Airworthiness Directives; The Boeing Company Airplanes

<RULE> DEPARTMENT OF TRANSPORTATION <SUBAGY>Federal Aviation Administration</SUBAGY> <CFR>14 CFR Part 39</CFR> <DEPDOC>[Docket No. FAA-2025-0474; Project Identifier AD-2024-00777-T; Amendment 39-23186; AD 2025-23-03]</DEPDOC> <RIN>RIN 2120-AA64</RIN> <SUBJECT>Airworthiness Directives; The Boeing Company Airplanes</SUBJECT> <HD SOURCE="HED">AGENCY:</HD> Federal Aviation Administration (FAA), DOT. <HD SOURCE="HED">ACTION:</HD> Final rule. <SUM> <HD SOURCE="HED">SUMMARY:</HD> The FAA is adopting a new airworthiness directive (AD) for certain The Boeing Company Model 757 airplanes. This AD was prompted by reports of precoolers that failed due to a wear-out condition, combined with latently failed overheat detection thermal switches. This AD requires an inspection for heat damage on the engine strut structure, repetitive tests of the thermal switch temperature and ground wires, replacement of the precooler on Model 757-300 airplanes, and applicable on-condition actions. The FAA is issuing this AD to address the unsafe condition on these products. </SUM> <EFFDATE> <HD SOURCE="HED">DATES:</HD> This AD is effective January 13, 2026. The Director of the Federal Register approved the incorporation by reference of a certain publication listed in this AD as of January 13, 2026. </EFFDATE> <HD SOURCE="HED">ADDRESSES:</HD> <E T="03">AD Docket:</E> You may examine the AD docket at <E T="03">regulations.gov</E> under Docket No. FAA-2025-0474; 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. <E T="03">Material Incorporated by Reference:</E> • 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 <E T="03">myboeingfleet.com.</E> • 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 <E T="03">regulations.gov</E> under Docket No. FAA-2025-0474. <FURINF> <HD SOURCE="HED">FOR FURTHER INFORMATION CONTACT:</HD> Kathryn Hill, Aviation Safety Engineer, FAA, 2200 South 216th St., Des Moines, WA 98198; phone: 206-231-3626; email: <E T="03">Kathryn.A.Hill@faa.gov.</E> </FURINF> <SUPLINF> <HD SOURCE="HED">SUPPLEMENTARY INFORMATION:</HD> <HD SOURCE="HD1">Background</HD> 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 757 airplanes. The NPRM was published in the <E T="04">Federal Register</E> on April 1, 2025 (90 FR 14346). The NPRM was prompted by reports of precoolers that failed due to a wear-out condition, combined with latently failed overheat detection thermal switches. In the NPRM, the FAA proposed to require an inspection for heat damage on the engine strut structure, repetitive tests of the thermal switch temperature and ground wires, replacement of the precooler on Model 757-300 airplanes, and applicable on-condition actions. The FAA is issuing this AD to address the combination of a failed precooler and latently failed overheat detection thermal switches. The unsafe condition, if not addressed, may result in prolonged high temperature heat exposure on the strut, which could lead to separation of the engine strut-to-wing box connection. <HD SOURCE="HD1">Discussion of Final Airworthiness Directive</HD> <HD SOURCE="HD1">Comments</HD> The FAA received comments from the Air Line Pilots Association, International (ALPA), ProTech Aero Services Limited, a named individual, and two anonymous commenters, who all supported the NPRM without change. The FAA received additional comments from Aviation Partners Boeing (APB), European Air Transport Leipzig GmbH, and United Parcel Service Co. (UPS Airlines). The following presents those comments and the FAA's response. <HD SOURCE="HD1">Effect of Winglets on Accomplishment of the Proposed Actions</HD> APB stated that the installation of winglets per Supplemental Type Certificate (STC) ST01518SE does not affect compliance with the required actions of the proposed rule. The FAA agrees. The installation of STC ST01518SE does not affect compliance with the required actions of this AD, specifically, the accomplishment of the manufacturer's service instructions. The FAA has not changed this AD in this regard. <HD SOURCE="HD1">Request To Revise Compliance Time for General Visual Inspection (GVI)</HD> UPS Airlines requested the FAA revise the proposed AD to only require a GVI of the strut if the thermal switch fails the test that is required to be accomplished within 9,000 flight hours or 27 months from the effective date of the AD, whichever occurs first. The commenter stated there are multiple maintenance program inspections of the strut that are accomplished at various intervals as specified in the Boeing 757 Maintenance Planning Data (MPD). The commenter also stated that MPD inspections look for corrosion, heat damage, and any obvious signs of damage. The commenter asserted the inspection of the strut in the proposed AD is redundant to the MPD inspections and will not provide an enhanced level of safety. The FAA does not agree with the request. The maintenance tasks provided in the Boeing 757 MPD are in general the manufacturer's recommended tasks so that each operator can develop a customized maintenance program. Also, the intervals of certain inspections can be escalated depending on the operator's existing maintenance practices. The FAA cannot be sure that all strut areas addressed by this AD will be inspected in a timely manner by all affected operators based on each operator's customized maintenance program. The FAA has determined that a latent failure mode identified on the thermal switches may leave hot air leakage due to precooler failure undetected for a prolonged period, and this condition could eventually result in critical thermal damage to the strut structure. The risk of having critical thermal damage becomes significantly high for those airplanes that continue to operate with latently failed thermal switches. Since the failure of thermal switches cannot be detected until the switches are tested, the FAA concluded that the strut should be inspected as soon as reasonably practical to mitigate the risk by detecting thermal damage to the strut structure. The FAA has not changed this AD in this regard. <HD SOURCE="HD1">Request To Correct Aircraft Maintenance Manual (AMM) Reference</HD> European Air Transport Leipzig GmbH requested that the FAA correct the reference to Boeing “757 AMM 26-12-01” for the thermal switch set temperature test in Boeing Alert Requirements Bulletin 757-26A0062 RB, dated January 17, 2025. The commenter stated that “757 AMM 26-12-00” is the correct reference for the accepted procedure. The FAA agrees that the AMM reference to the accepted procedure for the thermal switch set temperature test is incorrect, since 757 AMM 26-12-01 relates to the removal and installation of the strut overheat switch, and 757 AMM 26-12-00 relates to the thermal switch set temperature test. However, no change to the AD is necessary in this regard. The FAA notes that the incorrect AMM reference is in Boeing Alert Service Bulletin 757-26A0062, dated January 17, 2025, not Boeing Alert Requirements Bulletin 757-26A0062 RB, dated January 17, 2025. In addition to containing the same information that is required for compliance with the requirements bulletin, the service bulletin provides additional guidance, including references to accepted procedures for accomplishing the required actions. Since the reference to the incorrect AMM section in the service bulletin is “referred to” as additional guidance, operators are not required to use it to accomplish the thermal switch set temperature test. Operators may use the appropriate AMM sections to accomplish the required actions. The FAA has not changed this AD in this regard. <HD SOURCE="HD1">Conclusion</HD> 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, this AD is adopted as proposed in the NPRM. None of the changes will increase the economic burden on any operator. <HD SOURCE="HD1">Material Incorporated by Reference Under 1 CFR Part 51</HD> The FAA reviewed Boeing Alert Requirements Bulletin 757-26A0062 RB, dated January 17, 2025. This material specifies procedures for a GVI for heat damage on the left and right engine strut structure, repetitive thermal switch temperature tests and continuity tests of the ground wires, and, for Model 757-300 airplanes, replacement of the precooler at intervals not to exceed 45,000 total precooler flight hours. This material also specifies procedures for applicable on-condition actions including repair of structures with heat damage, replacement of the thermal switch, repair or replacement of failed circuit wires, and a system test of the strut overheat detection system, which includes doing applicable corrective actions until the test is passed. 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 <E T="02">ADDRESSES</E> section. <HD SOURCE="HD1">Interim Action</HD> The FAA consi ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Preview showing 10k of 21k characters. Full document text is stored and available for version comparison. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━