Discussion \n\n\n\tWe 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 737-600, -700, -700C, -800, -900, and -900ER series airplanes. The NPRM published in the Federal Register on March 29, 2016 (81 FR 17415) (''the NPRM''). The NPRM was prompted by an evaluation by the DAH indicating that the S-14L and S-14R lap splices are subject to WFD. The NPRM proposed to require repetitive low frequency eddy current inspections for cracking in the lower fastener row of the S-14L and S- 14R lap splices, and repair if necessary. We are issuing this AD to detect and correct widespread cracking in the S-14L and S-14R lap splices that could rapidly link up and result in possible rapid decompression and reduced structural integrity of the airplane. \n\nComments \n\n\n\tWe gave the public the opportunity to participate in developing this AD. The following presents the comments received onthe NPRM and the FAA's response to each comment. \n\nSupport for the NPRM \n\n\n\tBoeing and a commenter, Jordan Ibsen, supported the content of the NPRM. \n\nEffect of Winglets on Accomplishment of the Proposed Actions \n\n\n\tAviation Partners Boeing stated that accomplishing the supplemental type certificate (STC) ST00830SE does not affect compliance with the actions specified in the NPRM. \n\tWe agree with the commenter. We have redesignated paragraph (c) of the NPRM as (c)(1) and added a new paragraph (c)(2) to this final rule to state that installation of STC ST00830SE does not affect the ability to accomplish the actions required by this final rule. Therefore, for airplanes on which STC ST00830SE is installed, a ''change in product'' Alternative Method of Compliance (AMOC) approval request is not necessary to comply with the requirements of 14 CFR 39.17. \n\nRequest To Revise Compliance Time \n\n\n\tThe European Aviation Safety Agency (EASA) noted that Boeing Alert Service Bulletin737-53A1352, dated October 2, 2015, specifies that the existing 737-600/700/800/900 Maintenance Planning Document (MPD), Section 9, Airworthiness Limitation Instruction (ALI) Inspection Program, is not sufficient to prevent WFD in the S-14L and S-14R lap splice, lower fastener rows, between station (STA) 360-540 and STA 727- 887, as the airplane ages. EASA added that this service information specifies inspections at principal structural elements (PSEs) 53-30-04- 6, 53-30-04-6a, 53-60-04-6 and 53-60-04-6a before accumulating 54,000 total flight cycles. However, EASA noted that 737-600/700/800/900 MPD, Section 9, C626AOO 1-CMR Table 9-2, Revision August 2012, requires doing the inspections before accumulating 50,000 flight cycles; which contradicts the initial statement that the ALI inspection program is not sufficient to preclude WFD. EASA concluded that if the current ALI is not sufficient to preclude WFD, then the 50,000 flight cycles should be reduced, rather than increased to 54,000flight cycles. \n\tWe infer the commenter is requesting that we reduce the 54,000 flight-cycle compliance time specified in Boeing Alert Service Bulletin 737-53A1352, Revision 1, dated March 10, 2016 (which is the appropriate source of service information for accomplishing the required actions in this AD). We do not agree with the request to reduce the compliance time. However, we do agree to clarify the WFD analysis. Boeing uses a different methodology than the standard damage tolerance analysis for evaluating structure that is susceptible to WFD. This methodology can sometimes produce a longer initial inspection threshold than the baseline maintenance program, but requires more frequent repetitive inspections, as in the case of the S-14L and S-14R lap splices. Although, for certain airplanes, the initial WFD threshold specified in Boeing Alert Service Bulletin 737-53A1352, Revision 1, dated March 10, 2016, is 4,000 flight cycles more than the ALI threshold; the repetitive inspectioninterval is reduced by 6,000 flight cycles. \n\tOperators are still required to accomplish the ALI inspections in accordance with 14 CFR 91.403(c). However, if the inspections specified in Boeing Alert Service Bulletin 737-53A1352, Revision 1, dated March 10, 2016, and the ALI inspections overlap (same location, inspection detail, and technique) then the more restrictive of the two programs satisfies both requirements. Since a specific revision of the ALI inspections are required by AD 2013-19-23, Amendment 39-17605 (78 FR 61173, October 3, 2013), Boeing Alert Service Bulletin 737-53A1352, Revision 1, dated March 10, 2016, contains an approved AMOC to AD 2013- 19-23, for certain PSEs, after the initial inspections in that service bulletin are accomplished. We have not changed this AD in this regard. \n\nRequest for Clarification of Extent of Boeing Organization Designation Authorization (ODA) \n\n\n\tSouthwest Airlines (SWA) asked for clarification that the Boeing ODA identified in paragraph (i)(3) of the proposed AD can provide an AMOC for any ''repair, modification, or alteration'' that includes the authority to approve existing repairs in the inspection area that inhibit accomplishment of the AD requirements as terminating action to paragraph (g) of the proposed AD. SWA also asked if the ODA has the authority to provide alternative inspection procedures for repaired areas where the inspection in paragraph (g) of the proposed AD cannot be accomplished. Additionally, SWA asked that we clarify that the Boeing ODA identified in paragraph (i)(3) of the proposed AD is able to issue an AMOC for an existing repair at the S-14 lap joint (where the location of the repair inhibits accomplishing the initial inspection), provided the repair was approved by any FAA designation authority and there are a minimum of three fastener rows above and below the lap joint. SWA stated that neither Boeing Alert Service Bulletin 737- 53A1352, dated October 2, 2015, nor the NPRM clearly statehow to address existing repairs that prevent accomplishment of the inspections specified in paragraph (g) of the proposed AD. \n\tWe agree with the commenter that clarification of the extent of the authority of the Boeing ODA is necessary. The Boeing ODA includes the authority to evaluate existing repairs and provide alternative inspection programs in the repaired area, and includes approval of alternative inspections as AMOCs if accomplishment of the inspections required by paragraph (g) of this AD is inhibited. We have not changed this AD in this regard. \n\n((Page 83659)) \n\n\n\tWe infer that SWA is asking if the Boeing ODA can issue a global AMOC for the referenced repair. The Boeing ODA does not have that authority. We have not received any information from Boeing that defines such a repair that would be considered for a global AMOC. If Boeing provides supporting data, we will evaluate the data to determine if that repair and any associated inspections provide an acceptable levelof safety. We have not changed this AD in this regard. \n\nChange to This AD \n\n\n\tWe have reviewed Boeing Alert Service Bulletin 737-53A1352, Revision 1, dated March 10, 2016, and there are no substantial changes. Therefore, we have included Boeing Alert Service Bulletin 737-53A1352, Revision 1, dated March 10, 2016, as the appropriate source of service information for accomplishing in the actions required by paragraph (g) of this AD. We have also added a new paragraph (h) to this AD to provide credit for actions done prior to the effective date of this AD using Boeing Alert Service Bulletin 737-53A1352, dated October 2, 2015. We have redesignated subsequent paragraphs accordingly. \n\nConclusion \n\n\n\tWe reviewed the relevant data, considered the comments received, and determined that air safety and the public interest require adopting this AD with the changes described previously and minor editorial changes. We have determined that these minor changes: \n\tAre consistent with the intent that was proposed in the NPRM for correcting the unsafe condition; and \n\tDo not add any additional burden upon the public than was already proposed in the NPRM. \n\tWe also determined that these changes will not increase the economic burden on any operator or increase the scope of this AD. \n\nRelated Service Information Under 1 CFR Part 51 \n\n\n\tWe reviewed Boeing Alert Service Bulletin 737-53A1352, Revision 1, dated March 10, 2016. The service information describes procedures for low frequency eddy current inspections and repair for cracking in the lower fastener row of the S-14L and S-14R lap splices. 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\tWe estimate that this AD affects 1,513 airplanes of U.S. registry. We estimate 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 for Group 1 airplanes 84 work-hours x $85 $0 $7,140 per $10,502,940 per \n\t(1,471 airplanes). per hour = $7,140 inspection cycle. inspection cycle. \n\tper inspection \n\tcycle. Inspection for Group 2 airplanes 65 work-hours x $85 0 $5,525 per $232,050 per \n\t(42 airplanes). per hour = $5,525 inspection cycle. inspection cycle. \n\tper inspection \n\tcycle. ---------------------------------------------------------------------------------------------------------------- \n\n\n\tWe have received no definitive data that enables us toprovide 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\tWe are 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\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) Is not a ''significant rule'' under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979), \n\t(3) Will not affect intrastate aviation in Alaska, and \n\t(4) 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.