Aging Airplane Program: Widespread Fatigue Damage

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.

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 (§ 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 § 25.571 and Appendix H

1. Opposition to changes to § 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 § 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

Nominal value

($ millions)

7% Present

value

($ millions)

Benefits

9.8

4.8

Costs

3.8

3.6

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 § 25.571(b).

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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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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).

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

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airplane structure.

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Baseline structure means structure that is designed under the original type certificate or amended type certificate for that airplane model.

• 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 § 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.

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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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71 FR 19928

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 § 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, § 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 § 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)”

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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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69 FR 45936, July 30, 2004.

In July 2005, the FAA published a disposition of comments received in response to our request.

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Also in July 2005, the agency published a policy statement, “Safety-A Shared Responsibility-New Direction for Addressing Airworthiness Issues for Transport Airplanes,”

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that explains our reasons for adopting requirements for design approval holders.

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70 FR 40168, July 12, 2005: Fuel Tank Safety Compliance Extension (final rule) and Aging Airplane Program Update (Request for comments).

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70 FR 40166, July 12, 2005 (PS-ANM110-7-12-2005).

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

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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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71 FR 38540.

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.

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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 § 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 § 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 § 26.21.

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The final rule requires that design approval holders evaluate airplane configurations that include modifications mandated by airworthiness directive.

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 § 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),

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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.

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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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72 FR 63363, November 8, 2007.

10

Certification Procedures for Products and Parts.

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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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

Part 26 final rule

Proposed part 25

SUBPART C—Aging Airplane Safety—Widespread Fatigue Damage

Subpart I—Continued Airworthiness

§ 26.5 Applicability table

New

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§ 26.21 Limit of validity (LOV)

§ 25.1807 Initial operational limit: Widespread Fatigue Damage (WFD).

§ 25.1809 Changes to type certificates: Widespread Fatigue Damage (WFD).

§ 26.23 Extended limit of validity (LOV)

§ 25.1811 Extended operational limit: Widespread Fatigue Damage (WFD)

§ 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: §§ 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.

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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.

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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

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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.

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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.”

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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.

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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.

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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.

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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 § 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.”

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This final rule, which specifically addresses WFD, is intended to be the last element of the overall Aging Aircraft Program.

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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 § 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.

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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).

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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 §§ 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.

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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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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 (§ 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

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be combined and aligned in a single rulemaking. These commenters included industry representatives who are members of the AAWG,

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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).

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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,

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the damage tolerance provisions of § 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 § 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 § 25.571, Amendment 25-96 or later. Amendment 25-96 revised § 25.571 to require that full-scale fatigue test evidence

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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.

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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, § 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 § 25.1807(g), along with §§ 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 § 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 § 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, § 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.

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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 §§ 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 § 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 § 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 § 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 § 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 § 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 § 26.21, an LOV must be established within 60 months after the effective date of the rule. In Table 1 of § 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 § 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 §§ 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 § 121.1115 and § 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 § 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 § 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 § 121.1115 and § 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 § 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 § 25.1807 (now § 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.

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 § 25.571, Amendment 25-45, up to but not including Amendment 25-96 (1998).

• Group III—Airplanes certified to requirements of § 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 § 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 § 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 § 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 § 26.21(a).

Table 1 of § 26.21 is used to call out existing airplanes and assign compliance dates. Holders of type certificates for these models must comply with § 26.21(c)(1). The remainder of § 26.21(c) specifies additional people who must comply.

Under today's rule, the compliance times specified in § 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 § 25.571(b).

27

All applicants who must comply with § 26.21 may use this date as one option for compliance.

27

Under § 21.17, these applicants are subject to § 25.571 at Amendment 25-96. In addition to this certification basis, they are subject to the requirements of this final rule.

Applicants who have the same compliance times and the option to use the date specified in the § 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 § 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 § 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 § 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 § 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 § 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: § 25.571, Appendix H, and Operational Rules

This final rule revises § 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 § 25.1529.

Except for the change in terminology from initial operational limit to LOV, these revisions to § 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 § 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 § 25.571

Industry representatives on the AAWG and Airbus commented that no change is needed to § 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 § 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 § 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 § 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 § 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 ½ 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, and modifications will simplify the process for extending the LOV.

The FAA does not agree that establishment of an LOV for a future airplane model should wait until the high-time airplane approaches its design service goal. As discussed previously, establishing design approval holder compliance dates that are a function of when high-time airplanes reach their design service goal would introduce a level of complexity and uncertainty to the requirements of the operational rules that is unnecessary and inappropriate.

One manufacturer is already employing the concept of establishing LOVs based on the fatigue knowledge base available through the certification process. Airbus has already included an LOV in the applicable Airworthiness Limitations section approved by EASA for all of its models with the exception of the A340.

4. Operational Rules

For airplanes whose type certificate application is made after the effective date of this final rule, LOVs must be established by the date the certificate is issued or the date specified in the plan approved under § 25.571(b). The LOV will be included in the Airworthiness Limitations section of the Instructions for Continued Airworthiness and will apply regardless of how or by whom the airplane is operated.

As discussed above, the FAA may issue a type certificate for an airplane model before full-scale fatigue testing has been completed. In that case, the Airworthiness Limitations section of the Instructions for Continued Airworthiness must include a number equal to

1/2

the number of cycles accumulated on the fatigue test article. Under § 91.403(c), operators may not operate these airplanes beyond this number of cycles. Once the fatigue testing is completed and the LOV is established and approved, operators may revise this airworthiness limitation to include the LOV. This LOV will be higher than the airworthiness limitation specifying

1/2

the number of fatigue test article cycles.

H. How to Set LOVs

Section 26.21(b) of this final rule requires design approval holders to establish an LOV of the engineering data that supports the structural maintenance program. This LOV corresponds to the period of time, stated as a number of total accumulated flight cycles or flight hours, or both, during which the design approval holder is able to demonstrate that WFD will not occur in the airplane. This demonstration must include an

evaluation of airplane structural configurations and be supported by test evidence and analysis. If available, service experience, or service experience and teardown inspection results, may be added to the test evidence and analysis to provide additional substantiation. The service experience and teardown inspections must be of high-time airplanes of similar structural design, accounting for differences in operating conditions and procedures.

The NPRM proposed in § 25.1807(b) [adopted here as § 26.21(b)] that holders of design approvals for existing airplanes subject to the rule be required to evaluate airplane structural configurations to determine when WFD was likely to occur for structure susceptible to multiple site damage or multiple element damage. The results of the evaluation were to be used to support establishment of an initial operational limit (now the LOV.)

The Boeing Company and industry representatives on the AAWG commented that proposed § 25.1807 would require an “evaluation” that is not adequately defined and that there are no objective criteria for establishment of an LOV. These deficiencies could result in establishment of an LOV based solely on analyses of structure susceptible to multiple site damage and multiple element damage, without consideration of more relevant and reliable data, such as test evidence and service experience. These commenters concluded that, in these circumstances, airplanes could be operated well past the point to which the engineering data supports safe operation.

The commenters recommended that the required evaluation explicitly include the following tasks, which are described in the AAWG's 2003 report

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as necessary to establish or extend an LOV.

28

AAWG, Widespread Fatigue Damage Bridge Tasking Report, July 23, 2003.

1. Ensure that the basics of the Aging Aircraft Program are in existence.

2. Collect data necessary to extend fatigue test evidence.

3. Perform analysis of the structure for multiple site damage and multiple element damage.

4. Create and update maintenance documents to include maintenance actions and modifications for those areas where it has been predicted that multiple site damage and multiple element damage will occur before the proposed LOV.

In addition, industry representatives on the AAWG and Boeing recommended that the rule explicitly use the term “fatigue test evidence” to refer to the collective body of information that should be considered in establishing an LOV. The FAA agrees that the first task, having basics of the four elements of the Aging Aircraft Program in place,

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is an important element for continued safe operation out to LOV. However, as discussed in the NPRM, this final rule does not include requirements related to those initiatives because they are already mandated by airworthiness directives, operational rules, and airworthiness limitations.

29

Mandatory modification, corrosion prevention and control, supplemental structural inspection, and repair assessment.

The FAA considers that tasks 2 and 3 are implicit in the text of the proposed rule but agrees that proposed § 25.1807 could be misinterpreted and result in too much reliance on results of analysis to preclude WFD up to the LOV. This was not our intent. In fact, as discussed in the NPRM, our intent was consistent with the AAWG's recommendations regarding WFD.

In response to these commenters, the FAA has revised the proposed rule to clarify how the LOV is to be established. This final rule specifies that—for an LOV to be acceptable—the supporting evaluation must demonstrate that the fatigue characteristics and any specified maintenance actions for the airplane are sufficient to prevent WFD from occurring before the LOV.

The required demonstration typically involves an evaluation of the airplane structure to determine its susceptibility to WFD and, if the structure is susceptible, an evaluation indicating that WFD will not occur before the proposed LOV. The evaluation must be supported by test evidence and analysis. The design approval holder may augment the test evidence and analysis with any available service experience, or service experience and teardown inspection results of high-time airplanes. Service experience and teardown inspection results must be of airplanes of similar structural design and must account for differences in operating conditions and procedures. After seeing these changes to the rule as they were described in the Technical Document, Boeing stated that it supports the FAA's adoption of an airplane-level assessment of fatigue test evidence as the basis for both the determination and extension of LOV.

The FAA is using the term “test evidence” to align with the rule text of § 25.571 relative to WFD. Therefore, in the context of this final rule, test evidence is data derived from full-scale fatigue testing, which may be of the complete airplane, or of separate major sections of the airplane, or a combination of the two. The test evidence would be used to support the proposed LOV for an airplane model. The amount of test evidence required to show compliance would depend on where a design approval holder proposes to set an LOV and what data (such as test evidence or service experience) already exist.

For a new airplane model that is pending approval, there should be test evidence to address all WFD-susceptible structural areas of an airplane. The test duration should be at least two times the proposed LOV. The test evidence may be from prior full-scale fatigue tests performed by the applicant or others on similar structure. For derivative models, the applicant should compare the derivative model to the tested model. To use the test evidence from the original certification project or previous derivatives, the applicant should show that the derivative model does not significantly change the basic structural design concept, aerodynamic contour, and internal load distribution. Advisory Circulars 120-YY and 25.571-1X further describe considerations for when existing test evidence could be used.

For some older airplanes, fatigue test data may be limited to fuselage structure. This is because the pressurized fuselage has been considered to be the most fatigue-critical part of the airplane. The wing and empennage have typically been considered less critical and, as a result, relevant test data may not exist. However, for these same airplane models, significant service experience does exist. The FAA would accept a combination of test evidence and analysis as well as service experience as data to show compliance with this final rule.

For example, in the case of one of the pre-Amendment 25-45 airplane models, significant numbers of airplanes both in service and in storage have accumulated flight cycles in excess of the design service goal. For this model, there is significant existing test evidence for the fuselage, but very little for the wing. In this case, the FAA expects that demonstrating freedom from WFD for the wing would be based primarily on service experience; for the fuselage, it would be based primarily on service experience and test evidence. Advisory Circular 120-YY further describes considerations for when service experience could be used to supplement existing fatigue testing that is limited to certain major components of the airplane, such as the fuselage.

The FAA has used the term “analysis” to include fatigue and damage tolerance analyses. Teardown inspections of in-service airplanes and fatigue test articles should be performed to the degree necessary to validate that the test evidence, analysis, and service experience are representative of the fatigue performance of the airplane out to the LOV. Design approval holders must explain in their certification plan how they intend to substantiate their proposed LOV. The FAA has revised AC 120-YY to provide further guidance on the steps to take for establishing an LOV.

As discussed in the NPRM, design approval holders are not required to identify and develop maintenance actions if they can show that such actions are not necessary to prevent WFD before the airplanes reach LOV. If they choose to establish LOVs that rely upon maintenance actions to prevent WFD before the LOV, they must identify those actions and, unless the necessary service information already exists, develop the service information in accordance with a binding schedule approved by the FAA. Those actions would then be mandated, not by today's rule, but by future airworthiness directives.

To be approved, the “binding schedule” for necessary maintenance actions must ensure that the service information is provided in a “timely manner.” In the NPRM, the FAA explained that the purpose of this requirement was to enable the FAA to issue the necessary airworthiness directives in time to allow operators to accomplish these actions during normal maintenance. The intent is to allow design approval holders the flexibility to focus their efforts on initially developing service information on those maintenance actions that must be accomplished first. At the same time, the FAA expects design approval holders to devote sufficient resources to these efforts so that:

• The service information is available when the FAA needs it to initiate the airworthiness directive rulemaking process, including providing public notice and opportunity to comment; and

• The resulting airworthiness directives will provide sufficient compliance times so that the required actions can be accomplished without disrupting operators' normal maintenance schedules.

Airbus stated that the analysis is the driver for substantiating LOVs and that test evidence supports the analysis.

Analysis methods are used in combination with the engineering data to characterize WFD behavior to the degree necessary to determine if maintenance actions are required prior to the proposed LOV. As a result, test evidence and analysis are both required to demonstrate freedom from WFD. This is consistent with the existing requirements of § 25.571 at Amendment 25-96.

We agree that a design approval holder may not have both service experience and teardown inspection results available to use as part of its compliance data. We have modified the requirement so that a design approval holder may have either service experience or service experience and results of teardown inspections. The change is follows:

“This demonstration must include an evaluation of airplane structural configurations and be supported by test evidence and analysis at a minimum and, if available, service experience, or service experience and teardown inspection results, of high-time airplanes of similar structural design, accounting for differences in operating conditions and procedures.”

I. How To Extend LOVs

Proposed § 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. The configuration to be evaluated would consist of not only all model variations and derivatives approved under the type certificate for which the extension is sought, but also all structural repairs, alterations, and modifications to those airplanes, whether mandated by airworthiness directive or not.

Section 26.23(b) of this final rule (proposed as § 25.1811) contains requirements for obtaining approval of an extended LOV that corresponds to the period of time, stated as a number of total accumulated flight cycles or flight hours or both, beyond an existing LOV during which it is demonstrated that WFD will not occur in the airplane. This demonstration must include an evaluation of airplane structural configurations and be supported by test evidence and analysis at a minimum and, if available, service experience, or service experience and teardown inspection results of high-time airplanes of similar structural design, accounting for differences in operating conditions and procedures. Requirements for this section are the same as those for establishing an LOV. The FAA has removed the requirement to evaluate repairs, alterations, and modifications from § 26.23.

1. Change the Procedure for Extending LOVs

Industry representatives on the AAWG, ATA, Cessna, Airbus, United Parcel Service, FedEx, Boeing, and American Airlines stated that the means proposed in § 25.1811 for extending an operational limit is administratively difficult, impractical, and technically unachievable. The commenters expressed doubt that the proposed process could be realistically or uniformly accomplished because different operators will be involved in extending the LOV for the same airplane model. Furthermore, said the commenters, it is unlikely that any single operator has the information necessary to obtain an extended LOV. The cost, and uncertainty about the outcome of the evaluation, would make this process nearly impossible for an operator to attempt.

The commenters added that extending an LOV would need to be done by addressing each individual airplane, identified by tail number, whereas the maintenance actions which support the initial LOV are based on statistics pertaining to behavior of the entire fleet of a particular model. Thus, the method of determining maintenance actions to preclude WFD out to the LOV is not valid for a single airplane. The AAWG industry representatives recommended that establishing an extended LOV and evaluating repairs, alterations, and modifications be a sequential process. The first step would be to establish the extended LOV. The second step would be for each design approval holder for a modification to evaluate its own design relative to the extended LOV and obtain a separate, independent approval for its design. The operator would continue to be responsible for assembling all maintenance requirements, depending on actual airplane configuration, and for obtaining approval of the maintenance program from the principal maintenance inspector. Such a process is similar to industry proposals for compliance with the Aging Airplane Safety Final Rule.

Several commenters also remarked that the administrative process for obtaining an amended type certificate or supplemental type certificate will be extraordinarily difficult to manage because manufacturers, operators, and holders of supplemental type certificates do not necessarily have access to each other's proprietary information. The existing business and legal agreements in place did not contemplate the high degree of data disclosure that will be required to develop WFD guidance material and

data needed for an amended type certificate or supplemental type certificate. Furthermore, many transport airplanes are converted to operate in different roles than those for which they were originally designed. Often operators cannot obtain support or design data from design approval holders because the latter have concerns about liability, are no longer in business, or are more motivated to sell new airplanes than to support old ones.

Several commenters recommended that the FAA delete proposed § 25.1811 and revise proposed § 25.1807 to allow extension of an LOV by a process approved by the Administrator. They base their recommendation on the fact that the technical requirements for establishing an LOV are no different from those for establishing an extended LOV.

The FAA agrees that, given the extensive information required to develop guidelines for including a WFD evaluation of repairs, alterations, and modifications, the proposed requirements for extending the LOV needed to be changed. As discussed earlier, the FAA has removed those requirements. As a result, this final rule includes requirements for extending an LOV based on the original LOV airplane configuration plus all new structural modifications or replacements mandated by airworthiness directives. The FAA has revised requirements of § 26.23(b) to be consistent with § 26.21(b). As previously stated, if our research demonstrates that additional actions are needed to address risks for repairs, alterations, and modifications, the FAA will consider further rulemaking.

The FAA does not agree with the suggestions to allow extension of an LOV using a process approved by the Administrator. In this final rule, requirements for extending an LOV are similar to those for establishing the first LOV. However, the design approval holder is not required to develop the data to support an extended LOV because such extensions are optional. The extended LOV and associated maintenance actions (inspections, modifications, or replacements) must be defined within the Airworthiness Limitations section for the airplane. This requirement is unchanged from the proposed requirements of § 25.1811(b) of the NPRM. As stated in the NPRM, the FAA intends to use airworthiness directives to mandate any maintenance actions necessary to reach the LOV established under § 26.21, so that operators will have an opportunity to comment on the proposed maintenance actions. It is not necessary to use this process for extensions of the LOV, however, because the extended LOV would include all maintenance actions at the time of approval. For these reasons, the FAA has kept requirements for extending an LOV separate from § 26.21. The FAA has revised AC 120-YY to provide guidance on establishing an extended LOV.

2. Evaluation of Repairs, Alterations, and Modifications for an LOV Extension

EASA stated that certain existing repairs, alterations, and modifications should be evaluated for WFD when the LOV is being extended. EASA states that the risk of WFD increases for repairs, alterations, and modifications as airplanes age.

As discussed elsewhere in this document, an extension should be based on the airplane's structural configuration, just as the initial LOV is. Persons establishing extensions to LOVs may identify conditions or limitations in the Airworthiness Limitations section of the Instructions for Continued Airworthiness that apply to the extensions. For example, the LOV extension may only be valid for airplanes that operate at a certain cabin differential pressure or maximum takeoff gross weight. Operators may have to evaluate their airplanes and take certain actions prior to incorporating any extensions. AC 120-YY provides additional guidance on this.

3. Alternate Means of Compliance (AMOCs)

APA commented that operators should not be allowed alternate means of compliance (AMOCs) for the WFD rule because, it says, if the FAA allows AMOCs as it does with airworthiness directives, the ability to collect data and track compliance will be greatly complicated. Each operator, said the commenter, will comply in a manner with the least financial impact to its company. This may or may not be supported by the ongoing efforts of the original equipment manufacturers to develop analysis techniques and procedures. It will also add significant financial costs to the original equipment manufacturers and the FAA to support, track, and verify each AMOC.

The initial LOV is established and approved under § 26.21 or § 25.571. Any extension to the initial LOV or any subsequent LOV is established and approved under § 26.23. The FAA does not issue AMOCs for these regulations. Any deviation from a rule is handled via the procedures contained in 14 CFR part 11.

Under § 26.21, any maintenance actions needed to support the initial LOV will be mandated by airworthiness directives, and compliance with those airworthiness directives and the ability to apply for an AMOC for those maintenance actions will not involve procedures that are any different from those used for airworthiness directives today. An AMOC for the maintenance actions for an initial LOV will not affect the LOV itself.

Under § 26.23, however, any maintenance actions developed to support the extended LOV will be incorporated into the Airworthiness Limitations section of the Instructions for Continued Airworthiness. The maintenance actions for extended LOVs will not be published in airworthiness directives.

4. Extension Procedure Doesn't Allow Public Comment

ATA and Northwest Airlines stated that the proposed rule does not permit the public to comment on extensions to LOVs and the maintenance actions that support them. Extensions to LOVs mandated by airworthiness directive would allow the opportunity for public comments on extended LOVs.

Although mandating LOV extensions by airworthiness directive would allow the public the opportunity to comment, the FAA does not agree with the suggestions to use airworthiness directives to allow extension of an LOV. This is for two reasons:

• Approving an extended LOV isn't rulemaking; it's a finding of compliance with the applicable regulatory standard (

i.e.,

freedom from WFD).

• If the FAA doesn't extend the LOV, or subsequent extensions of that LOV, there's no unsafe condition justifying an airworthiness directive, because affected airplanes are grounded when they reach the LOV.

The FAA has revised AC 120-YY to provide guidance on establishing an extended LOV.

The AAWG recommended in its Task 3 Report that design approval holders and operators work together in establishing LOVs and LOV extensions. Under today's rule, the FAA expects that design approval holders and operators will work together when persons are seeking approval for extended LOVs.

J. Applicability for Existing Airplanes

The rule proposed in the NPRM would apply to existing transport category airplanes with a maximum takeoff gross weight greater than 75,000 pounds, by virtue of either the original type certification of the airplane or a later increase, that are operated under part 121 or 129.

This final rule applies to certain existing transport category, turbine-powered airplanes with a maximum takeoff gross weight greater than 75,000 pounds and a type certificate issued after January 1, 1958, regardless of whether the maximum takeoff gross weight is a result of an original type certificate or a later design change. In addition, it applies to transport category, turbine-powered airplanes with a type certificate issued after January 1, 1958, if a design change approval for which application is made after the effective date of the rule has the effect of reducing the maximum takeoff gross weight from greater than 75,000 pounds to 75,000 pounds or less. It also applies to operators of those airplanes being operated under part 121 or 129.

1. Type Certificates Issued After January 1, 1958

As proposed, applicability of the rule was not limited to turbine-powered airplanes with type certificates issued after January 1, 1958. Everts Air Cargo requested that McDonnell Douglas Model DC-6 airplanes be excluded from applicability, and Boeing requested that both the DC-6 and DC-7 be excluded. Everts Air Cargo stated that its airplanes are non-pressurized, which should reduce the risk that they would develop WFD. Both Boeing and Everts pointed out that §§ 121.370a and 129.16 of the Aging Airplane Safety Final Rule apply only to certain transport category, turbine-powered airplanes with a type certificate issued after January 1, 1958. The commenters recommended that the rule pertaining to WFD apply only to those same airplanes.

The FAA agrees that certain parts of the applicability of this final rule should align with the Damage Tolerance Data Rule and the Aging Airplane Safety Final Rule and other aging airplane rules, such as EAPAS/FTS. The McDonnell Douglas DC-6 and DC-7 airplanes have not had a damage tolerance assessment and have not been included in the Damage Tolerance Data Rule. In addition, the risk from excluding these airplanes is small because there are so few of them.

Therefore, in this final rule the FAA has added the phrase “transport category, turbine-powered airplanes with a type certificate issued after January 1, 1958” to the applicability provisions of § 26.21 and to the operating rules. The change means that the following airplanes, which would have been affected by the proposal, are not subject to this final rule:

• McDonnell Douglas Models DC-6 and DC-7.

• Lockheed Model 1649A-98.

• Lockheed Model 1049 Series.

• Lockheed Models 49-46, 149-46, 649-79, 649A-79, 749-79, and 749A-79.

2. Original Type Certification

The applicability provision in proposed § 25.1807 included airplanes with maximum takeoff gross weights exceeding 75,000 pounds, as approved during original type certification, as well as airplanes with lower weights that had been increased to greater than 75,000 pounds through later design changes. This applicability provision was intended to address two situations. In the past, some designers and operators avoided applying requirements mandated only for airplanes over a specific capacity by receiving a design change approval for a slightly lower capacity. By referencing the capacity resulting from original type certification, the NPRM removed this means of avoiding compliance.

Similarly, an airplane design could be originally certified with a capacity lower than the minimum specified in the rule, but through later design changes, the capacity has been increased above this minimum. The reference in the NPRM to a later increase in capacity was intended to ensure that, if this occurs, the design would have to meet the requirements of the rule.

The applicability proposed in the NPRM did not distinguish among design changes based on whether their date of application for design approval occurred before or after the rule's effective date. That provision in proposed § 25.1807 is similar to that for the EAPAS/FTS, Fuel Tank Flammability, and Damage Tolerance Data Rules. In addition, the reference to capacity resulting from original type certification is common to proposed § 25.1807 and the other rules. The agency has determined that the approach to applicability under today's rule should be slightly different from that used in previous rules. This is to avoid requiring design approval holders to establish LOVs for models that have maximum takeoff gross weights that were decreased to 75,000 pounds or less by an amended type certificate or supplemental type certificate before the effective date of today's rule. Applicants for such design changes in the past could not have designed the airplanes' capacities to avoid complying with today's requirements, and it is not our intent to include them in the applicability of this final rule.

The FAA has revised this section (now § 26.21) to apply to transport category, turbine-powered airplanes with a maximum takeoff gross weight greater than 75,000 pounds and a type certificate issued after January 1, 1958, regardless of whether the maximum takeoff gross weight is a result of an original type certificate or a later design change. This section also applies to transport category, turbine-powered airplanes with a type certificate issued after January 1, 1958, if a design change approval, for which application is made after the effective date of the rule, has the effect of reducing the maximum takeoff gross weight from greater than 75,000 pounds to 75,000 pounds or less.

The FAA has also revised the applicability of §§ 121.1115 and 129.115 to be consistent with the applicability of § 26.21 for existing airplanes. For future airplanes for which an LOV is approved in accordance with § 25.571 of today's rule, we have retained the requirement that §§ 121.1115 and 129.115 apply to operators of U.S.-registered transport category, turbine-powered airplanes, regardless of the maximum takeoff gross weight. For future design changes reducing the maximum takeoff gross weight from greater than 75,000 pounds to 75,000 pounds or less, the compliance date for operators is 30 months after the effective date of the rule, or the date of design change approval, or the date specified in the plan approved under § 25.571(b), whichever occurs latest. For these design changes, unless or until the design approval holder complies with § 26.21 by establishing a new LOV, the LOV applying to the airplane in the absence of the design change would still apply.

3. Airplane Configuration

This final rule requires that holders of type certificates for existing airplanes evaluate certain configurations of those airplanes for susceptibility to WFD and use the results of the evaluation to set LOVs for those airplanes. The configurations to be evaluated are:

• All model variations and derivatives approved under the type certificate, and

• All structural modifications and replacements to those airplanes which were mandated by airworthiness directives issued to address any configuration developed by the design approval holder.

In the NPRM, the FAA proposed evaluation of the same airplane configurations.

In their comments, the industry representatives on the AAWG, Boeing, and Airbus expressed concern about the proposed requirement to evaluate all structural modifications and

replacements mandated by airworthiness directives. Airbus stated that this approach deviates from all previous industry recommendations and will lead to a significant increase in configurations to be assessed. The industry representatives on the AAWG, Boeing, and Airbus requested that the FAA reconsider this requirement and focus only on airworthiness directives which have been issued specifically to address WFD.

The FAA issues many airworthiness directives which require structural modifications or replacements not intended to address WFD. These required modifications or replacements, however, may affect susceptibility of a structure to WFD. A modification might introduce new details that cause a structure which was previously not susceptible to WFD to become susceptible, or make a change that increases susceptibility so that previously established maintenance actions need to be modified. Because today's rule is intended to address the potential for WFD in airplanes as they are actually configured, we must address these required modifications. It would serve no useful purpose to evaluate structural configurations which no longer exist in service because airworthiness directives have required modifications to those configurations.

Modifications mandated by airworthiness directives are much fewer in number than other modifications, and they generally affect airplanes of the same model in the same way. Many modifications mandated by airworthiness directives would not affect the potential for WFD; others could.

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Therefore, the FAA is today issuing this requirement as proposed.

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Advisory Circular 120-YY provides guidance on which modifications mandated by airworthiness directives should be assessed by the design approval holder.

4. Weight Cutoff

In the preamble to the proposed rule, the FAA stated that the agency had considered applying the rule to all existing transport category airplanes, regardless of the maximum takeoff gross weight. The FAA acknowledged that using a weight cutoff of greater than 75,000 pounds excludes approximately 1,600 regional jets operating under parts 121 or 129, giving the impression that this rule might not align with our “One Level of Safety” initiative. However, the FAA justifies the proposed weight cutoff on the basis of the relatively young age of the regional jet fleet. Because those airplanes are younger, they have a low present risk for WFD.

Embraer agreed that existing regional jet airplanes should not be subject to the rule at this time, stating that the airplanes have typically been certificated to damage tolerance requirements. Other commenters—such as the National Transportation Safety Board, Transport Canada, the Air Line Pilots Association (ALPA), EASA, and an individual commenter—did not agree, because the regional jets are at risk of developing WFD as they accumulate flight cycles just as larger airplanes are. The ALPA recommended that the FAA form a study group to assess WFD in lighter airplanes. Pending a detailed risk analysis, the association suggested a weight cutoff of 12,000 pounds.

The 75,000 pound weight cutoff was based on recommendations from the AAWG for WFD rulemaking. The overwhelming majority of passengers and cargo are carried by airplanes with a maximum gross takeoff weight of greater than 75,000 pounds. Inclusion of airplanes below that limit and above 12,500 pounds is under study by the FAA and if service experience shows a need to include those airplanes, rulemaking will be considered to include them.

The FAA's highest priority is to address the oldest airplanes at highest risk of WFD—namely, airplanes with a maximum takeoff gross weight greater than 75,000 pounds. However, the FAA recognizes that the lighter and relatively younger regional jets will also be at risk of developing WFD as they accumulate flight cycles. We will reassess the fleet, including those airplanes below 75,000 pounds, after this rule has been implemented, to determine whether further rulemaking is necessary.

5. Default LOVs and Excluded Airplanes

a. Table 1—Default LOVs

In the proposed operational requirements in the NPRM, the FAA inadvertently created an ambiguity regarding the obligations of operators of airplanes for which the design approval holder might fail to establish an LOV as required. While the FAA fully anticipates that affected design approval holders will comply with the requirements of this final rule, there is a need to clearly provide for what happens if one or more does not. As proposed, paragraph (a) of §§ 121.1115 and 129.115 would apply to operators of airplanes for which an LOV “has been established.” Paragraph (b) of these sections requires that operators incorporate approved LOVs.

Our expectation was that, if a design approval holder failed to comply with the requirement to obtain approval for an LOV, the operator or operators, in order to continue to operate the affected airplanes, would themselves obtain the necessary approval. Because they would not have access to the design approval holder's data necessary to perform a WFD evaluation, they would likely have to rely on the design service goals and extended service goals set forth in Table 3 of the NPRM (

see

below). As stated in the NPRM, “After June 18, 2008, an affected operator could not operate an airplane unless the operator has incorporated an Airworthiness Limitations section approved under Appendix H to part 25 or § 25.1807 into its maintenance program.”

The FAA now recognizes that the final rule should explicitly define operators' obligations if the design approval holder fails to comply. Therefore, the FAA has revised the operational rules to state that, in the absence of an approved LOV, the operator must incorporate the applicable LOV specified in Table 1

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of either § 121.1115 or § 129.115. The table also adds flight hour numbers for design service goals for airplanes for which that information was available.

31

To develop Table 1, the FAA added airplanes to Table 3, deleted airplanes from Table 3, and split Boeing Models 737, 747, and 777 airplanes into two groups. These airplanes were added: Airbus A318 and A380; Bombardier CL-600 (2D15 and 2D24); and Embraer ERJ-170 and ERJ-190. The following airplane models were deleted: Boeing 707 and 720; Bombardier CL-44 and BD-700; British Aerospace Airbus, Ltd. BAC 1-11; British Aerospace (Commercial Aircraft) Ltd. Armstrong Whitworth Argosy A.W. 650 Series 101; BAE Systems (Operations) Ltd BAe 146A (all models), Avro 146 RJ70A, Avro RJ85A, and Avro RJ100A.

The inclusion of default LOVs in Table 1 does not prevent an operator from developing its own LOV under § 26.23 of this final rule. The rule specifies that—

• The design approval holder must establish an LOV, and

• If an LOV is not approved, an operator must use the default LOV in Table 1. If an operator later chooses to establish an LOV under § 26.23, that LOV will be considered an extended LOV.

This provision eliminates any need for operators to obtain a separate approval for these “default” LOVs. It also eliminates the risk that a relatively young airplane would be grounded as of an operator's compliance date simply because the FAA had not approved an LOV for that airplane.

Boeing stated that the default LOVs published in the Technical Document are without context and could be misused. Boeing said that it could provide more appropriate numbers to

use, but that these numbers should be removed from the rule because Boeing intends to comply with the rule.

The default LOVs in Table 2 of § 121.1115 and § 129.115 are intended to be used by persons who may choose to operate one of the excluded airplanes. They may also be used by other operators if a design approval holder is late in establishing an LOV, in order to prevent airplanes with fewer accumulated flight cycles and flight hours than the default LOV from being grounded. A few airplanes, such as the Airbus A380, already have an operational limitation included in their Airworthiness Limitations section. These are referenced in the table by a NOTE, and may be used as a default LOV.

Figure 3—Comparison of NPRM Design and Extended Service Goals and Final Rule Default

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