AD 2025-23-03

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.

AD Docket: You may examine the AD docket at regulations.gov 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.

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; 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-2025-0474.

Kathryn Hill, Aviation Safety Engineer, FAA, 2200 South 216th St., Des Moines, WA 98198; phone: 206-231-3626; email: Kathryn.A.Hill@faa.gov.

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 Federal Register 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.

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.

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.

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.

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.

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.

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 ADDRESSES section.

The FAA considers this AD to be an interim action. If final action is later identified, the FAA might consider further rulemaking then.

The FAA estimates that this AD affects 235 airplanes of U.S. registry. The FAA estimates the following costs to comply with this AD:

The FAA estimates the following costs to do any on-condition actions that would be required based on the results of the inspection and test. The agency has no way of determining the number of airplanes that might need these on-condition actions:

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.

This AD will not have federalism implications under Executive Order 13132. This AD will 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 this AD:

(1) Is not a “significant regulatory action” under Executive Order 12866,

(2) Will not affect intrastate aviation in Alaska, and

(3) Will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act.

Accordingly, under the authority delegated to me by the Administrator, the FAA amends 14 CFR part 39 as follows:

1. The authority citation for part 39 continues to read as follows:

2. The FAA amends § 39.13 by adding the following new airworthiness directive: