Widespread Fatigue FAR 2010-28363.pdf

7/27/2019 Widespread Fatigue FAR 2010-28363.pdf

1/44

69746 Federal Register / Vol. 75, No. 219 / Monday, November 15, 2010/ Rules and Regulations

DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Parts 25, 26, 121, and 129

[Docket No. FAA200624281; AmendmentNos. 25132, 265, 121351, 12948]

RIN 2120AI05

Aging Airplane Program: WidespreadFatigue Damage

AGENCY: Federal AviationAdministration (FAA), DOT.ACTION: Final rule.

SUMMARY: This final rule amends FAAregulations pertaining to certificationand operation of transport categoryairplanes to prevent widespread fatiguedamage in those airplanes. For certainexisting airplanes, the rule requiresdesign approval holders to evaluatetheir airplanes to establish a limit of

validity of the engineering data thatsupports the structural maintenanceprogram (LOV). For future airplanes, therule requires all applicants for typecertificates, after the affective date of therule, to establish an LOV. Designapproval holders and applicants mustdemonstrate that the airplane will befree from widespread fatigue damage upto the LOV. The rule requires thatoperators of any affected airplaneincorporate the LOV into themaintenance program for that airplane.Operators may not fly an airplane

beyond its LOV unless an extended LOVis approved.DATES: These amendments becomeeffective January 14, 2011.FOR FURTHER INFORMATION CONTACT: Ifyou have technical questionsconcerning this rule, contact WalterSippel, ANM115, Airframe/CabinSafety Branch, Federal AviationAdministration, 1601 Lind Avenue SW.,Renton, WA 980573356; telephone(425) 2272774; facsimile (425) 2271232; e-mail [email protected] have legal questions, contact DougAnderson, Office of Regional Counsel,Federal Aviation Administration, 1601Lind Avenue SW., Renton, WA 980573356; telephone (425) 2272166;facsimile (425) 2271007; [email protected].

SUPPLEMENTARY INFORMATION:

Authority for This Rulemaking

The FAAs authority to issue rules onaviation safety is found in Title 49 of theUnited States Code. Subtitle I, section106 describes the authority of the FAAAdministrator. Subtitle VIIAviationPrograms describes in more detail thescope of the agencys authority.

This rulemaking is promulgatedunder the authority described in subtitleVII, part A, subpart III, section 44701,General requirements. Under thatsection, the FAA is charged withpromoting safe flight of civil aircraft inair commerce by prescribing minimumstandards required in the interest ofsafety for the design and performance of

aircraft; regulations and minimumstandards in the interest of safety forinspecting, servicing, and overhaulingaircraft; and regulations for otherpractices, methods, and procedures theadministrator finds necessary for safetyin air commerce. This regulation iswithin 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 thoseairplanes and for other practices,

methods, and procedures relating tothose airplanes.

Contents

I. Executive SummaryII. Background

A. Summary of the NPRMB. Related ActivitiesC. Differences between NPRM and Final

Rule1. Substantive changes2. Regulatory Evaluation changes3. New part 26 for design approval holders

airworthiness requirements4. New subparts for airworthiness

operational rulesD. Summary of Comments

III. Discussion of the Final RuleA. Overview1. Widespread fatigue damage2. Final ruleB. Requests for Deferral or Withdrawal of

Rule1. Safety benefits dont justify rule2. Existing programs serve purpose of rule3. Divide rule into twoC. Concept of Operational Limits1. Requests for requiring maintenance

programs instead2. Single retirement point for a model3. Potentially adverse effect on safetyD. Change in Terminology (Initial

Operational Limit to LOV)1. Rationale for the term LOV

2. Refer to the structural maintenanceprogram

E. Repairs, Alterations, and Modifications1. Whether repairs, alterations, and

modifications pose WFD risks2. Relationship to damage tolerance

requirements ( 25.571)a. Pre-Amendment 2596 airplanes

b. Airplanes certified to Amendment 2596or later

3. Guidelines for repairs, alterations, andmodifications

4. Rely on the Changed Product RuleF. LOVs for Existing Airplanes1. NPRM compliance date

2. When to set LOVs for existing airplanesa. Pre-Amendment 2545 airplanes

b. Airplanes certified to Amendment 2545or later

3. Varying implementation strategies4. FAA review and approval timeG. LOVs for Future Airplanes: Revisions to

25.571 and Appendix H1. Opposition to changes to 25.571

2. Change to Appendix H3. When to set LOVs for future airplanesH. How to Set LOVsI. How to Extend LOVs1. Change the procedure for extending

LOVs2. Evaluation of repairs, alterations, and

modifications for LOV extensions3. Alternate means of compliance (AMOCs)4. Extension procedure doesnt allow

public commentJ. Applicability for Existing Airplanes1. Type certificates issued after January 1,

19582. Original type certification3. Airplane configuration

4. Weight cutoff5. Default LOVs and excluded airplanesa. Table 1Default LOVs

b. Table 2Airplanes excluded from26.21

6. Bombardier airplanes7. Intrastate operations in Alaska8. Composite structuresK. HarmonizationL. Regulatory Evaluation1. Benefits of proposed rule2. Costs of proposed rulea. Need to know LOVs to determine cost

b. Need to know maintenance actions todetermine cost

c. Costs to manufacturersd. Cost of failing to harmonize rulee. Cost to replace an airplanef. Residual value of airplanes3. Rotable parts4. Use of LOVs for financial evaluations

IV. Regulatory Notices and Analyses

I. Executive Summary

This final rule requires certain actionsto prevent catastrophic failure due towidespread fatigue damage (WFD)throughout the operational life ofcertain existing transport categoryairplanes and all those to be certificated

in the future. Existing airplanes subjectto the rule are turbine-poweredairplanes with a type certificate issuedafter January 1, 1958, which have amaximum takeoff gross weight greaterthan 75,000 pounds and are operatedunder part 121 or 129. The rule appliesto all transport category airplanes to becertificated in the future, regardless ofmaximum takeoff gross weight or howthey are operated. The benefits of thisrule are estimated at a present value of$4.8 million. The cost is estimated at apresent value of $3.6 million.

VerDate Mar2010 16:13 Nov 12, 2010 Jkt 223001 PO 00000 Frm 00002 Fmt 4701 Sfmt 4700 E:\FR\FM\15NOR2.SGM 15NOR2
mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]


2/44

69747Federal Register / Vol. 75, No. 219 / Monday, November 15, 2010/ Rules and Regulations

1After sustaining a certain level of damage, theremaining structure must be able to withstandcertain static loads without failure. In the contextof WFD, the damage is a result of the simultaneouspresence of fatigue cracks at multiple locations inthe same structural element (i.e., multiple sitedamage) or the simultaneous presence of fatiguecracks in similar adjacent structural elements (i.e.,multiple element damage).

2Baseline structure means structure that isdesigned under the original type certificate oramended type certificate for that airplane model. 371 FR 19928

FIGURE 1WFD FINAL RULEBENEFITS AND COSTS

Nominal value($ millions)

7% Presentvalue

($ millions)

Benefits ..... 9.8 4.8Costs ......... 3.8 3.6

Fatigue damage to a metallic structureoccurs when the structure is subjectedto repeated loads, such as thepressurization and depressurization thatoccurs with every flight of an airplane.Over time this fatigue damage results incracks in the structure, and the cracksmay begin to grow together. Widespreadfatigue damage is the simultaneouspresence of fatigue cracks at multiplestructural locations that are of sufficientsize and density that the structure willno longer meet the residual strengthrequirements of 25.571(b).1 Structuralfatigue characteristics of airplanes are

understood only up to the point whereanalyses and testing of the structure arevalid. There is concern about operatingan airplane beyond that point for severalreasons. One reason is that WFD isincreasingly likely as the airplane ages,and is certain if the airplane is operatedlong enough. Another is that existinginspection methods do not reliablydetect WFD because cracks are initiallyso small and may then link up and growso rapidly that the affected structurefails before an inspection can beperformed to detect the cracks.

To preclude WFD related incidents in

existing transport category airplanes,this final rule requires holders of designapprovals for those airplanes subject tothe rule to perform the followingactions:

1. Establish a limit of validity of theengineering data that supports thestructural maintenance program (LOV);

2. Demonstrate that WFD will notoccur in the airplane prior to reachingthe LOV; and

3. Establish or revise theAirworthiness Limitations section in theInstructions for ContinuedAirworthiness to include the LOV.

As used in this preamble, the termdesign approval holders includesholders of type certificates,supplemental type certificates, oramended type certificates, andapplicants for such approvals. In the

context of this final rule, the designapproval holder is generally the typecertificate holder. Requiring designapproval holders to perform the actionslisted above is intended to supportcompliance by operators with todaysamendments to parts 121 and 129. Thisfinal rule amends those parts to requirethat operators incorporate the LOV as

airworthiness limitations into theirmaintenance program for each affectedmodel that they operate.

The amendments to the operatingrules have the effect of prohibitingoperation of an airplane beyond itsLOV. However, todays rule provides anoption for any person to extend the LOVfor an airplane and to develop themaintenance actions which support theextended limit. Thereafter, to operate anairplane beyond the existing LOV, anoperator must incorporate the extendedLOV and associated maintenanceactions into its maintenance program.

The airplane may not be operatedbeyond the extended LOV.In response to comments on the

notice of proposed rulemaking, the FAAhas made a number of substantivechanges which significantly reduce thecosts presented in the proposal. TheFAA has

Eliminated the requirement toevaluate WFD associated with mostrepairs, alterations, and modifications ofthe baseline 2 airplane structure.

Simplified how an LOV may beextended.

Extended the compliance dates bywhich design approval holders mustestablish an LOV for existing airplanes.

Extended the time for operators toincorporate LOVs into theirmaintenance programs.

Limited the applicability of thefinal rule to transport category, turbine-powered airplanes with a typecertificate issued after January 1, 1958.

Todays rule requires that designapproval holders take the necessarysteps to preclude WFD in the future byrequiring that they establish LOVs.Although the rule allows designapproval holders to establish LOVswithout relying on maintenance actions,

the FAA expects most current designapproval holders to adopt LOVs thatwill rely on such actions. Since WFD is

by definition a condition in whichstructure will no longer meet theresidual strength requirements of 25.571(b), it could lead to acatastrophic failure. Thus the FAAwould mandate those maintenanceactions by airworthiness directive. The

agency expects these actions to greatlyreduce the number of unanticipatedinspections and repairs resulting fromemergency airworthiness directives theFAA issues when WFD is discovered inservice. The FAA estimates the value ofmanaging WFD with maintenanceactions developed under this final ruleversus the current practice of issuing

airworthiness directives as WFD isfound is worth $4.8 million in presentvalue. There are other benefits of thisrule that were not included in the final

benefit assessment. They includeprevention of accidents and a longereconomic life for the airplane. The FAAestimates that this rule will cause oneairplane to be retired because of itsreaching the anticipated LOV in the 20-year analysis period. The retirement ofthis one airplane will result in costs ofapproximately $3.8 million, with apresent value of approximately $3.6million. This operators cost is the only

cost attributed to the final rule, sincemanufacturer costs were found to beminimal.

Thus, as noted earlier, this final rulesestimated present value benefits of $4.8million exceed the estimated presentvalue costs of approximately $3.6million.

II. Background

A. Summary of the NPRM

On April 18, 2006, the FAA publisheda notice of proposed rulemaking(NPRM), entitled Aging AircraftProgram: Widespread Fatigue Damage.3

That proposal was based on arecommendation from the AviationRulemaking Advisory Committee(ARAC). The NPRM contained extensiverequirements for setting and supportingan initial operational limit for anairplane model. The FAA proposed thatthe rule apply to transport categoryairplanes with a maximum gross takeoffweight of greater than 75,000 pounds.The due date for comments was July 17,2006.

The FAA proposed that designapproval holders for those airplanes berequired to take actions to preclude

WFD. For new airplanes, the FAAproposed to amend 25.571 andAppendix H to part 25 to require thatapplicants for a new type certificateestablish an initial operational limit andinclude that limit in the AirworthinessLimitations section of the Instructionsfor Continued Airworthiness for theairplane. The agency also proposed thatapplicants develop guidelines forevaluating repairs, alterations, andmodifications for WFD.



3/44


469 FR 45936, July 30, 2004.

570 FR 40168, July 12, 2005: Fuel Tank SafetyCompliance Extension (final rule) and AgingAirplane Program Update (Request for comments).

670 FR 40166, July 12, 2005 (PSANM1107122005).

771 FR 38540.

8The final rule requires that design approvalholders evaluate airplane configurations thatinclude modifications mandated by airworthinessdirective.

Section 25.1807 proposed that holdersof design approvals for existingairplanes or applicants for suchapprovals be required to do thefollowing:

1. Establish an initial operationallimit; and

2. Establish a new AirworthinessLimitations section or revise an existing

Airworthiness Limitations section toinclude the initial operational limit.

Section 25.1807(g) proposed thatholders of design approvals for existingairplanes or applicants for suchapprovals be required to prepare thefollowing:

1. A list of repairs and modificationsdeveloped and documented by thedesign approval holder;

2. Service information formaintenance actions necessary topreclude WFD from occurring before theinitial operational limit; and

3. Guidelines for identifying,evaluating, and preparing serviceinformation for repairs, alterations, andmodifications for which no serviceinformation exists.

For existing airplanes for which aninitial operational limit is established, 25.1809 proposed that design changes

be evaluated for susceptibility to WFDand, if a change were susceptible, thatthe design approval holder identifywhen WFD is likely to occur andwhether maintenance actions would berequired. Section 25.1811 provided thatany person could apply to extend anoperational limit, using a processsimilar to that for establishing the initial

operational limit. Under 25.1813,certain repairs, alterations, andmodifications proposed for installationon airplanes with an extendedoperational limit would also beevaluated.

The FAA proposed to amend theoperating requirements of parts 121 and129 to require that no operator couldoperate an airplane unless the initialoperational limit or extendedoperational limit for the airplane had

been incorporated into the operatorsmaintenance program.

The NPRM contains the background

and rationale for this rulemaking and,except where the FAA has maderevisions in this final rule, should bereferred to for that information.

B. Related Activities

In July 2004, the FAA published thenotice entitled Fuel Tank SafetyCompliance Extension (Final Rule) andAging Airplane Program Update(Request for Comments) 4 to proposeairworthiness requirements for design

approval holders to support certainoperational rules. The FAA requestedcomments on the agencys proposal.

In July 2005, the FAA published adisposition of comments received inresponse to our request.5 Also in July2005, the agency published a policystatement, SafetyA SharedResponsibilityNew Direction for

Addressing Airworthiness Issues forTransport Airplanes, 6 that explains ourreasons for adopting requirements fordesign approval holders.

On May 22, 2006, the FAA publisheda Notice of Availability and request forcomments on proposed AdvisoryCircular (AC) 120YY, WidespreadFatigue Damage on Metallic Structure.The notice stated that the proposed ACcould be found on the Internet athttp://www.faa.gov/aircraft/draft_docs.This proposed advisory circularprovides guidance to design approvalholders on establishing initial andextended operational limits to precludeWFD for certain transport categoryairplanes and evaluating repairs,alterations, and modifications to theairplanes. The advisory circular alsoprovides guidance to operators onincorporating the initial or extendedoperational limit and any relatedairworthiness limitation items into theirmaintenance programs. The noticespecified that comments on theproposed advisory circular were to bereceived by July 17, 2006.

On July 7, 2006, at the request of anumber of commenters, the FAApublished a notice 7 extending the

comment period on both the NPRM andproposed AC 120YY to September 18,2006. On August 18, 2006, the agencyposted proposed AC 25.5711X, DamageTolerance and Fatigue Evaluation ofStructure, on the Internet at http://www.faa.gov/aircraft/draft_docs.Comments on this document, whichproposed revision of existing AC25.5711C, were due by October 21,2006.

On November 26, 2006, the FAA helda public meeting with the ARACTransport Airplane and Engine IssuesGroup. Under ARAC, the AirworthinessAssurance Working Group (AAWG) hadpreviously provided recommendationsto the FAA on how to addresswidespread fatigue damage. Because theFAA had received several commentsconcerning differences between theAAWGs recommendations and theNPRM, the meeting was held to discuss

the reasons for these differences. TheFAAs presentation at the meeting has

been placed in the docket for thisrulemaking. Except as discussed in thecontext of specific issues affecting thisfinal rule, the FAA will not revisit thosedifferences here.

On December 11, 2008, at the requestof the Acting Administrator, the FAA

held a public meeting to allowcomments on the changes that hadoccurred to the rule since it had beenproposed in the NPRM. A TechnicalDocument describing those changes wasposted in the docket, and theannouncement of the meeting andopening of the comment period for theTechnical Document was published inthe Federal Register on Nov. 7, 2008 (73FR 66205). The public was invited tosubmit comments on the TechnicalDocument either in person at themeeting or by sending them to thedocket. Seventy-one people attended the

meeting and Boeing, the Air TransportAssociation of America (ATA), andFedEx made presentations, along withthe FAA. Many attendees commented orasked questions. In addition, 12commenters submitted comments aboutthe Technical Document to the docket.The comment period closed onDecember 22, 2008.

While some of the comments receivedduring the comment period for theTechnical Document were new, manywere restatements of comments madeafter publication of the NPRM. Weaddress all of the comments, from bothcomment periods, in the section below.

Comments received during bothcomment periods are posted to thedocket. A transcript of the publicmeeting, including presentations givenand comments delivered there, may also

be found in the docket.

C. Differences Between NPRM and FinalRule

1. Substantive Changes

The FAA has eliminated therequirement to evaluate WFD associatedwith most repairs, alterations, andmodifications of the baseline airplanestructure.8 The agency has also made a

change in terminology. This final ruleuses the term limit of validity of theengineering data that supports themaintenance program (LOV) ratherthan the term initial operational limit.The FAA finds that the term limit ofvalidity is more appropriate than theterm initial operational limit indefining the point to which an airplane

http://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docshttp://www.faa.gov/aircraft/draft_docs


4/44


972 FR 63363, November 8, 2007.10Certification Procedures for Products and Parts.

11This section, which includes an applicabilitytable for part 26, was adopted as part of the EAPASfinal rule.

may be safely operated. Therequirements in this final rule forestablishing the LOV under 26.21 arethat it be supported by test evidence andanalysis at a minimum and, if available,

by service experience or serviceexperience and teardown inspectionresults for those airplanes of similarstructural design with the highest total

accumulation of flight cycles or flighthours (commonly referred to as high-time airplanes). This criterion is similarto the criterion used in 25.571(b). Thisfinal rule also clarifies how the LOVmay be extended, using the same typeof evaluation as that required for settingthe LOV under 26.21.

In response to requests for more time,the FAA has extended the compliancedates by which design approval holdersmust establish an LOV for existingairplanes. Those dates vary according tothe age of the airplanes, from 18 monthsafter the effective date for the oldest

airplanes to 60 months after theeffective date for the newest ones.Additionally, the agency has extendedthe time for operators to incorporateLOVs into their maintenance programs.These dates vary with the age of theairplanes as well, and are 12 monthslater than the related design approvalcompliance dates, thus giving operators12 months to incorporate the LOV intotheir maintenance programs. Operatorcompliance dates range from 30 to 72months after the effective date. The FAAhas also changed the proposedoperational rules to correct aninadvertent ambiguity in the NPRM

regarding obligations of operators ofairplanes for which the type certificateholder might fail to establish an LOV asrequired.

Another change involves applicabilityto existing transport category airplanes.This final rule applies to transportcategory, turbine-powered airplaneswith a type certificate issued after

January 1, 1958. This limitation wasadded to make applicability of todaysrule consistent with that of the otheraging airplane rules. The FAA also

added airplanes to the list of thoseexcluded from the LOV requirements of 26.21 because the airplanes are notoperated under parts 121 or 129. Eitherthey are being operated under differentparts of the Code of Federal Regulations(CFR) or they are not in service at thistime. The number of these airplanes stilloperating is very small, and the

probability of their retirement in thenear future is high.

2. Regulatory Evaluation Changes

The FAA has substantially revised theRegulatory Evaluation for severalreasons. One concerns differences

between the rule as proposed and thefinal rule. For example, the requirementto evaluate WFD associated withrepairs, alterations, and modifications ofthe baseline airplane structure, exceptfor those mandated by airworthinessdirectives, has been eliminated fromthis final rule. Another reason concernsinformation received during therulemaking process which indicatedthat some of the initial assumptionsabout benefits and costs of the rule werenot valid. For example, initially, theFAA assumed that design approvalholders would set the LOV for a specificairplane model at the design servicegoal for that model. However,subsequently, some design approvalholders indicated that they planned toset the LOV 33% to 180% higher. Thenet effect of these changes has been todramatically reduce the costs estimatedfor compliance with the rule.

Our revised Regulatory Evaluation

lists three potential sources of benefitsof the rule, namely (1) prevention ofaccidents; (2) extension of the economiclife of the airplane with correspondingrevenues from that additional economiclife; and (3) near elimination ofemergency airworthiness directives.

Preventing a WFD accident isestimated to have benefits ranging from$20 million to $680 million. There aremultiple factors, however, that make itdifficult to forecast that this ruleabsolutely would prevent accidents.

Among them are earlier FAArulemaking actions to prevent knownfatigue problems from reoccurring.

Similarly, although specificmaintenance actions designed to extendthe life of airplane structure have addedyears of service to the DC9 fleet,quantification of such values for othermodels is unnecessary, given that

benefits already exceed the nearlyminimal costs.

As a result, the quantified benefit ofthis final rule is based solely on the nearelimination of emergency ADspertaining to WFD. The analysisassumes the rule will prevent 1.5 daysof down time associated withemergency ADs.

3. New Part 26 for Design ApprovalHolders Airworthiness Requirements

In the WFD proposed rule, and inproposals for other Aging AirplaneProgram rules, the FAA placed the

airworthiness requirements for designapproval holders in part 25, subpart I.As explained in the EnhancedAirworthiness Program for AirplaneSystems/Fuel Tank Safety final rule(EAPAS/FTS),9 the FAA decided afterfurther review and input from industryand foreign aviation authorities to placethese requirements in a new part 26 andmove the enabling regulations into part21.10 The FAA determined that this wasthe best course of action because itkeeps part 25 applicable only toairworthiness standards for transportcategory airplanes. This is important

because it maintains harmonization andcompatibility among the United States,Canada, and the European Unionregulatory systems. Providing referencesto part 26 in part 21 clarifies how thepart 26 requirements will addressexisting and future design approvals.

In creating part 26, the FAArenumbered the proposed sections ofpart 25, subpart I, and incorporated thechanges discussed in this preamble. Atable of this renumbering is shown

below.

FIGURE 2TABLE SHOWING RELATIONSHIP OF PROPOSED PART 25 SUBPART I TO PART 26 FINAL RULE

Part 26 final rule Proposed part 25

SUBPART CAging Airplane SafetyWidespread Fatigue Damage ... ... Subpart IContinued Airworthiness 26.5 Applicability table .............................................................................. New11 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)



5/44


FIGURE 2TABLE SHOWING RELATIONSHIP OF PROPOSED PART 25 SUBPART I TO PART 26 FINAL RULEContinued

Part 26 final rule Proposed part 25

25.1813 Repairs, alterations, and modifications: Widespread Fa-tigue Damage (WFD).

4. New Subparts for Airworthiness

Operational RulesThe WFD NPRM was among several

Aging Airplane Program rulemakinginitiatives that proposed new subparts(subparts AA and B in parts 121 and129, respectively) for airworthinessrequirements, and redesignated certainsections of parts 121 and 129. Since theEAPAS/FTS final rule was the first ofthese rulemaking initiatives to becodified, the new subparts andredesignated sections were adopted inthat rule. Therefore, the FAA hasremoved the regulatory language andrelated discussion about these changes

from this final rule. This final rule addsnew sections that include WFD-relatedrequirements: 121.1115 and 129.115.

D. Summary of Comments

The FAA received comments aboutthe NPRM from 40 commenters,including airplane manufacturers,operators, aviation associations, andothers. The comments covered an arrayof topics and contained a range ofresponses. There was much supportfrom airplane manufacturers, operators,and associations for the concept ofprecluding WFD in aging airplanes.

There were also a number ofrecommendations for changes andrequests for clarification. As previouslydiscussed, at the December 11, 2008public meeting, Boeing, FedEx, andATA gave presentations of theirresponses to the Technical Document.

In addition, the FAA receivedcomments about airworthinessrequirements for design approvalholders. We addressed many of thesame or similar comments in the July2005 disposition of commentsdocument to the Fuel Tank SafetyCompliance Extension (Final Rule) and

Aging Airplane Program Update(Request for Comments). We alsoexplained in detail the need for theserequirements in our July 2005 policystatement. As a result, the FAA will notrevisit those comments here.

III. Discussion of the Final Rule

A. Overview

1. Widespread Fatigue Damage

Widespread fatigue damage is thesimultaneous presence of cracks atmultiple structural locations that are of

sufficient size and density that the

structure will no longer meet theresidual strength requirements of14 CFR 25.571(b). This may result incatastrophic structural failure and lossof the airplane.

Fatigue is the gradual deterioration ofa material subjected to repeatedstructural loads. When it occurs in morethan one location, cracks manifestthemselves as multiple site damage ormultiple element damage. Multiple sitedamage is the simultaneous presence offatigue cracks at multiple locations thatgrow together in the same structuralelement, such as a large skin panel or

lap joint. Multiple element damage isthe simultaneous presence of fatiguecracks in similar adjacent structuralelements, such as frames or stringers.Some structural elements aresusceptible to both types of damage, and

both types may occur at the same time.Cracks associated with multiple site

damage and multiple element damageare initially so small that they cannot bereliably detected with existinginspection methods. Widespread fatiguedamage is especially hazardous becausethese small, undetectable cracks inmetallic structure can link up andgrow very rapidly to bring about

catastrophic failure of the structure.Although operators perform routinestructural inspections to detect fatiguedamage, fatigue cracks related to WFDgrow so rapidly that operators cannotinspect susceptible structures oftenenough to detect the cracks before theycause structural failure. As a result,many of the findings of these types ofcracks have been fortuitous: mechanicsand others have observed fatigue crackswhile doing other work. For example,cracks have been found by workerswhile stripping and painting anairplane. Cracks have also been found

by mechanics conducting unrelatedinspections of skin anomalies on theexternal fuselage; further investigationrevealed multiple cracks in stringersand circumferential joints.

In other cases, undetected multiplesite damage in wing or fuselagestructure has eventually led tocatastrophic failure of the structure inflight. For example, wing failures haveresulted in losses of C130 and P4Y2airplanes. Failures of aft pressure

bulkheads have caused decompressionof B747, DC9, and L1011 airplanes.

Concern about WFD was brought to

the forefront of public attention in April1988, when an 18-foot-long section ofthe upper fuselage of a Boeing Model737 airplane separated from the airplaneduring flight. The airplane, operated byAloha Airlines, was en route from Hiloto Honolulu, Hawaii, at 24,000 feet.Onboard were 89 passengers and 6crewmembers. A flight attendant died asa result of the accident, and eightpassengers were injured.

The damage to the airplane consistedof a total separation and loss of a majorportion of the upper crown skin andother structure. The damaged area

extended from the main cabin entrancedoor aft for about 18 feet. At the timeof the accident, the airplane hadaccumulated 89,680 flight cycles and35,496 flight hours.

In the years after the Aloha Airlinesaccident, WFD was discovered in thefollowing airplanes:

Boeing 727:Cracking along a lapjoint.

In 1998, during maintenance, twocracks were found growing out fromunderneath the lap joint. Disassembly ofthe joint revealed a 20-inch hiddencrack from multiple site damage on the

lower row of rivet holes in the innerskin. Boeing 737:Cracking along a lap

joint.In July 2003, a mechanic preparing to

paint discovered extensive multiple sitedamage with up to 10 inches of locallink-up of cracks in one area.

Boeing 747:Cracking of the aftpressure bulkhead.

In 2005, Boeing issued serviceinformation to address multiple sitedamage of the aft pressure bulkheadradial lap splices. The serviceinformation was based on analysis andfatigue 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 asingle radial lap splice during aninspection of the aft pressure bulkhead.

McDonnell Douglas DC9:Crackingof the aft pressure bulkhead.

On June 22, 2003, widespread fatiguedamage on a DC9 airplane led to rapiddecompression at 25,000 feet. Laterinspection revealed multiple site



6/44


12The group was initially known as theAirworthiness Assurance Task Force.

13Task 3.Widespread Fatigue Damage (WFD) ofRepairs, Alterations, and Modifications. Provide awritten report providing recommendations on howbest to enable part 121 and 129 certificate holders

of airplanes with a maximum gross take-off weightof greater than 75,000 pounds to assess the WFDcharacteristics of structural repairs, alterations, andmodifications as recommended in a previous ARACtasking. The written report will include a proposedaction plan to address and/or accomplish theserecommendations including actions that should beaddressed in Task 4 [below]. The report is to besubmitted to the ARAC, Transport Airplane andEngine Issues Group, for approval. The ARAC,Transport Airplane and Engine Issues Group, willdetermine as appropriate the means by which theaction plan will be implemented. The proposedactions and implementation process approved bythe ARAC, Transport Airplane and Engine IssuesGroup, will be subject to FAA concurrence.Published in 69 FR 26641, May 13, 2004.

14Under 14 CFR 91.403(c), no person may operatean airplane unless applicable airworthinesslimitations have been complied with. By requiringoperators to incorporate the LOV airworthinesslimitations developed by the design approval

Continued

damage with extensive link-up ofcracks.

Lockheed C130A:Fatigue cracks inthe wing structure.

On August 13, 1994, whileresponding to a forest fire in theTahachapi Mountains near Pearblossom,California, the airplane experienced anin-flight separation of the right wing. All

3 flight crewmembers were killed, andthe airplane was completely destroyed.

Lockheed C130A:Fatigue cracks inthe wing structure.

On June 17, 2002, while executing afire retardant drop over a forest fire nearWalker, California, the airplanes wingsfolded upward at the center wing-to-fuselage attachment point, and theairplane broke apart. All three flightcrewmembers were killed, and theairplane was completely destroyed.

Consolidated-Vultee P4Y2:Fatiguecracks in the wing structure.

On July 18, 2002, the airplane wasmaneuvering to deliver fire retardantover a forest fire near Estes Park,Colorado, when its left wing separatedfrom the airplane. Both flightcrewmembers were killed, and theairplane was destroyed. An examinationof other Consolidated-Vultee P4Y2airplanes revealed that the area wasdifficult to inspect because of itslocation relative to fuselage structure.

Lockheed L1011:Failure in-flightof the aft pressure bulkhead stringerattach fittings.

In August 1995, an L1011 airplaneexperienced a rapid decompression at33,000 feet. Twenty stringer end fittings

were found severed and the aft pressurebulkhead was separated from thefuselage crown by a crackapproximately 12 feet long. The flightcrew was unable to maintain cabinpressure control until after rapiddescent.

Boeing 747:Cracking of adjacentfuselage frames.

In 2005, during an overnightmaintenance visit, missing skinfasteners common to a fuselage framewere discovered in the upper deck area.Further inspection revealed that theframe was severed. Substantial cracking

was also found in the adjacent left andright frames. Airbus A300:Cracking of adjacent

fuselage frames.In 2002, investigations conducted as a

result of fatigue cracks found on a testarticle and later in service revealed thatcracking of certain adjacent fuselageframes could result in multiple elementdamage. The determination was basedon analysis, service experience, andfatigue testing.

Since 1988, the FAA has issuedapproximately 100 airworthiness

directives to address WFD in airplanes.Approximately 25 percent of theseairworthiness directives were too urgentto allow the public an opportunity tocomment in advance. Theseairworthiness directives requiredinspections, and the FAA latersuperseded the majority of them toexpand the inspections or require

modifications because inspections werenot enough to preclude WFD.

Shortly after the Aloha Airlinesaccident, the AAWG 12 was formed toidentify procedures to ensure continuedstructural airworthiness of agingtransport category airplanes. Basicapproaches defined by the group andaccepted by the FAA includedrecommending procedures to precludeWFD in those airplanes. When ARACwas formed in 1991 to provide adviceand recommendations on safety-relatedmatters to the FAA, the AAWG becamea working group under its auspices. In

2003 the AAWG completed itsrecommendation on WFD.In 2004, the FAA tasked ARAC to

provide a written report on part 121and 129 certificate holders operatingairplanes with a maximum takeoff grossweight of greater than 75,000 pounds toassess the WFD characteristics ofstructural repairs, alterations, andmodifications as recommended in aprevious tasking of the AviationRulemaking Advisory Committee. 13During the comment period on theNPRM for this final rule, the AAWG wasworking to complete Task 3, torecommend how an operator would

include consideration of WFD forrepairs, alterations, and modifications toairplanes operated under part 121 or129.

On April 17, 2007, the AAWGpresented its final report on Task 3 toARAC. Many of the conclusions andrecommendations in the final report arethe same as those provided in the

comments on the proposed rule whichare discussed in this preamble.

2. Final Rule

This final rule requires actions topreclude WFD in transport categoryairplanes. It applies to both existingtransport category airplanes that have amaximum takeoff gross weight greater

than 75,000 pounds and to all transportcategory airplanes to be certified in thefuture, regardless of the maximumtakeoff weight.

Todays rule imposes requirements onthose holding design approvals forexisting transport category airplanesthat are subject to the rule. The designapproval holders are required toevaluate the structural configuration ofeach model for which they hold a typecertificate to determine its susceptibilityto WFD and, if it is susceptible, todetermine that WFD would not occur

before the proposed LOV. Theevaluation would be based on testevidence and analysis at a minimumand, if available, service experience orservice experience and teardowninspection results of airplanes with ahigh number of total accumulated flightcycles or flight hours or both, which arefrequently referred to as high-timeairplanes. The evaluation would beperformed on airplanes of similarstructural design, accounting fordifferences in operating conditions andprocedures. Using the results of theevaluation, the design approval holdermust then establish an LOV.

Holders of approvals for design

changes that increase an airplanesmaximum takeoff gross weight to morethan 75,000 pounds, or decrease it frommore than 75,000 pounds to 75,000pounds or less after the effective date ofthe rule, must also evaluate the affectedairplanes for WFD and establish LOVsfor those airplanes.

The final rule amends Appendix H topart 25 to require that the LOV whichis established by the design approvalholder be included in the AirworthinessLimitations section of the Instructionsfor Continued Airworthiness. It alsoamends operating rules in parts 121 and

129 to require that operators of anaffected airplane incorporate into theirmaintenance programs an AirworthinessLimitations section that includes anLOV for that airplane.

The amendments to parts 121 and 129have the effect of prohibiting operationof an airplane beyond its LOV.14 For



7/44


8/44


fact remains that WFD is difficult, if notimpossible, to detect. Small cracks thatcan lead to WFD often cannot bedetected until they suddenly increase insize and link up, to cause catastrophicdamage. Dramatic crack growth canoccur quite suddenly and quickly, after

being undetectable for long periods oftime. That is why maintenance

inspections cannot be relied on to detectand repair such cracking. Airplanemaintenance programs includeinspections that are designed to detectobvious damage and irregularities.WFD, by its nature, is usually hidden,and not readily detectable. Discovery ofWFD in some airplanes by mechanicshas been a purely random occurrence,where damage detected was the result ofWFD that had progressed to the point offailure of structural members. Anexample is discovery of WFD on aBoeing 747, with adjacent framecracking and separations. It was

detected because of loose rivets on theskin. Mechanics happened upon theWFD damage by chance, becauseinspections had not uncovered anyproblem. Improving a maintenanceprogram by adding or modifyinginspections would not necessarily havethe effect of improving detection ofWFD. In general, the only way toaddress WFD is by modifying orreplacing structure.

The National Transportation SafetyBoard report stated the following:

It is probable that numerous small fatiguecracks in the lap joint along S10L joined to

form a large crack (or cracks) similar to thecrack at S10L that a passenger saw whenboarding the accident flight. The damagediscovered on the accident airplane, damageon other airplanes in the Aloha Airlines fleet,fatigue striation growth rates, and the servicehistory of the B737 lap joint disbondproblem led the Safety Board to concludethat, at the time of the accident, numerousfatigue cracks in the fuselage skin lap jointalong the S10L linked up quickly to causecatastrophic failure of the large section of thefuselage.

The AAWG worked on varioussolutions to the safety problemsencountered by aging airplanes and was

instrumental in developing the fourprograms listed earlier in thisdocument. However, they decided thatadditional actions were needed topreclude WFD in airplanes, and thesteps they outlined included:

Setting limits of validity of themaintenance program.

Deciding whether WFD can beinspected for, and, if so, for how longsuch inspections would be effective.

Defining when WFD-susceptiblestructure should be modified orreplaced.

Lynden Air Cargo stated that itsupported an approach that usedairworthiness directives to addressWFD-susceptible structural componentsinstead of an LOV approach for theentire airplane. Lynden Air Cargofurther stated that the unique design ofthe L382G allows for the wholeairframe to be renewed by replacing

WFD-susceptible sections (e.g., centerwing and outer wing).

The FAA agrees with Lynden AirCargo that WFD-susceptible structurecan be replaced when the engineeringdata determines it should be replaced topreclude WFD. However, as airplanesage, other areas may also need to bereplaced. The only way to determinethat is to evaluate the engineering data(analyses, tests, service experience) forthe entire airplane. Without the LOV,the operational life of an airplane isundefined. As a result, the list of areasto inspect, modify, replace, or any

combination of these may be extensive,since the data would need tosubstantiate an indefinite life.

3. Divide Rule into Two

FedEx, Northwest Airlines,Continental Airlines, NACA, and ATAstated that the proposed draft final ruledoes not allow the public anopportunity to comment on the LOVsthat design approval holders propose ascompliance to part 26. They suggestedthe rule be divided into two rules: onefor design approval holders and one foroperators. The commenters noted thatthis two-step process would provide the

public the opportunity to comment ondesign approval holders proposedLOVs. Deferral of the operator rulewould also allow for public comment onthe WFD maintenance actions at thesame time LOVs are established. Insupport of this approach, FedExspecifically argued that the incrementalcosts for the part 26 work to designapproval holders is minimal, as designapproval holders have confirmed intheir comments to this docket.

The FAA has determined thatcomplementary, concurrentrequirements for design approval

holders and operators are necessary toachieve the safety benefits of theproposed rule in a timely manner.Although design approval holderswould be required to develop LOVs foraffected airplanes under part 26, thesafety benefit for this rulemakinginitiative is not met until operatorsincorporate LOVs and only operateairplanes up to the point in time forwhich it can be shown that the airplanewill be free from WFD. Until designapproval holders actually comply withpart 26, its not possible to identify the

precise LOV for any particular airplane.However, operators have had adequategeneral notice of the objectives of thisrulemaking and the proposed methodsfor achieving those objectives in theform of the design approval holdersanticipated LOVs. Since the publicmeeting, both Boeing and Airbus haveprovided revised information about

where they anticipate those LOVs willbe set.

If additional, multiple rulemakingsare necessary to require operators toincorporate LOVs into theirmaintenance programs, there is a risk ofairplanes exceeding LOVs before thoserules become effective. The FAAconcludes that, to achieve our safetyobjectives, design approval holders andoperators must have a sharedresponsibility on certain safety issuesaffecting the existing fleet. We alsoconclude, from reviews such as theCommercial Airplane Certification

Process Study (March 2002), that weneed to facilitate more effectivecommunication of safety information

between design approval holders andoperators. As both technology andairworthiness issues become morecomplex, certain fleet-wide safety issuesrequire the FAA to implementcomplementary requirements for designapproval holders and operators, whenappropriate.

C. Concept of Operational Limits

This final rule requires designapproval holders to establish limits ofvalidity of the engineering data that

supports the maintenance program. Theproposed rule would have required thatdesign approval holders establish initialoperational limits beyond whichairplanes may not be operated. Theinitial operational limit would be basedon the demonstration of freedom fromWFD up to that initial operational limit.

Several commenters supported theconcept of early detection of WFD foraging airplanes but opposed therequirement to establish initialoperational limits beyond which theairplanes could not be operated. Thesecommenters equated establishment of

such limits with mandatory retirementof airplanes and suggested that, instead,the FAA enhance current maintenanceprograms and practices.

1. Requests for Requiring MaintenancePrograms Instead

An aircraft leasing and tradingcompany named AWAS recommendedthat an inspection-based maintenanceprogram become mandatory as airplanesreach their design service goal or theiroperational limit. Lynden Air Cargostated that there are better, less intrusive



9/44


methods to achieve early detection ofWFD than the application of onerousinitial and extended operational limits.According to the commenter, thesemethods include proper establishment,accomplishment, and enforcement ofcurrent airplane maintenance programs,such as the maintenance programsrequired by parts 121 and 135. Lynden

Air Cargo said it is continuouslyrevising its Continuous AirworthinessMaintenance Program to include adesign approval holder inspectionprogram of Structural Significant Itemsand recommended structural service

bulletins.These commenters raise some of the

same issues as did those who opposedthe rule altogether. They suggest thatcurrent programs for aging airplanes ornew maintenance programs to detectWFDalong with issuance ofairworthiness directives when WFD isdetectedwould obviate the need for

setting operational limits.As stated in the NPRM, the structuralfatigue characteristics of airplanes areonly understood up to a point in timeconsistent with the analyses performedand amount of testing accomplished.Structural maintenance programs aredesigned with this in mind. The LOV isdefined as the limit of the engineeringdata that supports the structuralmaintenance program and the currentregulatory maintenance requirements ofparts 121 and 129 do not require thatWFD be specifically addressed.

Also as discussed previously, WFDcannot be detected reliably by existing

inspection methods. Therefore, the FAAconsiders that WFD in existing airplanesneeds to be proactively addressed byrequiring design approval holders to userelevant engineering data to project thenumber of flight cycles or flight hoursor both which the airplanes canaccumulate without incurring WFD. Theengineering data may include theevaluation and establishment ofmaintenance actions that address WFD.

2. Single Retirement Point for a Model

The Modification and ReplacementParts Association (MARPA) opposed a

single, mandatory retirement age forairplanes because of the vastdifferences possible between aircraftmodels, missions, and maintenance. Ina similar vein, a company named Safair,which is based in South Africa,commented that the difference instructural integrity of aging airframeslies in their use and abuse during theirlives and is largely dependent on thespecific load factors to which theairframe is subjected. Safair added thatthe proposed rule may be based oninadequate technical evaluation of the

actual operational experience,considering the number of older aircraftthat have been safely operated well

beyond the actual cycles listed in theproposed rule.

It is true that there may be differencesbetween airplanes of the same modelwhich 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 missionprofile to account for the differentloads the airplane may be subjected tothat must be addressed in their design.In setting the LOV, manufacturers willtake this information into account, alongwith service experience of the particularairplane model and fatigue testevidence. The LOV must apply to anairplane model, because it is based onanalysis of the service experience of theentire fleet of affected airplanes.

3. Potentially Adverse Effect on Safety

Lynden Air Cargo, MARPA, and theairplane leasing and trading companyAWAS also suggested that mandatoryretirement of airplanes may have anadverse effect on safety which has not

been considered by the FAA.Specifically, AWAS envisioned thatoperators of airplanes approaching theiroperational limit may perform minimalmaintenance on airframes to savemoney. MARPA said that mandatoryretirement could have a negative

influence on the degree and timing ofsafety-related investment, particularlyas the aircraft nears its throwawayyears. The owner and operator may notintend to be unsafe, suggested MARPA,

but the question Why invest now? willarise. A similar comment from LyndenAir Cargo anticipated that operators areunlikely to apply the same level ofmaintenance effort for an airplane 1,000flight hours from the scrap heap as onewith 20,000 flight hours remaining.

Under existing operating rules,operators are responsible formaintaining their airplanes in an

airworthy condition. These maintenancerequirements apply equally to new andold airplanes. Even without this finalrule, operators have always planned toretire airplanes, and service experienceindicates that they generally continue tomaintain them safely up to that point.The purpose of this final rule is toensure that airplanes are retired beforethe point where they can no longer besafely maintained with respect to WFD.

D. Change in Terminology (InitialOperational Limit to LOV)

1. Rationale for the Term LOV

The NPRM proposed to establish aninitial operational limit, expressed inflight cycles, flight hours, or both,

beyond which an airplane could not beoperated. Several commenters,

including industry representatives onthe AAWG and Boeing, objected to thisterm and suggested that instead the FAArefer to the limit of validity of theengineering data that supports themaintenance program, or LOV. Thisfinal rule uses the term LOV to expressthe point beyond which an airplanecannot be operated (unless an extendedLOV has been approved).

In recommending that the FAA referto the limit of validity of theengineering data that supports themaintenance program, or LOV,industry representatives on the AAWGstated that the term initial operationallimit implies that the use of an airplaneis limited in operation. According to thecommenters, the limitation is actually

based on the engineering knowledge ofthe structural behavior of the airplanemodel and is intended to ensure thatrequired inspections are sufficient toensure safe operations until a certainnumber of flight cycles or flight hoursor both have been reached. Theengineering data that support suchinspection requirements change withtime due to knowledge gained from in-service experience and additionaltesting.

Boeing defined LOV as the point(usually measured in flight cycles) inthe structural life of an airplane wherethe engineering basis for themaintenance actions contained in theAirworthiness Limitations section of theInstructions for ContinuedAirworthiness is no longer a validpredictor of future structural behavior.

Our intent, as stated in the NPRM,was to ensure that large transportcategory airplanes not be operated

beyond their initial operational limit,unless operators had incorporated anextended operational limit and the

service information necessary to supportit into their maintenance programs. Justas the structural fatigue characteristicsof airplanes are understood only up toa point consistent with analysesperformed, testing accomplished, andin-service experience gained, theengineering data used to developinspections and modifications topreclude WFD is valid only to a certainpoint.

For these reasons, the FAA finds theterm limit of validity more appropriatethan the term initial operational limit



10/44


16March 31, 1998, 63 FR 15708. 1772 FR 70486, December 12, 2007.

in defining the point to which anairplane may be safely operated inrelation to WFD. The LOV issubstantiated by test evidence andanalysis. This test evidence and analysismay be augmented by serviceexperience, or by service experience andteardown inspection results, if available.The service experience and teardown

inspection results must be for high-timeairplanes of similar structural design,accounting for differences in operatingconditions and procedures. Additionalengineering data would be necessary tosupport operation of an airplane beyondthe LOV. The legal effect of the termsinitial operational limit and limit ofvalidity is the same. Therefore, this finalrule uses the term limit of validityinstead of the term initial operationallimit.

2. Refer to the Structural MaintenanceProgram

Airbus stated that the term limit ofvalidity of the engineering data thatsupports the maintenance programshould be revised for clarification.Because WFD is addressed byperforming inspections or modificationsor replacements of airframe structure,the phrase maintenance programshould be changed to structuralmaintenance program.

The FAA agrees with Airbus and thatchange is reflected here.

E. Repairs, Alterations, andModifications

This final rule requires design

approval holders to establish LOVs forairplane models subject to this rule.However, it does not include separaterequirements to address WFD forrepairs, alterations, and modifications tothose airplanes or to develop guidelinesto address repairs, alterations, ormodifications. The proposed rule wouldhave required evaluation of repairs,alterations, and modifications of the

baseline structure of the airplane. Theproposed rule would have also requireddevelopment of guidelines for repairs,alterations, and modifications. Personsrepairing or altering airplanes certified

to 25.571 at Amendment 2596 or laterare already required to show the repairor alteration to be free from WFD up tothe airplanes design service goal. Thisrequirement has not changed sinceadoption of Amendment 2596 in1998.16

1. Whether Repairs, Alterations, andModifications Pose WFD Risks

The Technical Document, discussedearlier, stated that the FAA, in response

to comments, had removed theproposed requirements for repairs,alterations, and modifications. Inresponse to the Technical Document,Lynden Air Cargo, Northwest Airlines,ATA, Continental Airlines, and FedExstated that they support removal ofrequirements for repairs, alterations,and modifications from the draft final

rule. These commenters stated thatrepairs, alterations, and modificationspresent a reduced risk for WFD becausethey will be surveyed and assessedunder the Aging Airplane Safety FinalRule and the Damage Tolerance Data forRepairs and Alterations Rule (hereafterreferred to as the Damage ToleranceData Rule).17 Commenters often usedthe term Aging Airplane Safety Rule torefer to the Damage Tolerance Data Ruleor the Aging Airplane Safety Final Rule,or both. In instances where this occurs,to avoid confusion, the name of thespecific rule has been inserted in

parentheses.These commenters expressed thebelief that a new WFD requirement forrepairs, alterations, and modifications isunnecessary because of these otherrequirements, which are already inplace. Lynden Air Cargo stated that,although it supports removal ofrequirements to evaluate repairs,alterations, and modifications for WFD

because the Damage Tolerance DataRule already adequately addresses them,it does not understand how each designapproval holder is going to establish thevalidity of its maintenance programwithout validating the repairs and

alterations it has established under thatprogram. Northwest Airlines said that itsupported the conclusion of the AAWGthat the costs of including repairs,alterations, and modifications in therule outweighed the benefits that sucha requirement would have.

Boeing, Airbus, and the EuropeanAviation Safety Agency (EASA) said theFAA should reconsider its decision toremove from the rule the requirementsfor evaluating certain repairs,alterations, and modifications. All threecommenters stated that removing thoserequirements could affect safety because

certain alterations could affect the LOVand the structural maintenance programthat supports the LOV. An example ofan alteration that could affect the LOVand structural maintenance program,the commenter maintained, is one thatwould cause a global loading increase,such as an alteration allowing a highercabin differential pressure. Airbusstated that, although the ChangedProduct Rule (14 CFR 21.101) mayaddress future alterations and

modifications, it does not cover existingones.

Boeing recommended that the FAArevise subpart E of part 26, the DamageTolerance Data Rule, for repairs andalterations, and 121.1109 and129.109, the Aging Airplane SafetyFinal Rule, to include requirements forevaluating repairs, alterations, and

modifications for WFD. Boeingsrecommendation contains two parts.First, it requests that the FAA extendthe compliance date for both rules by 18months after the effective date of theWFD rule. Second, it says the FAAshould incorporate the 2007 ARACrecommendations on evaluating repairs,alterations, and modifications into thoserules.

Boeing, Airbus, EASA, and the AlliedPilots Association (APA) stated thatcertain repairs, alterations, andmodifications need to be evaluated forWFD. APA stated that eliminating therequirement to evaluate WFD associatedwith most repairs, alterations andmodifications from the final rule isrisky, because many high-time airplanesfall into this category and will not haveany current analysis done on theirmodified airframes.

In its final report to ARAC concerningTask No. 3, the AAWG stated that it hasreviewed the accident record and hasobserved thatwhile there is atechnical possibility of a WFD-relatedaccident involving a repair oralterationthere are no recordedaccidents attributed to WFD occurringin properly-installed repairs or

alterations. The group added that areview of certain repairs, alterations,and modifications is necessary, becausesome of them have the potential todevelop WFD.

The FAA agrees with the commentersthat some repairs, alterations, andmodifications may pose a risk ofdeveloping WFD. However, the riskappears to be less than that for baselineairplane structure because all adverseservice experience to date has beenlimited to baseline airplane structure.Type certificate holders design repairs,alterations, and modifications using the

same design philosophies and loadcases as for baseline airplane structure.As they do with the baseline airplanestructure, type certificate holders re-evaluate their repairs, alterations, andmodifications as service experience isgained. Therefore, these repairs,alterations, and modifications should beacceptable up to the LOV.

The repairs, alterations, andmodifications developed by personsother than type certificate holders maypresent a slightly greater risk, becausethose persons typically do not have the



11/44


12/44


13/44


14/44


25Aviation Rulemaking Advisory Committee;Transport Airplane and Engine IssuesNew Task,dated April 11, 2007.

26A Structures Task Group is a model-specificgroup that consists of type certificate holders andoperators responsible for the development of agingairplane model-specific programs. It also includesregulatory authorities which approve and monitorthose programs.

airworthiness limitations in theirmaintenance program:

The LOV that has been specified inthe Airworthiness Limitations section ofthe Instructions for ContinuedAirworthiness; or

If the LOV has not yet beenestablished, a number equal to 12 thenumber of cycles accumulated on the

fatigue test article if a type certificate isissued prior to completion of full-scalefatigue testing.

Comments received during the NPRMcomment period were responding to theone specific compliance date publishedin the NPRM. Comments receivedduring the comment period for theTechnical Document, which describedchanges that had occurred to the rulesince it had been proposed in theNPRM, were in response to the phasedcompliance dates published in theTechnical Document, which are thedates cited in todays rule.

1. NPRM Compliance DateCommentersincluding industry

representatives on the AAWG, Cessna,Continental Airlines, Embraer, AWAS,the CAA, American Airlines, Boeing,Airbus, and FedExobjected to theproposed compliance date of December18, 2007, for both technical andpractical reasons. Several commentersstated that hard compliance dates andan expected final rule issuance inDecember 2006 would leave designapproval holders with less than 12months to comply with the subpart Irequirements (now part 26). These

commenters requested that the FAArevise the compliance dates to representa number of months after the effectivedate of the rule rather than a hard date.This approach would prevent the FAAsschedule for issuing the final rule fromaffecting compliance by design approvalholders.

We have revised the compliance datesin this final rule to specify that personsmust comply either by a datedetermined as a specified number ofmonths after the effective date of thefinal rule or (for applicants) by the dateof approval of the related certificate.

2. When to Set LOVs for ExistingAirplanes

Industry representatives on theAAWG, Boeing, Continental Airlines,Northwest Airlines, ATA, Lynden AirCargo, and FedEx stated that thereshould be a phased approach to settingLOVs, with the oldest airplane models

being addressed first. The industryrepresentatives on the AAWG suggestedthat existing airplane models subject tothe rule be divided into two groups: (1)Pre-Amendment 2545 airplanes and (2)

airplanes certified to Amendment 2545or later. The commenters stated thatperforming WFD evaluations onairplane models before the high-timeairplane reaches its design service goal,as proposed in 25.1807 (now 26.21)and as specified in the TechnicalDocument, would not significantlyincrease operational safety. This is

because WFD is typically not a concernuntil later in an airplanes operationallife. As discussed earlier, thesecommenters objected to the proposedcompliance date of December 18, 2007.Commenters also objected to thecompliance times identified in theTechnical Documentthat is, 18months for pre-Amendment 2545airplanes, 48 months for Amendment2545 up to but not includingAmendment 2596 airplanes, and 60months for Amendment 2596airplanes.

Boeing said that the final rule should

provide the greatest amount of time fordesign approval holders to developLOVs, so that LOVs provide the greatestflexibility for the fleet. Severalcommenters argued that requiringcompliance prior to or concurrent withthe Aging Airplane Safety Rule (DamageTolerance Data Rule) would not bepractical because of limited industryand FAA resources. In addition, Boeingand Northwest Airlines argued thatestablishing an LOV for an airplanemodel before significant serviceexperience had been accumulatedwould result in an erroneous LOV.

We agree that it makes sense to have

compliance dates for establishing LOVsfor existing airplanes based on therelative safety risk (i.e., addressing theoldest airplanes first) and on availableresources. However, the agency does notagree that early establishment of anLOV will result in an erroneous LOV.Setting an LOV without benefit ofsignificant service experience mightresult in an LOV that sets the limit ata lower number of flight hours or flightcycles than one that benefits fromsignificant service experience, but itwould be incorrect to characterize it aserroneous. This is because the LOV is

a function of the fatigue knowledge baseavailable at the time it is established.

a. Pre-Amendment 2545 Airplanes

Industry representatives on theAAWG, Boeing, Continental Airlines,Northwest Airlines, ATA, and FedExpointed out that the first group ofairplanes is collectively at the highestrisk because of cumulative time inservice and the limited fatigue test dataavailable for these models. Theyrecommended that the compliance datefor the first group of airplanes should be

by a certain date after the effective dateof the rule. The AAWGs final report 25recommends that LOVs be establishedfor the first group of airplanes by June2009, or 18 months prior to theoperators compliance date for the finalrule, whichever occurs later. This wouldalso provide sufficient time forStructures Task Groups 26 includingoperators of affected airplanes, toparticipate in establishing the LOVs. Alater Boeing comment, however,requested that the compliance dates forthose airplanes be 36 months, instead of18 months (as stated in the technicaldocument), from the effective date of therule. Boeing stated that this additionaltime would allow them to have the FAAreview and accept the Boeingproprietary LOV methodology, prepareLOV fleet proposals, and coordinatethem within Boeing and with operators

before submitting them to the FAA forreview and approval.

The FAA agrees that pre-Amendment2545 airplanes should be addressedfirst because they are among the oldestairplanes and at the highest risk fordeveloping WFD. In fact, most high-timepre-Amendment 2545 airplanes haveexceeded their design service goals.While the FAA understands that LOVshave been developed for a number ofaffected airplanes, the agency alsounderstands that not all design approvalholders have begun or completed thisactivity on all affected models. The FAArecognizes the benefits of allowing

Structures Task Groups to participate insetting LOVs. Therefore, the FAA hasdetermined that the compliance periodfor the oldest affected airplanes should

be increased to 18 months to allowsufficient time for design approvalholders to show compliance withtodays rule. This increases by sixmonths the amount of time designapproval holders have to comply overwhat was anticipated in the NPRM. The2007 AAWG Task 3 Report furthersupports the compliance date of 18months. In its report, the AAWG statedthat most of the work for the pre-

Amendment 2545 airplanes hasalready been completed. As a result, wedo not concur with the commenter that36 months is necessary to establishLOVs.



15/44


27Under 21.17, these applicants are subject to 25.571 at Amendment 2596. In addition to thiscertification basis, they are subject to therequirements of this final rule.

b. Airplanes Certified to Amendment2545 or Later

For the second group of airplanes(certified to Amendment 2545 or later),industry representatives on the AAWG,Boeing, Continental Airlines, NorthwestAirlines, ATA, and FedExrecommended setting a compliance datefor design approval holders to establishLOVs that are tied to both the designservice goal and the cumulative time onthe high-time airplanes of that model.Specifically, the industryrepresentatives on the AAWG proposedthat within 180 days of the effectivedate of the rule, the type certificateholders provide design service goals forall affected airplane models to the FAAfor approval. Once approved, thesedesign service goals would be placed inan appropriate certification document.Other commentersincluding Cessna,Continental Airlines, Embraer, AWAS,the CAA, American Airlines, Boeing,

Airbus, and FedExagreed withindustry representatives on the AAWGthat the compliance date for settingLOVs should take into account both thedesign service goal and the cumulativetime on the high-time airplanes of thatmodel.

The industry representatives on theAAWG proposed that the designapproval holder prepare a complianceplan with a binding schedule for a WFDevaluation when the high-time airplanereaches a point five years from itsdesign service goal. The AAWGindustry representatives suggested that a

means of determining this time shouldbe included in AC 120YY. FedEx andLynden Air Cargo suggested that theFAA use the design service goals thatare being developed under the DamageTolerance Data Rule to establishcompliance dates for establishing LOVsand associated WFD maintenanceactions. The commenters said that if nodesign service goal or design serviceobjective exists, the LOV should beestablished when the high-time airplaneof a particular model reaches 20 yearsof age.

In contrast, United Parcel Service and

Technical Data Analysis, Inc. supportedestablishing LOVs for all affectedairplane models as soon as possible,

because of the uncertainty associatedwith estimating future operating costsand the length of time that airplanes can

be operated.The WFD risk for these newer

airplane models is lower than for thepre-Amendment 2545 airplanes

because these airplanes are generallyyounger and have been certified todamage tolerance requirements.Therefore, the FAA agrees with the

industry representatives on the AAWGand other commenters that thecompliance times can be longer forthese airplanes. On the other hand, theproposal of the AAWG industryrepresentatives would add a level ofcomplexity and uncertainty todetermining compliance times that theFAA considers unnecessary and

inappropriate and that would makeoperators long-term planning difficult.

Therefore, as discussed earlier, toaccommodate the need for a longercompliance time for these airplanes, thisfinal rule creates three groups ofairplane models for determiningcompliance dates.

Group IPre-Amendment 2545(1978) airplanes.

Group IIAirplanes certified to therequirements of 25.571, Amendment2545, up to but not includingAmendment 2596 (1998).

Group IIIAirplanes certified torequirements of 25.571, Amendment2596 or later.

Group II airplane models were allsubjected to full-scale fatigue testprograms. In addition, all the models inthis group have been in service for aperiod of time. There should, therefore,

be a reasonable knowledge base readilyavailable on which to base an LOV.Todays rule requires establishment ofan LOV for all these models within 48months of the effective date of the rule,as indicated in Table 1 of 26.21. Thiswould allow design approval holders toschedule development of these LOVsafter the more urgent development of

LOVs for pre-Amendment 2545airplanes, so project schedules wouldnot conflict. At the same time, thiscompliance time would ensure thatLOVs are established long before thehigh-time airplanes of these modelswould reach their anticipated LOVs.

Design approval holders of thosemodels in Group III have had todemonstrate or will have to demonstratewith sufficient full-scale test evidencethat WFD will not occur within thedesign service goal of the airplane.Therefore, the design service goal would

be a valid LOV that is based on theknowledge base considered. However,

because these airplanes have notaccumulated much time in service,there is less urgency in establishing anLOV. As a result, the final rule provides60 months after the effective date of therule to establish an LOV for thesemodels. (See Table 1 of 26.21.) Thisprovides time to re-evaluate the fatiguedata and to establish an LOV which mayexceed the design service goal.Extending the compliance date forGroup III airplanes beyond thecompliance date for Group II airplanes

reduces the resource concerns aboutdeveloping LOVs for multiple airplanemodels at the same time.

Table 1 of 26.21 includes acompliance date for airplanes that donot appear in the table but may havehad a type certificate approved by theeffective date. These have a complianceperiod of 60 months. Some type

certificates are pending and may beapproved shortly. This last row of thetable is meant to capture any additionalairplanes that fit the applicabilitycriteria of 26.21(a).

Table 1 of 26.21 is used to call outexisting airplanes and assigncompliance dates. Holders of typecertificates for these models mustcomply with 26.21(c)(1). Theremainder of 26.21(c) specifiesadditional people who must comply.

Under todays rule, the compliancetimes specified in 26.21(c) for whenapplicants must establish an LOV

include the date specified in theapplicants plan for completion of thefull-scale fatigue testing and analyses ofthe testing to demonstrate compliancewith 25.571(b).27 All applicants whomust comply with 26.21 may use thisdate as one option for compliance.

Applicants who have the samecompliance times and the option to usethe date specified in the 25.571(b) planare:

Applicants for type certificates forwhich the application is pending as ofthe effective date.

Applicants for amendments to typecertificates (with the exception of thosethat change the weight of the airplane).

All of these applicants are required toestablish LOVs at the latest of thefollowing dates:

The date the type certificate oramended type certificate is issued,

Within 60 months after the effectivedate of the rule, or

The date specified in the planapproved under 25.571(b) indicatingwhen the full-scale fatigue testing andevaluation will be complete.

Among these applicants, WFD is ofless immediate concern because theirhigh-time airplanes will have

accumulated relatively few flight cyclesor flight hours by the compliance date.Establishing LOVs early in the servicelife of these airplanes will assistoperators in their long-term planning.This approach also serves as a transitionto 25.571 as amended by this finalrule, which requires establishing LOVsas part of initial type certification.



16/44


Maximum takeoff gross weightchanges to an airplane are treatedseparately in this rule. Holders of eithersupplemental type certificates oramendments to type certificates thatincrease maximum takeoff gross weightsfrom 75,000 pounds or less to greaterthan 75,000 pounds must comply nolater than 18 months after the effective

date.Applicants for supplemental type

certificates or amended type certificatesthat increase the maximum takeoff grossweight to greater than 75,000 poundsmust comply by the latest of thefollowing:

Within 18 months after the effectivedate of the rule,

The date the certificate is issued, or The date specified in the plan

approved under 25.571(b) indicatingwhen the full-scale fatigue testing andevaluation will be complete.

The option of 18 months after theeffective date as a compliance choice forthis group represents a six-monthincrease in the time to comply overwhat was originally proposed. We basedthese compliance dates on the length oftime given for design approval holdersof Group I airplanes to comply.

The NPRM did not specify acompliance time for applicants fordesign change approvals that, after theeffective date of the rule, decrease themaximum takeoff gross weight to 75,000pounds or less. This is because theapplicability provision in the NPRMincluded airplanes with maximumtakeoff gross weights exceeding 75,000

pounds, as approved during the originaltype certification. By referencing thecapacity resulting from original typecertification, the NPRM requiredapplicants to establish LOVs for designchange approvals that, after the effectivedate of the rule, decrease the maximumtakeoff gross weight to 75,000 pounds orless. Although not explicitly stated inthe NPRM, the LOV for those airplanesis required to be established by thecompliance date for the original typecertification or, in the case ofapplicants, by the date the approval ofthe design change has been issued.

Because the NPRM was not clear aboutwhen those applicants must comply, theFAA has revised todays rule.Applicants for design change approvalsthat decrease the maximum takeoff grossweight to 75,000 pounds or less after theeffective date of the rule must complywithin 18 months after the effective dateof the rule or by the date the certificateis issued or by the date specified in theplan approved under 25.571(b),whichever occurs latest.

The FAA has also revised thecompliance times to require those

applicants who would decrease thegross weight of their airplanes after theeffective date of the rule to submit acompliance plan within 90 days afterthe date of application.

3. Varying Implementation Strategies

APA suggested a way to addressconcerns about the time needed to

develop an LOV. The commenter statedthat the initial LOVs underconsideration, as defined in theTechnical Document, appear to beextremely liberal and based on limiteddata and minimal analysis. APAassumed that manufacturers would needmore time to develop their analysisprocedures, and said that a betterapproach for establishing the initialLOV would be to increase the designservice goal by 10% to 15% andmandate inspections of high-timeairplanes that are over their designservice goal. APA based its suggestionon an assumption that the designservice goals were based on hard testand engineering data. The commentersuggested halving the interval betweenmaintenance checks for airplanes overtheir design service goal. Then, thecommenter suggested, results of theseinspections could be given to themanufacturer for use in substantiatingthe engineering WFD analysis. This datacould be used to validate futureincremental LOV increases.

Although this commenter maintainedthat design service goals are based onhard test and engineering data, that hasnot always been the criteria by which

design service goals have been set.Amendment 2596 to 25.571introduced requirements that applicantsshow freedom from WFD up to thedesign service goal. Prior to Amendment2596, however, there was norequirement for setting a designapproval holders design service goal orfor validating it. Design approvalholders have always used engineeringdata to substantiate their designs. Mostdesign approval holders set designservice goals for their airplanes, eventhough they were not required to do so.But since there were no requirements

prior to Amendment 2596 about whatcriteria must be used to set the designservice goal, they have often been set forpurposes driven more by sales andmarketing than by engineering data.

Some design approval holders havestated that LOVs may be established ata point anywhere from 33% to 180%higher than the airplanes design servicegoal for certain models. This is because,for those design approval holders, thereis a large body of in-service data tosupport these higher LOVs. Otherdesign approval holders have taken an

approach similar to APAsrecommendation, in that they have beenincrementally increasing their airplanemodels LOV as the data supports it.Todays rule allows for animplementation strategy that providesflexibility to design approval holders indetermining the timing of serviceinformation development (with FAA

approval), while providing operatorswith certainty regarding the LOVapplicable to their airplanes. However,no matter how the design approvalholder chooses to manage LOVdevelopment, those LOVs must still besubstantiated by engineering data.

4. FAA Review and Approval Time

Industry representatives on theAAWG, Boeing, Airbus, and CAArequested that the rule include requiredtime periods for FAA review andapproval activities. These commentersnoted that the rules do not currentlylimit the amount of time the FAA willtake to review and approve documentsand that this will negatively affect theircompliance time. Several commentersalso noted that the amount of time theFAA will take to review and approvedesign approval holders LOVs couldreduce operator compliance timesignificantly.

We are not including required timeperiods for FAA review and approval ofthe required compliance activities.Instead, expectations for FAA personnelhave been defined in FAA Order8110.104, which directs the AircraftCertification and Flight Standards

Services in their roles andresponsibilities for implementing theseinitiatives. The order includes expectedtimes (6 weeks) for reviewing andapproving design approval holdercompliance plans, plans to correctdeficiencies, and draft and finalcompliance data and documents. Tofacilitate implementation, the FAA willtrain affected personnel in their rolesand responsibilities and provide in-depth familiarization with requirementsof the regulations and associatedguidance. Ultimately, however, thetiming of FAA approvals will be

determined by the quality of the designapproval holder submissions and theirresponsiveness to issues raised by theFAA.

We have structured the requirementsof the design approval holder rule anddeveloped complementary guidance tofacilitate timely review and approval ofdesign approval holder submittals (suchas compliance plans). An increase inoperator compliance time would helpensure that operators are not affected bythe FAA review and approval process.We have revised the WFD compliance



17/44


date for operators from 6 months to 12months after the relevant designapproval holder compliance date. Thisdate is measured after the effective dateof the final rule. As previously noted,for Group I, II, and III airplanes, t

Widespread Fatigue FAR 2010-28363.pdf

Documents