Airworthiness Directives; The Boeing Company Airplanes

SUPPLEMENTARY INFORMATION:

Background

The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to certain The Boeing Company Model 777-200, -200LR, -300, -300ER, and 777F series airplanes. The NPRM published in the Federal Register on March 28, 2024 (89 FR 21446). The NPRM was prompted by a report indicating multiple findings of cracks in the fuselage skin common to the UWL. In the NPRM, the FAA proposed to require external or internal (depending on configuration) inspections for any cracking of the left and right side fuselage skin common to the UWL, and applicable on-condition actions. The FAA is issuing this AD to address fuselage skin cracking caused by cold work surface upset that is not removed from the mating parts and high joint load transfer or significant local bending stresses at critical fastener locations. The unsafe condition, if not addressed, could result in an inability of a principal structural element (PSE) to sustain limit load, leading to reduced structural integrity of the airplane and possible loss of control of the airplane.

Discussion of Final Airworthiness Directive

Comments

The FAA received a comment from FedEx Express, who found no issues that could interfere with the timely implementation of the proposed actions in the NPRM.

The FAA received additional comments from three commenters, including American Airlines, Boeing, and United Airlines. The following presents the comments received on the NPRM and the FAA's response to each comment.

Request To Clarify Locations Specified in Paragraph (h)(3) of the Proposed AD

Boeing requested that paragraph (h)(3) of the proposed AD be clarified to specify that only the fuel tank side of fastener locations that penetrate the fuel tank boundary require cap seal dimensions to meet the dimensions given in Figure 1 to paragraph (h)(3) of the proposed AD. Boeing stated that the service information referenced in Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023, contain instructions for applying cap seals to fasteners that penetrate a fuel tank boundary and fasteners that do not penetrate a fuel tank boundary. Boeing added that for fasteners that do not penetrate a fuel tank boundary, fastener sealing is used as a pressure seal to help mitigate corrosion and have sealing specifications that include minimum thickness requirements less than the amount shown in paragraph (h)(3) of the proposed AD that are acceptable for their intended function outside of the fuel tank. Boeing stated that for fasteners that penetrate the fuel tank, the cap seals on the interior side of the fuel tank act as a primary fuel seal, providing fault tolerance against electromagnetic effects (electrical fault currents and lightning currents), and are also cap sealed on the exterior side for corrosion prevention.

Boeing concluded that only the cap seals on the interior side of the fuel tank require the dimensions specified in Figure 1 of paragraph (h)(3) of the proposed AD to perform their intended function, which is consistent with the labels shown in Figure 1 of paragraph (h)(3) of the proposed AD. Boeing recommended replacing the text “applying a cap seal (sealant) to a fastener, fastener head, and fastener threads and collars, for this AD, during application of any cap seal to a fastener, fastener head, or fastener threads and collars” with the text “applying a cap seal (sealant) to a fastener location that penetrates the fuel tank boundary, for this AD, during application of any cap seal to a fastener, fastener head, or fastener threads and collars inside the fuel tank.”

The FAA agrees to clarify paragraph (h)(3) of this AD for the reasons provided by the commenter. Locations that do not penetrate the fuel tank boundary do not require the same sealant application procedures. The FAA has added the text “inside the fuel tank” to paragraph (h)(3) of this AD. However, the FAA did not replace the text “to a fastener, fastener head, and fastener threads and collars” with the text “to a fastener location that penetrates the fuel tank boundary,” as recommended by the commenter, because the text “to a fastener, fastener head, and fastener threads and collars inside the fuel tank” better aligns with the language within the service information.

Request To Clarify Service Information in Paragraph (h)(3) of the Proposed AD

American Airlines requested that the FAA provided a comprehensive list of all service information that will be impacted by paragraph (h)(3) of the proposed AD. American Airlines stated that the statement “Where any service information referenced in Boeing Alert Requirements Bulletin 777-53A0100 RB” is vague and could potentially lead to confusion.

The FAA agrees to specify the service information affected by paragraph (h)(3) of this AD. In addition, the FAA notes that paragraph (h)(2) of this AD has similar language (“Where . . . any service information referenced in Boeing Alert Requirements Bulletin 777-53A0100 RB”). The FAA has added a note to paragraphs (h)(2) and (3) of this AD to specify that Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023, refers to Boeing Service Bulletin 777-53-0084, Revision 2, dated December 9, 2020; Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020; and Boeing Alert Requirements Bulletin 777-57A0122 RB, dated October 8, 2021.

Request To Add a Note to Paragraph (h)(3) of the Proposed AD

Boeing requested that the FAA add a note to paragraph (h)(3) of the proposed AD to refer to Boeing Model 777 Aircraft Maintenance Manual (AMM) section 28-11-00 as an acceptable cap sealing procedure to accomplish the actions required in that paragraph. Boeing stated it has received numerous queries from operators on Figure 1 to paragraph (h)(4) of AD 2023-17-14, which is identical to Figure 1 to paragraph (h)(3) of the proposed AD. Operators have asked whether Boeing Model 777 AMM section 28-11-00 would be an acceptable procedure for accomplishing the requirements of paragraph (h)(4) of AD 2023-17-14. Boeing stated it believes that AMM 28-11-00 is an acceptable procedure for performing cap sealing inside the fuel tank because the AMM is the source material for the contents in Figure 1 to paragraph (h)(3) of the proposed AD and contains the same minimum seal thickness dimensions as shown in Figure 1.

The FAA agrees. Boeing 777 AMM 28-11-00 meets the sealant requirements of Figure 1 to paragraph (h)(3) of this AD. As such, it would be an acceptable procedure to follow when sealing inside the fuel tank in accordance with paragraph (h)(3) of this AD. The FAA has added a note to paragraph (h)(3) of this AD accordingly.

Request for Removal of Certain Exceptions That Are Related to Other ADs

United Airlines (United) requested removal of the exceptions in paragraphs (h)(4) through (6) of the proposed AD that are for service information other than Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023. United stated that it agrees with the intention of the proposed AD, but found the exceptions stated in paragraphs (h)(4) through (6) of the proposed AD to be unclear. United stated that it has other ongoing projects that use the instructions of Boeing Service Bulletin 777-53-0084, Revision 2, dated December 9, 2020; Boeing Service Bulletin 777 53-0087, Revision 1, dated March 4, 2020; and Boeing Alert Requirements Bulletin 777-57A0122 RB, dated October 8, 2021; to comply with the requirements of AD 2019-11-02, Amendment 39-19648 (84 FR 28722, June 20, 2019) (AD 2019-11-02) and AD 2023-17-14, Amendment 39-22541 (88 FR 60111, August 31, 2023) (AD 2023-17-14). United pointed out that ADs 2019-11-02 and 2023-17-14 have not been updated to include the exceptions in those AD requirements and recommends that the exceptions removed from paragraphs (h)(4) through (6) of the proposed AD be added to the requirements of ADs 2019-11-02 and 2023-17-14.

The FAA disagrees with the request to remove the exceptions in paragraphs (h)(4) through (6) of this AD. If cracking is found during certain inspections required by Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023, then certain actions in Boeing Service Bulletin 777-53-0084, Revision 2, dated December 9, 2020; Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020; or Boeing Alert Requirements Bulletin 777-57A0122 RB, dated October 8, 2021; might be required as corrective actions. In order to address the identified unsafe condition, the service information, as applicable, must be accomplished with the exceptions specified in (h)(4) through (6) of this AD. There are no new requirements for AD 2019-11-02 and AD 2023-17-14. However, as stated in paragraph (i) of this AD, for airplanes on which a front spar lower chord modification specified in Boeing Alert Requirements Bulletin 777-57A0122 RB is done as part of the requirements of paragraphs (g) and (h)(6) of this AD, the modification requirements of paragraph (g) of AD 2023-17-14 are terminated for the applicable side (left or right) on which the modification was done.

In addition, in the “Approval” paragraphs of Boeing Service Bulletin 777-53-0084, Revision 2, dated December 9, 2020; Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020; and Boeing Alert Requirements Bulletin 777-57A0122 RB, dated October 8, 2021; it specifies that certain actions are an alternative method of compliance (AMOC) to the inspection and corrective action requirements of paragraph (g) of AD 2019-11-02, for modified longerons only. The FAA has not changed this AD in this regard.

Request To Add Paragraph for Terminating Action for AD 2019-11-02

American Airlines requested that the FAA add a paragraph similar to paragraph (i) of the proposed AD for terminating action for AD 2023-17-14 that refers to Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020, as the terminating action to AD 2019-11-02.

The FAA does not agree. As previously stated, the “Approval” paragraph of Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020, already includes an approval paragraph for the requirements of AD 2019-11-02. The FAA has not revised this AD in this regard.

Request To Add Clarifying Paragraph for Repetitive Inspections

American Airlines requested that the FAA add a paragraph clarifying that “If SB 777-53-0087 Rev 1 is accomplished due to findings from the inspections described in SB 777-53A0081 R02 (AD 2019-11-02), and NO skin repair is done, the repetitive inspections IAW SB 777-53A0100 will still be required since terminating action for those inspections is only valid if the underwing longeron AND fuselage skin modification is accomplish in accordance with SB 777-53-0087 R01.”

The FAA concurs with American Airlines' statement that repetitive inspections continue until the fuselage skin modification is done. The UWL with fuselage skin modification is terminating action to the repeat inspections in Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023. If Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020, is accomplished without the fuselage skin modification, repeat inspections in accordance with Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023, continue to apply. The FAA notes that flagnote (a) in the tables in the “Compliance” paragraph of Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023, specifies “Accomplishment of the left side UWL with fuselage skin modification in accordance with Revision 1 of Boeing Service Bulletin 777-53-0087 is terminating action for this repeat inspection.”

The FAA also notes where in the second paragraph of “Other Relevant Rulemaking” of the NPRM states “The accomplishment of the longeron modification specified in Boeing Service Bulletin 777-53-0084, Revision 2, dated December 9, 2020, or Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020” the text should state “The accomplishment of the longeron modification with fuselage skin modification specified in Boeing Service Bulletin 777-53-0084, Revision 2, dated December 9, 2020, or Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020.” However, the “Other Relevant Rulemaking” paragraphs are not restated in this AD. The FAA has not revised this AD in this regard.

Request To Allow Later Revisions of Service Information

American Airlines requested that the FAA allow the use of future revisions of Boeing Service Bulletin 777-53-0087, Revision 1, dated March 4, 2020. American Airlines stated the Revision 2 of Boeing Service Bulletin 777-53-0087 will add new replacement and supplemental kits, add fastener installation instructions, update bracket installation instructions, correct fastener callouts and correct typographical errors.

The FAA does not agree. Revision 2 of Boeing Service Bulletin 777-53-0087 has not yet been FAA approved nor been published. The FAA may not refer to any document that does not yet exist in an AD. To allow operators to use later revisions of the referenced document (issued after publication of the AD), either the FAA must revise the AD to reference specific later revisions, or operators must request approval to use later revisions as an alternative method of compliance with this AD under the provisions of paragraph (j) of this AD.

Request To Clarify Paragraph (h)(5) of the Proposed AD

American Airlines requested that the FAA clarify if paragraph (h)(5) of the proposed AD is exclusive to the center wing fuel tank location, or whether adjustments should also be made to the other steps that offer the choice to utilize Boeing Material Specification (BMS) 5-95 sealant or other sealants.

The FAA agrees to clarify. The exception in paragraph (h)(5) of this AD is applicable to sealant application in the center wing fuel tank only. The use of BMS 5-95 sealant instead of BMS 5-45 sealant is only a concern when the location being sealed is inside a fuel tank such that the sealant is directly exposed to fuel. Only Figure 13 and Figure 49 describe sealing operations inside a fuel tank using BMS 5-95 sealant. As such, paragraph (h)(5) of this AD is specifically limited to flagnote (f) of Figure 13 and Figure 49. Other locations ( i.e., other flagnotes and figures) in the service information that provide an option of BMS 5-45 and BMS 5-95 are acceptable as written. The FAA also notes that a similar rationale applies to the exception in paragraph (h)(4) of this AD. The FAA has not changed this AD in this regard.

Conclusion

The FAA reviewed the relevant data, considered any comments received, and determined that air safety requires adopting this AD as proposed. Accordingly, the FAA is issuing this AD to address the unsafe condition on these products. Except for minor editorial changes, and any other changes described previously, this AD is adopted as proposed in the NPRM. None of the changes will increase the economic burden on any operator.

Material Incorporated by Reference Under 1 CFR Part 51

The FAA reviewed Boeing Alert Requirements Bulletin 777-53A0100 RB, dated March 16, 2023. This material specifies procedures for external or internal (depending on configuration) detailed and ultrasonic or surface high frequency eddy current (HFEC) inspections for any cracking of the left and right side fuselage skin common to the UWL, and applicable on-condition actions. On-condition actions include, among other things, modification of the fuselage skin, and post-modification inspections and applicable corrective actions (repairs of cracking). Compliance times for on-condition actions depend on inspection type, inspection findings, and modification status.

The FAA also reviewed Boeing Multi Operator Message MOM-MOM-24-0054-01B, dated January 26, 2024. This material specifies corrections for Boeing Alert Requirements Bulletin 777-57A0122 RB, dated October 8, 2021, that address a non-destructive test manual (NDTM) error, fastener callout errors, inadequate cap seal instructions, figure orientation errors, minimum gap errors, missing fasteners on certain figures, affected groups missing from certain figures, and typographical errors.

This material is reasonably available because the interested parties have access to it through their normal course of business or by the means identified in the ADDRESSES section.

Costs of Compliance

The FAA estimates that this AD affects 272 airplanes of U.S. registry. The FAA estimates the following costs to comply with this AD: