# [Amended]
**AGENCY:**
Federal Aviation Administration (FAA), DOT.
**ACTION:**
Notice of proposed rulemaking (NPRM).
**SUMMARY:**
The FAA proposes to supersede Airworthiness Directive (AD) 2017-19-26, which applies to certain The Boeing Company Model 737-600, -700, -700C, -800, and -900 series airplanes. AD 2017-19-26 requires repetitive inspections for cracks along the chem-milled steps of the fuselage skin and missing or loose fasteners in the area of the preventive modification or repairs; replacement of the time-limited repair with a permanent repair, if applicable; and applicable corrective actions that end certain repetitive inspections. AD 2017-19-26 also requires repetitive inspections for modified airplanes. Since the FAA issued AD 2017-19-26, the FAA has determined that the compliance times are not adequate. This proposed AD would continue to require certain actions in AD 2017-19-26 but at reduced compliance times. This proposed AD would also revise instructions for the preventative modification and remove instructions for the permanent and time-limited repairs. 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 January 2, 2026.
**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 *regulations.gov.* Follow the instructions for submitting comments.
• *Fax:* 202-493-2251.
• *Mail:* U.S. Department of Transportation, Docket Operations, M-30, West Building Ground Floor, Room W12-140, 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.
*AD Docket:* You may examine the AD docket at *regulations.gov* under Docket No. FAA-2025-3429; 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.
*Material Incorporated by Reference:*
• For Boeing material in this proposed 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; website *myboeingfleet.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 at *regulations.gov* under Docket No. FAA-2025-3429.
**FOR FURTHER INFORMATION CONTACT:**
Luis Cortez-Muniz, Aviation Safety Engineer, FAA, 2200 South 216th St., Des Moines, WA 98198; phone: 206-231-3958; email: *[email protected]* .
**SUPPLEMENTARY INFORMATION:**
**Comments Invited**
The FAA invites you to send any written relevant data, views, or arguments about this proposal. Send your comments using a method listed under the *ADDRESSES* section. Include “Docket No. FAA-2025-3429; Project Identifier AD-2024-00722-T” 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 the 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 *regulations.gov,* including any personal information you provide. The agency will also post a report summarizing each substantive verbal contact received about this proposed AD.
**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 Luis Cortez-Muniz, Aviation Safety Engineer, FAA, 2200 South 216th St., Des Moines, WA 98198; phone: 206-231-3958; email: *[email protected]* . Any commentary that the FAA receives that is not specifically designated as CBI will be placed in the public docket for this rulemaking.
**Background**
The FAA issued AD 2017-19-26, Amendment 39-19056 (82 FR 44504, September 25, 2017) (AD 2017-19-26), for The Boeing Company Model 737-600, -700, -700C, -800, and -900 series airplanes with certain line numbers. AD 2017-19-26 was prompted by a fatigue test that revealed numerous cracks in the upper skin panel at the chem-milled step above the lap joint, followed by an evaluation by the design approval holder (DAH) that indicated that the upper skin panel at the chem-milled step above the lap joint is subject to widespread fatigue damage (WFD) if the preventative modification was installed after 30,000 total flight cycles. AD 2017-19-26 requires repetitive inspections for cracks along the chem-milled steps of the fuselage skin and missing or loose fasteners in the area of the preventive modification or repairs; replacement of the time-limited repair with a permanent repair, if applicable; and applicable corrective actions which would end certain repetitive inspections. AD 2017-19-26 also requires repetitive inspections for modified airplanes. The FAA issued AD 2017-19-26 to detect and correct cracking of the upper skin panel at the chem-milled step above the lap joint, which could result in reduced structural integrity of the airplane.
**Actions Since AD 2017-19-26 Was Issued**
Since the FAA issued AD 2017-19-26, the FAA received reports of five chemically milled skin cracks in an area just above the S-4 lap joint on airplanes with 38,000 and 53,000 total flight cycles. One of the cracks was found before the airplane reached the next repeat interval specified in Boeing Alert Service Bulletin 737-53A1232, Revision 3, dated July 27, 2015. As a result of these findings, the FAA has determined that reduced inspection intervals for the chem-mill areas and post-modification inspections (for airplanes on which the optional preventative modification is accomplished), as well as revised instructions for the preventative modification and removal of the instructions for the permanent and time-limited repairs, are now necessary to address the unsafe condition.
In addition, the FAA has determined that the end-level effect for the unsafe condition is rapid decompression of the airplane. The FAA has revised this NPRM accordingly.
The FAA is considering superseding similar ADs for Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes common to different locations, which have crown skin panels that are of a similar design as those on Model 737-600, -700, -700C, -800, and -900 series airplanes addressed by this proposed AD and may be subject to the same unsafe condition.
**FAA's Determination**
The FAA is issuing this NPRM after determining that the unsafe condition described previously is likely to exist or develop on other products of the same type design.
**Material Incorporated by Reference Under 1 CFR Part 51**
The FAA reviewed Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024. This material specifies procedures for repetitive external detailed inspections and either (1) external medium frequency eddy current (MFEC), magneto optic imager (MOI), or C-scan inspections or (2) external ultrasonic phased array (UTPA) inspections for cracks in the fuselage skin at chem-milled steps and pockets at certain locations, and repairing any cracks, as well as repetitive post-permanent repair inspections, and post-time-limited repair inspections. This material specifies that accomplishment of the preventative modification terminates the repetitive inspections. The preventative modification includes an external eddy current inspection of the skin and fastener holes for any crack and installation of a doubler. 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.
**Proposed AD Requirements in This NPRM**
This proposed AD would require accomplishing the actions specified in the material already described, except for any differences identified as exceptions in the regulatory text of this proposed AD. For information on the procedures and compliance times, see this material at *regulations.gov* by searching for and locating Docket No. FAA-2025-3429.
**Costs of Compliance**
The FAA estimates that this AD, if adopted as proposed, would affect 307 airplanes of U.S. registry. The FAA estimates the following costs to comply with this proposed AD:
| Action | Labor cost | Parts cost | Cost per product | Cost on U.S. operators |
| --- | --- | --- | --- | --- |
| Inspections | Up to 25 work-hours × $85 per hour = $2,125 per inspection cycle | $0 | Up to $2,125 per inspection cycle | Up to $652,375 per inspection cycle. |
| Action | Labor cost | Parts cost | Cost per product |
| --- | --- | --- | --- |
| Preventative Modification | Up to 956 work-hours × $85 per hour = $81,260 | Minimal | $81,260. |
| Post-modification inspections | Up to 92 work-hours × $85 per hour = $7,820 per inspection cycle | $0 | 7,820 per inspection cycle. |
The extent of cracking found during the inspections could vary significantly from airplane to airplane. The FAA has no way of determining which conditions may be found on each airplane, the cost to correct or repair each airplane, or the number of airplanes that may require repair.
**Authority for This Rulemaking**
Title 49 of the United States Code specifies the FAA's authority to issue rules on aviation safety. Subtitle I, section 106, describes the authority of the FAA Administrator. Subtitle VII, Aviation Programs, describes in more detail the scope of the Agency's authority.
The FAA is issuing this rulemaking under the authority described in Subtitle VII, Part A, Subpart III, Section 44701, General requirements. Under that section, Congress charges the FAA with promoting safe flight of civil aircraft in air commerce by prescribing regulations for practices, methods, and procedures the Administrator finds necessary for safety in air commerce. This regulation is within the scope of that authority because it addresses an unsafe condition that is likely to exist or develop on products identified in this rulemaking action.
**Regulatory Findings**
The FAA has determined that this proposed AD would not have federalism implications under Executive Order 13132. This proposed AD would not have a substantial direct effect on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government.
For the reasons discussed above, I certify that the proposed regulation:
(1) Is not a “significant regulatory action” under Executive Order 12866,
(2) Would not affect intrastate aviation in Alaska, and
(3) Would not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act.
**List of Subjects in 14 CFR Part 39**
Air transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.
**The Proposed Amendment**
Accordingly, under the authority delegated to me by the Administrator, the FAA proposes to amend 14 CFR part 39 as follows:
**PART 39—AIRWORTHINESS DIRECTIVES**
1. The authority citation for part 39 continues to read as follows:
**Authority:**
49 U.S.C. 106(g), 40113, 44701.
§ 39.13
2. The FAA amends § 39.13 by:
a. Removing Airworthiness Directive (AD) 2017-19-26, Amendment 39-19056 (82 FR 44504, September 25, 2017), and
b. Adding the following new AD:
*The Boeing Company:* Docket No. FAA-2025-3429; Project Identifier AD-2024-00722-T.
**(a) Comments Due Date**
The FAA must receive comments on this airworthiness directive (AD) by January 2, 2026.
**(b) Affected ADs**
This AD replaces AD 2017-19-26, Amendment 39-19056 (82 FR 44504, September 25, 2017) (AD 2017-19-26).
**(c) Applicability**
(1) This AD applies to The Boeing Company Model 737-600, -700, -700C, -800, and -900 series airplanes, certificated in any category, as identified in Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024.
(2) Installation of Supplemental Type Certificate (STC) ST00830SE does not affect the ability to accomplish the actions required by this AD. Therefore, for airplanes on which STC ST00830SE is installed, a “change in product” alternative method of compliance (AMOC) approval request is not necessary to comply with the requirements of 14 CFR 39.17.
**(d) Subject**
Air Transport Association (ATA) of America Code 53, Fuselage.
**(e) Unsafe Condition**
This AD was prompted by a fatigue test that revealed numerous cracks in the upper skin panel at the chem-milled step above the lap joint, followed by an evaluation by the design approval holder (DAH) that indicated that location is subject to widespread fatigue damage (WFD) on airplanes on which the preventative modification was installed after 30,000 total flight cycles, and by reports of fuselage fatigue cracks just above the S-4 lap joint. The FAA is issuing this AD to detect and correct fatigue cracking of the upper skin panel at the chem-milled step above the lap joint. The unsafe condition, if not addressed, could result in rapid decompression of the airplane.
**(f) Compliance**
Comply with this AD within the compliance times specified, unless already done.
**(g) Inspections of the Upper Skin Panel at Chem-Milled Steps and Pockets**
At the applicable times specified in tables 1, 2, and 3 of paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, except as required by paragraph (h) of this AD: Do an external detailed inspection and an external nondestructive inspection (a medium frequency eddy current (MFEC), magneto optic imager (MOI), C-scan, or ultrasonic phased array (UTPA) inspection) for any crack in the fuselage skin at the chem-milled steps, post-permanent repair inspections, and post-time-limited repair inspections, at certain locations specified in, and in accordance with, paragraphs 3.B.1, 3.B.3., and 3.B.4, as applicable, of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024. Repeat the inspections thereafter at the applicable time specified in tables 1, 2, and 3 of paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024.
**(h) Exceptions to Service Information Specifications**
(1) Where ACTION 2 of Part IV and ACTION 2 of Part VII of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, specifies to contact Boeing for work instructions or replacement instructions, this AD requires doing the instructions using a method approved in accordance with the procedures specified in paragraph (n) of this AD.
(2) Where the Compliance Time column in the tables under the “Compliance” paragraph of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, refers to the Revision 4 date of this service bulletin, this AD requires using the effective date of this AD.
(3) For airplanes on which the actions specified in paragraph (g) of this AD are required: Inspections specified in table 1 of paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, are not required in areas that are spanned by an FAA-approved repair that has a minimum of 3 rows of fasteners above and below the chem-milled step, provided that the repair was installed before October 30, 2017 (the effective date of AD 2017-19-26). Operators must accomplish post-repair inspections at the applicable time specified in table 2 of paragraph 1.E, “Compliance,” of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, except as required by paragraph (h)(5) of this AD.
(4) For any airplane that has an external doubler covering the chem-milled step, but the doubler does not span the step by a minimum of 3 rows of fasteners above and below the chem-milled step and the doubler was installed before October 30, 2017 (the effective date of AD 2017-19-26): One method of compliance with the inspection requirement of paragraph (g) of this AD for the inspections specified in table 1 of paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, is to inspect all chem-milled steps covered by the repair using non-destructive test (NDT) methods approved in accordance with the procedures specified in paragraph (n) of this AD. These repairs are to be considered time-limited and are subject to the post-repair supplemental inspections and replacement at the times specified in table 3 of paragraph 1.E., “Compliance,” of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, except as required by paragraph (h)(5) of this AD.
*Note 1 to paragraph (h)(4):* Guidance for the procedures for the alternative inspection specified in paragraph (h)(4) of this AD can be found in the Boeing 737 NDT Manual, Part 6, Subject 53-30-20.
(5) For airplanes on which Supplemental Type Certificate (STC) ST01697SE has been incorporated, all initial compliance times (thresholds) specified in flight cycles must be reduced to 1/2 and all repeat interval compliance times specified in flight cycles must be reduced to 1/4 of those specified in Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024.
**(i) Repair**
If any cracking is found during any inspection required by paragraph (g) or (h) of this AD, before further flight, repair the cracking using a method approved in accordance with the procedures specified in paragraph (n) of this AD.
**(j) Additional Actions for Modified Airplanes**
For airplanes which have installed STC ST01697SE and the preventive modification has been installed after 15,000 total flight cycles: Before the accumulation of 25,000 total flight cycles, do all applicable investigative and corrective actions using a method approved in accordance with the procedures specified in paragraph (n) of this AD.
**(k) Optional Terminating Action**
For airplanes that have accumulated 30,000 total flight cycles or fewer, or for airplanes on which STC ST0l697SE was installed and that have accumulated 15,000 total flight cycles or fewer: Accomplishment of the actions in paragraphs (k)(1) through (3) of this AD terminates the repetitive inspections required by paragraph (g) of this AD that are specified in table 1 of paragraph 1.E., Compliance, of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, for the modified area only.
(1) Do the preventive modification, including inspections for cracking, specified in Part V of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024.
(2) If any cracking is found during any inspection required by paragraph (k)(1) or (3) of this AD, before further flight, repair the cracking found using a method approved in accordance with the procedures specified in paragraph (n) of this AD.
(3) Do the post-modification repetitive inspections specified in table 1a and table 1b of paragraph 1.E., Compliance, and in accordance with Part VII of the Accomplishment Instructions of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024, except as required by paragraphs (h)(1) and (2) of this AD. The inspections must be performed and repeated at the applicable times specified in table 1a and table 1b of paragraph 1.E., Compliance, of Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024. For airplanes on which STC ST01697SE has been incorporated, all initial compliance times (thresholds) specified in flight cycles must be reduced to 1/2 and all repeat interval compliance times specified in flight cycles must be reduced to 1/4 of those specified in Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024.
**(l) Installation Limitations of Preventive Modification**
As of October 30, 2017 (the effective date of AD 2017-19-26), installation of the preventive modification specified in Boeing Service Bulletin 737-53A1232 is prohibited on the airplanes identified in paragraphs (l)(1) and (2) of this AD.
(1) Airplanes that have accumulated more than 30,000 total flight cycles.
(2) Airplanes which have installed STC ST01697SE and that have accumulated more than 15,000 total flight cycles.
**(m) Credit for Previous Actions**
(1) This paragraph provides credit for the optional actions in paragraph (k)(1) of this AD, if those actions were performed before the effective date of this AD using Boeing Alert Service Bulletin 737-53A1232, Revision 3, dated July 27, 2015.
(2) This paragraph provides credit for the optional actions in paragraph (k) of this AD, if those actions were performed before October 30, 2017 (the effective date of AD 2017-19-26) using the service information identified in paragraph (m)(2)(i) through (iii) of this AD.
(i) Boeing Special Attention Service Bulletin 737-53A1232, dated April 2, 2007.
(ii) Boeing Special Attention Service Bulletin 737-53A1232, Revision 1, dated May 18, 2012.
(iii) Boeing Special Attention Service Bulletin 737-53A1232, Revision 2, dated July 26, 2013.
**(n) Alternative Methods of Compliance (AMOCs)**
(1) The Manager, AIR-520, Continued Operational Safety Branch, FAA, has the authority to approve AMOCs for this AD, if requested using the procedures found in 14 CFR 39.19. In accordance with 14 CFR 39.19, send your request to your principal inspector or responsible Flight Standards Office, as appropriate. If sending information directly to the manager of the certification office, send it to the attention of the person identified in paragraph (o)(1) of this AD. Information may be emailed to: *[email protected]* . Before using any approved AMOC, notify your appropriate principal inspector, or lacking a principal inspector, the manager of the responsible Flight Standards Office.
(2) An AMOC that provides an acceptable level of safety may be used for any repair, modification, or alteration required by this AD if it is approved by The Boeing Company Organization Designation Authorization (ODA) that has been authorized by the Manager, AIR-520, Continued Operational Safety Branch, FAA, to make those findings. To be approved, the repair method, modification deviation, or alteration deviation must meet the certification basis of the airplane, and the approval must specifically refer to this AD.
(3) AMOCs approved for AD 2017-19-26 are not approved as AMOCs for the corresponding provisions of this AD.
**(o) Related Information**
(1) For more information about this AD, contact Luis Cortez-Muniz, Aviation Safety Engineer, FAA, 2200 South 216th St., Des Moines, WA 98198; phone: 206-231-3958; email: *[email protected]* .
(2) Material identified in this AD that is not incorporated by reference is available at the address specified in paragraph (p)(3) of this AD.
**(p) Material Incorporated by Reference**
(1) The Director of the Federal Register approved the incorporation by reference of the material listed in this paragraph under 5 U.S.C. 552(a) and 1 CFR part 51.
(2) You must use this material as applicable to do the actions required by this AD, unless the AD specifies otherwise.
(i) Boeing Alert Service Bulletin 737-53A1232, Revision 4, dated May 22, 2024.
(ii) [Reserved]
(3) 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; website *myboeingfleet.com* .
(4) 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.
(5) You may view this material at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, visit *www.archives.gov/federal-register/cfr/ibr-locations* or email *[email protected]* .
Issued on October 3, 2025.
Peter A. White,
Deputy Director, Integrated Certificate Management Division, Aircraft Certification Service.