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AGENCY:
Federal Aviation Administration (FAA), DOT.
ACTION:
Final rule.
SUMMARY:
The FAA is superseding Airworthiness Directive (AD) 2009-01-02, which applied to certain Boeing Model 737-600, -700, -700C, -800, and -900 series airplanes. AD 2009-01-02 required an inspection of frames between body station (BS) 360 and BS 907 to determine if certain support brackets of the air conditioning (A/C) outlet extrusions are installed, inspections for cracking of the frames around the attachment holes of the subject brackets, and repair if necessary. AD 2009-01-02 also requires installing new, improved fittings for all support brackets of the A/C outlet extrusions between BS 360 and BS 907. This AD was prompted by numerous reports of multiple cracks in the frame around the attachment holes of the support bracket of the A/C outlet extrusion, and the determination that certain repairs might develop fatigue cracks that could result in the inability of the frame to sustain limit load and therefore must be inspected. This AD would continue to require the actions specified in AD 2009-01-02 and would also require repetitive inspections for cracking of certain repairs, and repair if necessary. The FAA is issuing this AD to address the unsafe condition on these products.
DATES:
This AD is effective November 12, 2024.
The Director of the Federal Register approved the incorporation by reference of a certain publication listed in this AD as of November 12, 2024.
The Director of the Federal Register approved the incorporation by reference of a certain other publication listed in this AD as of February 27, 2009 (74 FR 4117, January 23, 2009).
ADDRESSES:
AD Docket: You may examine the AD docket at regulations.gov under Docket No. FAA-2023-1886; 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 final rule, any comments received, and other information. The address for Docket Operations is U.S. Department of Transportation, Docket Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New Jersey Avenue SE, Washington, DC 20590.
Material Incorporated by Reference:
- For Boeing material identified in this AD, contact Boeing Commercial Airplanes, Attention: Contractual & Data Services (C&DS), 2600 Westminster Blvd., MC 110 SK57, Seal Beach, CA 90740-5600; telephone 562-797-1717; websitemyboeingfleet.com.
- You may view this material 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 206-231-3195. It is also available atregulations.gov under Docket No. FAA-2023-1886.
FOR FURTHER INFORMATION CONTACT:
Owen F. Bley-Male, Aviation Safety Engineer, FAA, 2200 South 216th Street, Des Moines, WA 98198; phone: 206-231-3992; email: Owen.F.Bley-Male@faa.gov.
SUPPLEMENTARY INFORMATION:
Background
The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to supersede AD 2009-01-02, Amendment 39-15780 (74 FR 4117, January 23, 2009) (AD 2009-01-02). AD 2009-01-02 applied to certain Boeing Model 737-600, -700, -700C, -800, and -900 series airplanes. The NPRM published in the Federal Register on September 25, 2023 (88 FR 65637). The NPRM was prompted by numerous reports of multiple cracks in the frame around the attachment holes of the support bracket of the A/C outlet extrusion and the determination that certain repairs might develop fatigue cracks that could result in the inability of the frame to sustain limit load and therefore must be inspected. In the NPRM, the FAA proposed to continue to require a one-time general visual inspection of frames between BS 360 and BS 907 to determine if certain support brackets of the A/C outlet extrusions are installed; medium- and high-frequency eddy current inspections for cracking of the frames around the attachment holes of the subject brackets; repair if necessary; and installation of new, improved fittings for all support brackets of the A/C outlet extrusions between BS 360 and BS 907. The NPRM also proposed to require repetitive inspections for cracking of certain repairs, and repair if necessary. The FAA is issuing this AD to prevent frame cracking, which, if not corrected, could lead to a severed frame that, combined with cracking of the chemically milled steps in the skin above the Stringer 10 lap splice, could result in rapid decompression of the airplane.
Discussion of Final Airworthiness Directive
Comments
The FAA received a comment from an individual who supported the NPRM without change.
The FAA received additional comments from Aviation Partners Boeing, The Boeing Company (Boeing), Southwest Airlines (SWA), United Airlines (UAL), and an individual. The following presents the comments received on the NPRM and the FAA's response to each comment.
Request To Determine Estimated Cost
An individual commented that the NPRM stated that there is no definitive data on which to estimate the labor cost (for the repairs), so the basis for this estimated cost is unclear. The commenter suggested an estimate using cost data for operations similar to those required by this AD, making estimates more robust.
The FAA cannot pre-determine on-condition actions before the inspections required by this AD are actually completed. The work-hours to complete repairs will be unique to each airplane. For the other actions required by this AD, the FAA used estimated costs that were provided by the design approval holder. No changes have been made to this AD in this regard.
Effect of Winglets on Accomplishment of the Proposed Actions
Aviation Partners Boeing stated that incorporation of Supplemental Type Certificate (STC) ST00830SE for installation of blended or split scimitar winglets does not affect the accomplishment of the manufacturer's service instructions.
The FAA agrees with the commenter that STC ST00830SE does not affect the accomplishment of the manufacturer's service instructions. Therefore, the installation of STC ST00830SE does not affect the ability to accomplish the actions required by this AD. The FAA has not changed this AD in this regard.
Request for Correction of the Skin Lap Splice Location
Boeing stated that the NPRM incorrectly identified the location of the skin lap splice. The NPRM referred to “cracking of the skin lap splice above stringer 10.” Boeing requested that language be corrected to “cracking of the chemically milled steps in the skin above Stringer 10 lap splice” to match the language in Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022.
The FAA agrees with the request and has revised the “Background” section of this final rule and the “Unsafe Condition” in paragraph (e) of this AD accordingly.
Request To Add a General Visual Inspection for Cracks
Boeing noted that Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022, requires a general visual inspection (GVI) to detect cracks, not just to determine if certain support brackets of the A/C outlet extrusions are installed at certain frames. Boeing requested that a GVI be required to also detect cracks.
The FAA agrees with the request. The inspections specified in Part 2 of Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022, include a GVI to detect cracks. The “Material Incorporated by Reference under 1 CFR part 51” section of this final rule and paragraph (g)(2) of this AD have been revised to include a GVI as an inspection method.
Request To Revise the Applicability Statement
Boeing stated that paragraph (c), “Applicability,” of the proposed AD incorrectly identified the referenced service information as Boeing “Special Attention” Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022, and explained that Revision 4 of this service information is an “alert” service bulletin. The correct reference is Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022.
The FAA agrees and has revised paragraph (c) of this AD accordingly.
Request To Correct Special Attention Service Bulletin
Boeing stated that a certain Boeing special attention service bulletin is incorrectly identified under “Actions Since AD 2009-01-02 Was Issued” of the NPRM. Boeing pointed out that the NPRM incorrectly identified the service bulletin number as “737-25A1544.” The correct service bulletin number is “737-25-1544.”
The FAA partially agrees. The FAA agrees that the specified service bulletin number was stated incorrectly in the NPRM. However, since that section of the NPRM is not restated in this final rule, no change is necessary in this regard.
Request To Revise AMOC Provisions
SWA stated that paragraph (j) of the proposed AD requires inspection of repairs installed on the BS 907 frame at the compliance times given in Table 2 of Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022. SWA mentioned that Table 2 requires post-repair inspections for repairs that were installed per Boeing Special Attention Service Bulletin 737-25-1544, Revision 3, dated May 16, 2016; or Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022; and does not address repairs that were installed by any other means. SWA proposed revising paragraph (m)(4) of the proposed AD to extend alternative method of compliance (AMOC) approval to paragraph (j) of the proposed AD, provided the AMOC approval contains a damage tolerance evaluation. SWA believes that individually approved repairs that contain an AMOC to AD 2009-01-02, with a damage tolerance analysis as a Category A or Category B repair (as defined in Boeing SRM 51-00-06), provide an equivalent level of safety.
The FAA disagrees with the request. Only repairs installed per Boeing Special Attention Service Bulletin 737-25-1544, Revision 3, dated May 16, 2016; or Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022; are subject to the requirements of paragraph (j) of this AD. Repairs installed by any other means are not subject to the requirements of paragraph (j) of this AD, so the proposed AMOC to paragraph (j) would not be applicable. The FAA has not changed this AD in this regard.
Request To Clarify Airplanes Affected by New Requirements
UAL supported the NPRM, and requested clarification of the airplanes affected by paragraph (j) of the proposed AD. UAL stated that some airplanes had repairs to the STA 907 frame performed before release of Boeing Special Attention Service Bulletin 737-25-1544, Revision 3, dated May 16, 2016. UAL added that these repairs were coordinated with Boeing and received Form 8100-9 approvals as AMOCs to paragraph (g) of AD 2009-01-02 and are considered Category B damage tolerance repairs that also require additional inspections as defined in the 8100-9 approval. UAL stated that Table 2 of the “Compliance” paragraph of Boeing Alert Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022 (as specified in paragraph (j) of the proposed AD), applies only to airplanes on which a frame repair has been done at STA 907 in accordance with Boeing Special Attention Service Bulletin 737-25-1544, Revision 3, dated May 16, 2016; or Boeing Alert Service Bulletin 737-25-1544, Revision 4, dated February 15, 2022. UAL therefore stated that Category B repairs via Form 8100-9 should be excluded from paragraph (j) of the proposed AD.
The FAA provides the following clarification. As stated previously, only repairs installed per Boeing Special Attention Service Bulletin 737-25-1544, Revision 3, dated May 16, 2016; or Boeing Alert Service Bulletin 747-25A1544, Revision 4, dated February 15, 2022; are subject to the requirements of paragraph (j) of this AD. Repairs installed by any other means are not subject to the requirements of paragraph (j) of this AD. Therefore, no change to this AD is necessary.
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 Service Bulletin 737-25A1544, Revision 4, dated February 15, 2022. This material specifies procedures for a one-time general visual inspection of frames between BS 360 and BS 907 to determine if certain support brackets of the A/C outlet extrusions are installed; a general visual inspection and low-, medium-, and high-frequency eddy current inspections for cracking of the frames around the attachment holes of the subject brackets, and repair if necessary; and installation of new, improved fittings for all support brackets of the A/C outlet extrusions between BS 360 and BS 907. This material also specifies procedures for repetitive detailed and high-frequency eddy current inspections for cracking of certain repairs at BS 907 and repair if necessary.
This AD also requires Boeing Special Attention Service Bulletin 737-25-1544, Revision 1, dated January 16, 2008, which the Director of the Federal Register approved for incorporation by reference as of February 27, 2009 (74 FR 4117, January 23, 2009).
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 ADDRESSES.
Costs of Compliance
The FAA estimates that this AD affects 738 airplanes of U.S. registry. The FAA estimates the following costs to comply with this AD:
Estimated Costs
| Action | Labor cost | Parts cost | Cost per product | Cost on U.S. operators |
|---|---|---|---|---|
| General visual inspection (retained actions from AD 2009-01-02) | 1 work-hour × $85 per hour = $85 | $0 | $85 | $62,730. |
| Eddy current inspections (retained actions from AD 2009-01-02) | Up to 216 work-hours × $85 per hour = Up to $18,360 | $0 | Up to $18,360 | Up to $13,549,680. |
| Replace support fittings (retained actions from AD 2009-01-02) | Up to 346 work-hours × $85 per hour = Up to $29,410 | Up to $28,789 | Up to $58,199 | Up to $42,950,862. |
| Post-repair inspections (new action) | 42 work-hours × $85 per hour = $3,570 per inspection cycle | $0 | $3,570 per inspection cycle | $2,634,660 per inspection cycle. |