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Federal Register / Vol. 86, No. 161 / Tuesday, August 24, 2021 / Proposed Rules
The FAA proposes to adopt a new airworthiness directive AD for certain The Boeing Company Model 737300, 400, and 500 series airplanes. This proposed AD was prompted by an evaluation by the design approval holder DAH
indicating that the frame splice between certain stringers is subject to widespread fatigue damage WFD. This proposed AD would require an inspection of certain fuselage frame splices for existing repairs, repetitive inspections of certain fuselage frame splices for cracking, and applicable oncondition actions. The FAA is proposing this AD to address the unsafe condition on these products.
DATES: The FAA must receive comments on this proposed AD by October 8, 2021.
ADDRESSES: You may send comments, using the procedures found in 14 CFR
11.43 and 11.45, by any of the following methods:
Federal eRulemaking Portal: Go to https www.regulations.gov. Follow the instructions for submitting comments.
Fax: 2024932251.
Mail: U.S. Department of Transportation, Docket Operations, M30, West Building Ground Floor, Room W12140, 1200 New Jersey Avenue SE, Washington, DC 20590.
Hand Delivery: Deliver to Mail address above between 9 a.m. and 5
p.m., Monday through Friday, except Federal holidays.
For service information identified in this NPRM, contact Boeing Commercial Airplanes, Attention: Contractual & Data Services C&DS, 2600 Westminster Blvd., MC 110SK57, Seal Beach, CA
907405600; telephone 5627971717;
internet https
www.myboeingfleet.com. You may view this referenced service information at the FAA, Airworthiness Products Section, Operational Safety Branch, 2200 South 216th St., Des Moines, WA.
For information on the availability of this material at the FAA, call 206231
3195. It is also available on the internet at https www.regulations.gov by searching for and locating Docket No.
FAA20210609.

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SUMMARY:

Examining the AD Docket You may examine the AD docket at https www.regulations.gov by searching for and locating Docket No.
FAA20210609; or in person at Docket Operations between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The AD docket contains this NPRM, any comments received, and other information. The street address for Docket Operations is listed above.
FOR FURTHER INFORMATION CONTACT:
Wayne Ha, Aerospace Engineer,
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Airframe Section, FAA, Los Angeles ACO Branch, 3960 Paramount Boulevard, Lakewood, CA 907124137;
phone: 5626275238; fax: 562627
5210; email: wayne.ha@faa.gov.
SUPPLEMENTARY INFORMATION:

Comments Invited The FAA invites you to send any written relevant data, views, or arguments about this proposal. Send your comments to an address listed under ADDRESSES. Include Docket No.
FAA20210609; Project Identifier AD
202100274T at the beginning of your comments. The most helpful comments reference a specific portion of the proposal, explain the reason for any recommended change, and include supporting data. The FAA will consider all comments received by the closing date and may amend this proposal because of those comments.
Except for Confidential Business Information CBI as described in the following paragraph, and other information as described in 14 CFR
11.35, the FAA will post all comments received, without change, to https
www.regulations.gov, including any personal information you provide. The agency will also post a report summarizing each substantive verbal contact received about this NPRM.
Confidential Business Information CBI is commercial or financial information that is both customarily and actually treated as private by its owner.
Under the Freedom of Information Act FOIA 5 U.S.C. 552, CBI is exempt from public disclosure. If your comments responsive to this NPRM
contain commercial or financial information that is customarily treated as private, that you actually treat as private, and that is relevant or responsive to this NPRM, it is important that you clearly designate the submitted comments as CBI. Please mark each page of your submission containing CBI
as PROPIN. The FAA will treat such marked submissions as confidential under the FOIA, and they will not be placed in the public docket of this NPRM. Submissions containing CBI
should be sent to Wayne Ha, Aerospace Engineer, Airframe Section, FAA, Los Angeles ACO Branch, 3960 Paramount Boulevard, Lakewood, CA 907124137;
phone: 5626275238; fax: 562627
5210; email: wayne.ha@faa.gov. Any commentary that the FAA receives which is not specifically designated as CBI will be placed in the public docket for this rulemaking.

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Background Fatigue damage can occur locally, in small areas or structural design details, or globally, in widespread areas.
Multiple-site damage is widespread damage that occurs in a large structural element such as a single rivet line of a lap splice joining two large skin panels.
Widespread damage can also occur in multiple elements such as adjacent frames or stringers. Multiple-site damage and multiple-element damage cracks are typically too small initially to be reliably detected with normal inspection methods. Without intervention, these cracks will grow, and eventually compromise the structural integrity of the airplane. This condition is known as WFD. It is associated with general degradation of large areas of structure with similar structural details and stress levels. As an airplane ages, WFD will likely occur, and will certainly occur if the airplane is operated long enough without any intervention.
An FAA final rule Aging Airplane Program: Widespread Fatigue Damage;
75 FR 69746, November 15, 2010
became effective on January 14, 2011, and amended 14 CFR parts 25, 26, 121, and 129 commonly known as the WFD
rule. The WFD rule requires certain actions to prevent structural failure due to WFD throughout the operational life of certain existing transport category airplanes and all of these airplanes that will be certificated in the future. DAHs of existing and future airplanes subject to the WFD rule are required to establish a limit of validity LOV of the engineering data that support the structural maintenance program.
Operators affected by the WFD rule may not fly an airplane beyond its LOV, unless an extended LOV is approved.
The WFD rule does not require identifying and developing maintenance actions if the DAHs can show that such actions are not necessary to prevent WFD before the airplane reaches the LOV. Many LOVs, however, do depend on accomplishment of future maintenance actions. As stated in the WFD rule, any maintenance actions necessary to reach the LOV will be mandated by airworthiness directives through separate rulemaking actions.
In the context of WFD, this action is necessary to enable DAHs to propose LOVs that allow operators the longest operational lives for their airplanes, and still ensure that WFD will not occur.
This approach allows for an implementation strategy that provides flexibility to DAHs in determining the timing of service information development with FAA approval,
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