Background \n\n\n\tThe FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 by adding an AD that would apply to all PW PW4164, PW4164- 1D, PW4168, PW4168-1D, PW4168A, PW4168A-1D, and PW4170 model turbofan engines with a certain outer combustion chamber assembly and 3rd stage LPT duct segments installed. The NPRM published in the Federal Register on June 12, 2020 (85 FR 35812). The NPRM was prompted by multiple reports of damaged or failed 3rd stage LPT duct segments that resulted in engine surges, in-flight shutdowns, diversions, and air turnbacks. The reports were attributed to elevated gas path temperature at the outer diameter of the turbine flowpath and high-pressure turbine (HPT) 2nd stage blade outer air seal spallation, which led to the distortion and liberation of 3rd stage LPT duct segments. In the NPRM, the FAA proposed to require removing and replacing certain 3rd stage LPT duct segments. The FAA is issuing this AD \n\n((Page 79409)) \n\nto address the unsafe condition on these products. \n\nDiscussion of Final Airworthiness Directive \n\nComments \n\n\n\tThe FAA received comments from two commenters. The commenters were Air Line Pilots Association, International (ALPA) and Delta Air Lines, Inc. (Delta). ALPA supported the proposal without change. Delta supported the proposal but recommended certain changes. The following presents the comments received on the NPRM and the FAA's response to each comment. \n\nRequest to the Revise Compliance or Definition \n\n\n\tDelta requested that the FAA revise paragraph (g) of this AD to require replacement of duct segments that have operated with more than 2,500 cycles with a Talon IIB combustor. Alternatively, Delta requested the FAA revise paragraph (i) of this AD to define an ''engine shop visit'' as ''removal and disassembly of the HPT module.'' Delta noted several industry events have occurred related to the failure of 3rd stage LPT duct segments, also commonly called outer transition ducts (OTDs), on PW4000-100 engines with Talon IIB combustors. Prior information from the design approval holder indicates that industry failures have occurred only on parts that have operated more than 3,000 flight cycles with a Talon IIB combustor. \n\tDelta further noted that paragraph (g) of the proposed rule would require that duct segments with certain part numbers be removed and replaced with new parts at every engine shop visit, defined in paragraph (i) as the ''induction of an engine into the shop for maintenance involving the separation of pairs of major mating flanges.'' Per this definition, engines with few cycles in service since prior OTD replacement would require installation of new hardware, even for minor repairs where maintenance of the LPT would not have otherwise been required. \n\tAdditionally, Delta commented that OTD replacement requires significant teardown of the LPT module, which will result substantial fallout and repair costs for unrelatedhardware. Delta reasoned that the proposed rule would require repetitive replacement of low-time duct segments at substantial financial burden to achieve minimal risk reduction. Since industry experience has demonstrated duct segment liberation to occur on hardware above 3,000 flight cycles, Delta concluded that sufficient data should exist to establish an allowable service life that maintains an acceptable level of safety. \n\tDelta also noted that while a cycle-based requirement would be preferable, defining an engine shop visit as ''removal and disassembly of the HPT module'' would maintain an acceptable level of safety. Delta commented that requiring replacement of low-time duct segments during minor repairs presents a substantial financial burden for minimal risk reduction. \n\tThe FAA disagrees with revising paragraph (g) or (i) of this AD. Requiring removal of the 3rd stage LPT duct segments at the next HPT overhaul does not adequately address the unsafe condition. The FAA's riskassessment assumed that the 3rd stage LPT duct segments would be replaced at every shop visit, which provides an acceptable level of safety. Operators may propose to the FAA an alternative method of compliance in accordance with paragraph (j) of this AD. The FAA did not change this AD. \n\nRequest To Require Removed Duct Segments be Discarded, Scrapped, or Mutilated \n\n\n\tDelta requested that the FAA revise paragraph (g) of this AD to require that 3rd stage LPT duct segments that have been removed from service be discarded, scrapped, or mutilated. Delta reasoned that the rule would apply only to PW4000-100 engines with certain combustors, but the subject part numbers are certified for installation in other engine models. Delta expressed concern that without a requirement to discard or scrap the removed duct segments, they could potentially be installed in a PW4000-94 or non-Talon-IIB PW4000-100 engine or sold to an operator without knowledge of their prior operation with a Talon IIBcombustor. \n\tThe FAA disagrees with Delta's request. When parts are removed from service due to an AD, they are unserviceable unless the AD specifies otherwise. Unserviceable parts are not airworthy and should be disposed of in a manner that does not allow them to be returned to service. Operators must ensure only serviceable parts are installed on engines before approving the aircraft for return to service. The FAA did not change this AD. \n\nRequest To Include Available Fleet Data \n\n\n\tDelta requested that the FAA update the proposed rule to reflect the available fleet data, which shows that failure of 3rd stage LPT duct segments has only occurred after 3,000 cycles or greater in operation on engines with a Talon IIB combustor. Delta stated that the Discussion paragraph of the proposed rule does not provide prior service history of LPT duct segments that have failed in-service. Delta further noted that prior communications from the design approval holder indicate that industry failures occurred on parts that operated 3,000 flight cycles or more under exposure to the higher-than-expected temperatures with a Talon IIB combustor installed. \n\tThe FAA disagrees. The Discussion paragraph of the NPRM provides an adequate discussion of the failure in service of 3rd stage LPT duct segments. The FAA did not change this AD. \n\nRequest To Update Service Information \n\n\n\tDelta requested that the FAA add PW Service Bulletin (SB) PW4G-100- 72-220 to the Related Service Information paragraph of this AD. Delta noted that the accomplishment of SB PW4G-100-72-220 installs a Talon IIB combustor, and 3rd stage duct segments operated on a post-SB engine would be subject to the proposed rule as well. \n\tThe FAA agrees and has added PW SB PW4G-100-72-220 to the Related Service Information paragraph of this AD. \n\nConclusion \n\n\n\tThe 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. \n\nRelated Service Information \n\n\n\tThe FAA reviewed PW SB No. PW4G-100-72-214, dated December 15, 2011; PW SB No. PW4G-100-72-219, Revision 1, dated October 5, 2011; PW SB No. PW4G-100-72-253, dated November 24, 2014; and PW SB No. PW4G- 100-72-220, Revision 4, dated September 30, 2011. PW SB No. PW4G-100- 72-214 introduces the Talon IIB outer combustion chamber assembly that reduces the combustor exit temperature levels at the outer diameter of the combustor. PW SB No. PW4G-100-72-219 describes procedures for installing the Advantage70 engine upgrade kit to improve engine reliability and fuel consumption, and to reduce maintenance costs. PW SB No. PW4G- \n\n((Page 79410)) \n\n100-72-253 describes procedures forreplacing the outer combustion chamber assembly waspaloy nuts. PW SB PW4G-100-72-220 describes procedures for installing the Advantage70 engine upgrade kit to improve engine reliability and fuel consumption, reduce maintenance costs, and convert engine thrust rating. \n\nCosts of Compliance \n\n\n\tThe FAA estimates that this AD affects 99 engines installed on airplanes of U.S. registry. \n\tThe FAA estimates the following costs to comply with this AD: \n\n\n\tEstimated Costs ---------------------------------------------------------------------------------------------------------------- \n\tCost per Cost on U.S. \n\tAction Labor cost Parts cost product operators ---------------------------------------------------------------------------------------------------------------- Remove and replace 3rd stage LPT duct 56 work-hours x $85 per $85,000 $89,760 $8,886,240 \n\tsegments. hour = $4,760. ---------------------------------------------------------------------------------------------------------------- \n\nAuthority for This Rulemaking \n\n\n\tTitle 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. \n\tThe 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. \n\nRegulatoryFindings \n\n\n\tThis 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. \n\tFor the reasons discussed above, I certify that this AD: \n\t(1) Is not a ''significant regulatory action'' under Executive Order 12866, \n\t(2) Will not affect intrastate aviation in Alaska, and \n\t(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. \n\nList of Subjects in 14 CFR Part 39 \n\n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety.