a reference to the limitation
that an airplane may accumulate a number of cycles not greater than \1/
2\ the number of cycles accumulated on the fatigue test article until
such testing is completed.
3. When to Set LOVs for Future Airplanes
    Industry representatives on the AAWG, Boeing, and American Airlines
commented that design approval holders should not be required to
establish an LOV for a future airplane until the high-time airplane
approaches its design service goal. United Parcel Service, on the other
hand, recommended that the initial LOV be established during the
initial certification process, and before the first airplane enters
service. The ATA recommended that LOVs should be estimated at the time
of airplane certification but should be reassessed when the high-time
airplane approaches 75% of the estimate.
    The LOV is a function of the fatigue knowledge base available at
the time it is established. There should be sufficient data to
establish an LOV for a new airplane model being certificated once full-
scale fatigue test evidence is completed and assessed, normally several
years after the airplane enters service. We agree that an LOV
established for a new airplane model could be reassessed later when
service information could be used with other data necessary to extend
the LOV. Eliminating the requirement to address repairs, alterations,