Discussion \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 The Boeing Company Model MD-90-30 airplanes. The NPRM published in the Federal Register on June 21, 2019 (84 FR 29105). The NPRM was prompted by reports indicating that based on Model MD-80 airplane service experience, certain center wing stringers and skins are potentially susceptible to fatigue-related cracking on Model MD-90 airplanes. The Model MD-80 and Model MD-90 wings share the same basic design and experience similar stresses, but no such cracking has been found on Model MD-90 airplanes. Cracks on Model MD-80 airplanes were found in the center wing lower stringers, at the inboard end where they are joined to the airplane centerline by end fittings; in the lower stringer end fittings, at the outboard end where they attach to stringers; and in the lower forward and aft skins, underneath cracked stringers. If not addressed, this cracking could result in the inability of the structure to sustain limit loads, and adversely affect the structural integrity of the airplane. The NPRM proposed to require repetitive ETLF inspections of the left and right side fastener holes for any crack; repetitive ETHF inspections of the lower skin for any crack; and repair if any crack is found. \n\nComments \n\n\n\tThe FAA gave the public the opportunity to participate in developing this final rule. The following presents the comments received on the NPRM and the FAA's response to each comment. \n\nRequest for Clarification of Other Relevant Rulemaking Section \n\n\n\tBoeing requested clarification regarding the number of cracking occurrences reported in areas outside of those addressed by AD 2016-07- 28, Amendment 39-18473 (81 FR 21253, April 11, 2016) (''AD 2016-07- 28''), or Boeing Alert Service Bulletin MD80- \n\n((Page 70001)) \n\n57A244, dated March 3, 2016. Boeing noted that in the NPRM, the Other Relevant Rulemaking section stated that since AD 2016-07-28 was issued, cracking was found at fastener holes common to stringers (S) S-11 through S-22, and around the external bracket angle at S-18 and S-19. Boeing emphasized that there was only one case of cracking, at S-13. \n\tIn addition, Boeing requested that the Other Relevant Rulemaking section in the NPRM be revised to explain that the new service information that Boeing is developing for Model MD-80 airplanes is to address potential cracking in new stringer locations and is not in response to actual in-service reports of cracking. \n\tThe FAA agrees with the commenter's assessment of crack findings. Of the cracks found since issuance of AD 2016-07-28, only one was found at S-13--the only area not addressed by AD 2016-07-28. All remaining crack findings were within the scope of the requirements of AD 2016-07- 28. As a result of these cracks, Boeing made the determination to expand the inspection to stringers S-11 through S-22. This AD addresses those stringers on Model MD-90-30 airplanes. \n\tThe FAA acknowledges that the service information for the Model MD- 80 airplanes, Boeing Service Bulletin MD80-57A244, dated March 3, 2016, which is mandated by AD 2016-07-28, has been revised. Boeing Service Bulletin MD80-57A244, Revision 1, dated October 1, 2019, updates the inspection method and expands the inspection area. The FAA may consider further rulemaking in the future to mandate these changes for Model MD- 80 airplanes. \n\tBecause the Other Relevant Rulemaking section in SUPPLEMENTARY INFORMATION is not retained in final rules, the FAA has not revised this final rule in regard to these issues. \n\nRequest To Revise Cost Information \n\n\n\tDelta Air Lines (DAL) requested clarification regarding the number of airplanes on the U.S. registry that would be affected by the NPRM. DAL stated that the Costs of Compliance paragraph indicated that an estimated 43 airplanes would be affected. However, DAL is the only U.S. operator of the affected airplanes, and DAL reports that there are 65 airplanes in their operations specification. \n\tThe FAA agrees with the commenter and has revised the Costs of Compliance paragraph in this final rule accordingly. \n\nRequest for Legible Service Information \n\n\n\tDAL requested that better quality copies of certain sheets of Boeing Drawing SN09570007 be provided to operators. The commenter stated that the poor quality of these sheets renders them useless in terms of doing inspections and is concerned about showing compliance with the requirements specified in the proposed AD. \n\tThe FAA acknowledges the commenter's concern regarding the legibility of certain sheets of Boeing Drawing SN09570007. Since Boeing Alert Service Bulletin MD90-57A031, dated March 19, 2019, does not include Boeing Drawing SN09570007 in the ''Required for Compliance (RC)'' section, the drawing is not required to comply with this AD. The drawing is referenced in Paragraph 1.J.2., Planning Information, References, Data Supplied with the Service Bulletin, in Boeing Alert Service Bulletin MD90-57A031, dated March 19, 2019. Operators notified Boeing about the illegible drawing sheets, and in response, Boeing issued Boeing Multi Operator Message MOM-MOM-19-0549-01B, dated October 4, 2019, which provides clearer images of Boeing Drawing SN09570007. The FAA recognizes that Boeing Drawing SN09570007 may provide helpful information to operators. Therefore, the FAA has added Note 1 to paragraph (g) of this AD to notify operators that if they have illegible pages of Boeing Drawing SN09570007, additional guidance can be found in Boeing Multi Operator Message MOM-MOM-19-0549-01B, dated October 4, 2019. \n\nConclusion \n\n\n\tThe FAA reviewed the relevant data, considered the comments received, and determined that air safety and the public interest require adopting this final rule with the changes described previously and minor editorial changes. The FAA determined that these minor changes: \n\tAre consistent with the intent that was proposed in the NPRM for addressing the unsafe condition; and \n\tDo not add any additional burden upon the public than was already proposed in the NPRM. \n\tThe FAA also determined that these changes will not increase the economic burden on any operator (other than the estimated number of affected airplanes as explained under the ''Request to Revise Cost Information'' section of this final rule) or increase the scope of this final rule. \n\nRelated Service Information Under 1 CFR Part 51 \n\n\n\tThe FAA reviewed Boeing Alert Service Bulletin MD90-57A031, dated March 19, 2019. This service information describes procedures for repetitive ETLF inspections of the left and right side fastener holes for any crack, repetitive ETHF inspections of the lower skin for any crack, and repair if any crack is found. This service information 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. \n\nCosts of Compliance \n\n\n\tThe FAA estimates that this AD affects 65 airplanes of U.S. registry. The FAA estimates the following costs to comply with this AD: \n\n\n\tEstimated Costs ---------------------------------------------------------------------------------------------------------------- \n\tCost on U.S. \n\tAction Labor cost Parts cost Cost per product operators ---------------------------------------------------------------------------------------------------------------- Inspection.............. 30 work-hours x $85 per $0 $2,550 per $165,750 per \n\thour = $2,550 per inspection cycle. inspection cycle. \n\tinspection cycle. ---------------------------------------------------------------------------------------------------------------- \n\n\n\tThe FAA has received no definitive data that would enable the agency to provide cost estimates for the on-condition actions specified in this AD. \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 \n\n((Page 70002)) \n\nwith 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\tThis AD is issued in accordance with authority delegated bythe Executive Director, Aircraft Certification Service, as authorized by FAA Order 8000.51C. In accordance with that order, issuance of ADs is normally a function of the Compliance and Airworthiness Division, but during this transition period, the Executive Director has delegated the authority to issue ADs applicable to transport category airplanes and associated appliances to the Director of the System Oversight Division. \n\nRegulatory Findings \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 economicimpact, 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.