Airworthiness Directives; The Boeing Company Airplanes

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]</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> Notice of proposed rulemaking (NPRM). <SUM> <HD SOURCE="HED">SUMMARY:</HD> The FAA proposes to adopt a new airworthiness directive (AD) for certain The Boeing Company Model 757 airplanes. This proposed AD was prompted by reports of precoolers that failed due to a wear-out condition, combined with latently failed overheat detection thermal switches. This proposed AD would 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 proposing this AD to address the unsafe condition on these products. </SUM> <EFFDATE> <HD SOURCE="HED">DATES:</HD> The FAA must receive comments on this proposed AD by May 16, 2025. </EFFDATE> <HD SOURCE="HED">ADDRESSES:</HD> You may send comments, using the procedures found in 14 CFR 11.43 and 11.45, by any of the following methods: • <E T="03">Federal eRulemaking Portal:</E> Go to <E T="03">regulations.gov</E> . Follow the instructions for submitting comments. • <E T="03">Fax:</E> 202-493-2251. • <E T="03">Mail:</E> 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">Hand Delivery:</E> Deliver to Mail address above between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. <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 NPRM, any comments received, and other information. The street address for Docket Operations is listed above. <E T="03">Material Incorporated by Reference:</E> • For Boeing material identified 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 <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">Comments Invited</HD> The FAA invites you to send any written relevant data, views, or arguments about this proposal. Send your comments to an address listed under the <E T="02">ADDRESSES</E> section. Include “Docket No. FAA-2025-0474; Project Identifier AD-2024-00777-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 this 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 <E T="03">regulations.gov,</E> including any personal information you provide. The agency will also post a report summarizing each substantive verbal contact received about this NPRM. <HD SOURCE="HD1">Confidential Business Information</HD> 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 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> Any commentary that the FAA receives that is not specifically designated as CBI will be placed in the public docket for this rulemaking. <HD SOURCE="HD1">Background</HD> The FAA has received a report of a Model 757-300 airplane that was taken out of service due to heat damage found on the engine number 1 thrust reverser access door panel. There have been seven instances of precoolers installed on Model 757-300 airplanes with Rolls-Royce Deutschland Ltd. & Co. KG Model RB211-535-series engines that failed due to a wear-out condition. The result of a failed precooler is leakage of hot air to the strut due to a cracked or ruptured precooler core near the sideplates. The overheat detection system within an RB211-535-series engine strut for Model 757-200,-200PF,-200CB, and -300 airplanes contains thermal switches with a latent failure mode. The combination of a failed precooler and latently failed overheat detection thermal switches 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">FAA's Determination</HD> 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. <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 general visual inspection 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">Proposed AD Requirements in This NPRM</HD> 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 <E T="03">regulations.gov</E> under Docket No. FAA-2025-0474. <HD SOURCE="HD1">Interim Action</HD> The FAA considers that this proposed AD would be an interim action. If final action is later identified, the FAA might consider further rulemaking then. <HD SOURCE="HD1">Costs of Compliance</HD> The FAA estimates that this AD, if adopted as proposed, would affect 235 airplanes of U.S. registry. The FAA estimates the following costs to comply with this proposed AD: <GPOTABLE COLS="5" OPTS="L2,nj,i1" CDEF="s50,r50,r25,r25,r50"> <TTITLE>Estimated Costs</TTITLE> <CHED H="1">Action</CHED> <CHED H="1">Labor cost</CHED> <CHED H="1">Parts cost</CHED> <CHED H="1">Cost per product</CHED> <CHED H="1">Cost on U.S. operators</CHED> <ROW> <ENT I="01">Inspection</ENT> <ENT>12 work-hours × $85 per hour = $1,020</ENT> <ENT>$0</ENT> <ENT>$1,020</ENT> <ENT>$239,700.</ENT> </ROW> <ROW> <ENT I="01">Temperature and continuity test</ENT> <ENT>20 work-hours × $85 per hour = $1,700</ENT> <ENT>$0</ENT> <ENT>$1,700</ENT> <ENT>$399,500 per test cycle.</ENT> </ROW> <ROW> <ENT I="01">Precooler replacement (21 Model 757-300 airplanes)</ENT> <ENT>34 work-hours × $85 per hour = $2,890</ENT> <ENT>Up to $96,675</ENT> <ENT>Up to $99,565</ENT> <ENT>Up to $2,090,865 per replacement cycle.</ENT> </ROW> </GPOTABLE> The FAA estimates the following costs to do any on-condition actions that would be required based on the results of the proposed inspection and proposed test. The agency has no way of determining the number of airplanes that might need these on-condition actions: <GPOTABLE COLS="4" OPTS="L2,i1" CDEF="s50,r40,12,12"> <TTITLE>On-Condition Costs *</TTITLE> <CHED H="1">Action</CHED> <CHED H="1">Labor cost</CHED> <CHED H="1">Parts cost</CHED> <ENT I="01">Strut overheat detection system test</ENT> <ENT>2 work-hours × $85 per hour = $170</ENT> <ENT>$0</ENT> <ENT>$170</ENT> </ROW> <ROW> <ENT I="01">Thermal switch part number (P/N)-003 replacement</ENT> <ENT>2 work-hours × $85 per hour = $170</ENT> <ENT>939</ENT> <ENT>1,109</ENT> </ROW> <ROW> <ENT I="01">Thermal ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Preview showing 10k of 19k characters. Full document text is stored and available for version comparison. ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━