Aging Airplane Program: Widespread Fatigue Damage, 69746-69789 [2010-28363]

Agencies

• Federal Aviation Administration
• DEPARTMENT OF TRANSPORTATION

[Federal Register Volume 75, Number 219 (Monday, November 15, 2010)]
[Rules and Regulations]
[Pages 69746-69789]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2010-28363]



[[Page 69745]]

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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

[[Page 69746]]


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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 (425) 227-2774; facsimile (425) 227-
1232; e-mail walter.sippel@faa.gov. 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 (425) 227-1007; e-mail 
douglas.anderson@faa.gov.

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.

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               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.

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    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 https://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 https://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

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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)

[[Page 69750]]

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

[[Page 69751]]

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

[[Page 69752]]

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

[[Page 69753]]

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

[[Page 69754]]

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 airpla