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 The Boeing Company Model 737-900ER series airplanes. The NPRM published in the Federal Register on January 10, 2020 (85 FR 1290). The NPRM was prompted by reports of significant corrosion of electrical connectors located in the MLG wheel well. The NPRM proposed to require repetitive records checks to determine exposure to certain deicing fluids or repetitive inspections for corrosion of the electrical connectors, and corrective actions if necessary. \n\tThe FAA is issuing this AD to address corrosion and subsequent moisture ingress that may lead to electrical shorting of the connectors and incorrect functioning of critical systems necessary for safe flight and landing. \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\nSupport for the NPRM \n\n\n\tAir Line Pilots Association, International (ALPA) and two other commenters supported the NPRM. \n\nRequest To Revise Resistance Values \n\n\n\tBoeing requested that Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003, which incorrectly specified a maximum electrical bonding resistance of 5 milliohms for aluminum and 10 milliohms for stainless steel, be replaced with Boeing Alert Service Bulletin 737-24A1148, Revision 2, dated September 14, 2020, which updates the maximum allowable resistance values to 3 milliohms for both aluminum and stainless steel, per AWL No. 28-AWL-04, as identified in Subsection G of Boeing Temporary Revision (TR) 09-020, dated March 2008, to the Boeing 737-600/700/800/900 Maintenance Planning Document (MPD), D626A001-Certification Maintenance Requirements (CMR), Revision March 2008. Boeing also advised that operators who have incorporated Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003, should restore the fuel quantity indicating system (FQIS) aluminum and stainless steel connectors to a maximum resistance of 3 milliohms at the next inspection interval per Boeing Alert Service Bulletin 737-24A1148, Revision 2, dated September 14, 2020. Boeing observed that the maximum allowable resistance values of 5 milliohms (aluminum) and 10 milliohms (stainless steel) specified in Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003, are greater than the values specified in AWL No. 28-AWL-04, and that the proposed AD is in conflict with \n\n((Page 20446)) \n\nAD 2008-10-10 R1, Amendment 39-16164 (75 FR 1529, January 12, 2010) (AD 2008-10-10 R1), and AD 2018-20-24, Amendment 39-19458 (83 FR 51815, October 15, 2018) (AD 2018-20-24), both of which reference AWL No. 28- AWL-04. ADs 2008-10-10 and 2018-20-24 currently require revising the Airworthiness Limitations (AWLs) section of the Instructions for ContinuedAirworthiness by incorporating new limitations for fuel tank systems to satisfy Special Federal Aviation Regulation No. 88 (in 14 CFR part 21) for FQIS connectors in unpressurized area. The critical design configuration control limitations (CDCCLs) require that if the FQIS connector D4850 is disturbed or repaired, the electrical bonding resistance value from the backshell to the structure must be 3 milliohms or less. \n\tThe FAA agrees with the request to require the revised service information, which updates the maximum allowable resistance values to 3 milliohms. These values are consistent with AD 2008-10-10 R1 and AD 2018-20-24 and SFAR No. 88 requirements. The FAA has revised paragraph (g)(2) of this AD to specify Boeing Alert Service Bulletin 737-24A1148, Revision 2, dated September 14, 2020, and has also updated the ''Related Service Information under CFR part 51'' paragraph in this final rule. \n\nRequest To Specify Initial Compliance Time \n\n\n\tDelta Air Lines (DAL) suggested that paragraph (g) specify the compliance time as within 24 months of issuance of the original certificate of airworthiness or 12 months after the effective date of this AD, whichever comes later. DAL noted that newer airplanes having accumulated less than 24 months since the issuance of the original certificate of airworthiness should not require inspection prior to the accumulation of 24 months since the issuance of the original certificate of airworthiness, because these aircraft were in a known corrosion-free condition upon completion of production. This condition, DAL asserted, is equivalent to an aircraft that has just been inspected as required by this AD. DAL maintained that because the repeat interval after initial inspection given in the proposed rule is 24 months, an equivalent level of safety would be provided if the initial inspection were accomplished within 24 months of delivery or 12 months after the effective date of this AD, whichever occurs later. \n\tThe FAA agrees with the request. An affected airplane that has just received an original airworthiness certificate or original export certificate of airworthiness is in a condition equivalent to that of completing the repetitive inspection; therefore, conducting an initial inspection within 24 months is appropriate to address the concern of the AD for that airplane. The initial compliance time specified in paragraph (g) of this AD has been changed to include this information. \n\nRequest To Modify Compliance Times \n\n\n\tDAL suggested that paragraph (g)(2) of the proposed AD be changed to add a new paragraph (g)(2)(iv) specifying ''If connectors or contacts show signs of corrosion or connector resistance measurements are greater than the specified milliohm limit given in Boeing Alert Service Bulletin 737-24A1148, Revision 1, accomplish appropriate corrective action(s) prior to further flight in accordance with Boeing Alert Service Bulletin 737-24A1148, Revision 1.'' DAL observed that the applicable times specified in paragraphs (g)(2)(i) through (iii) of the proposed AD are specific to corrosion of the backshells, and that the proposed AD does not clearly define corrective actions for the connectors and contacts, which would also be inspected in paragraph (g)(2) of the proposed AD. DAL contended that the intended action of the proposed AD is for discrepant connectors and contacts to have corrective action accomplished prior to further flight. \n\tThe FAA disagrees with the request to modify paragraph (g)(2) of this AD. The service information adequately specifies when to perform corrective actions for connectors and contacts. The FAA has not changed this AD with regard to this request. \n\nRequest To Address Discrepancies in NPRM \n\n\n\tDAL noted the following discrepancies with Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003: The Work Instructions, paragraph 3.B of the Accomplishment Instructions, indicate that the accomplishment instructions are divided into two parts; however, they are actually divided into three parts. Also, part 3 of the Work Instructions does not give the instruction to remove electrical power from the airplane, yet Figure 5, Step 10, of the ASB instructs to restore electrical power. DAL noted that if electrical power was not removed, it cannot be restored. The FAA infers a request to address the discrepancies in the proposed AD. \n\tThe FAA agrees with addressing the stated discrepancies in this AD. The 57 discrepancies have been rectified in Boeing Alert Service Bulletin 737-24A1148, Revision 2, dated September 14, 2020, and this AD has been changed to reference that revision. \n\nRequest To Include AMOC Notices \n\n\n\tDAL requested that the proposed AD be revised to include references to two FAA-approved AMOCs to the mandated service information: AMOC Notice 737-24A1148-AMOC-01, dated May 06, 2013, and FAA AMOC Letter 130S- 09-9, dated April 28, 2009. DAL observed that the AMOCs are applicable service documentation that should be specifically included in the final rule to facilitate operators in correctly incorporating the requirements of the final rule. \n\tThe FAA partially agrees with the request. To facilitate operators in correctly incorporating the requirements of the final rule, paragraph (i)(4) of this AD has been changed to include reference to FAA AMOC Letter 130S-09-9, dated April 28, 2009. However, Boeing AMOC Notice 737-24A1148-AMOC-01, dated May 6, 2013, has been incorporated into the revised service bulletin that is mandated by this AD and therefore is not included in paragraph (i)(4) of this AD. \n\nRequest To Give Credit for Previous Actions \n\n\n\tUnited Airlines (UAL) generally supported the NPRM and requested that credit be given for actions previously accomplished in their maintenance inspection program per their publication EA 2400-01516, addressing Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003. UAL stated it has incorporated inspections of its737- 900ER fleet starting May 2010, using their publication EA 2400-01516. \n\tThe FAA agrees that operators should get credit for performing actions of Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003, referenced in the NPRM. As stated previously, this final rule has been revised to refer to the latest service bulletin, Boeing Alert Service Bulletin 737-24A1148, Revision 2, dated September 14, 2020, which contains updated service information to address the unsafe condition. Operators who previously performed the work before the effective date of this AD using Boeing Alert Service Bulletin 737- 24A1148, dated December 6, 2001, or Revision 1, dated July 10, 2003, may receive credit for accomplishment of the initial detailed inspection specified in paragraph (g)(2) of this AD; however, the updated FQIS connector D4850 resistance values must be used at the \n\n((Page 20447)) \n\nnext repetitive inspection interval, in accordance with Boeing Alert Service Bulletin737-24A1148, Revision 2, dated September 14, 2020. The FAA has added paragraph (h) of this AD to provide credit for Boeing Alert Service Bulletin 737-24A1148, dated December 6, 2001, and Boeing Alert Service Bulletin 737-24A1148, Revision 1, dated July 10, 2003. \n\nEffect of Winglets on Accomplishment of the Proposed Actions \n\n\n\tAviation Partners Boeing stated that the installation of winglets per Supplemental Type Certificate STC ST00830SE does not affect the accomplishment of the manufacturer's service instructions. Blended winglets are part of the production type certificate for the 737-900ER and are not an STC installation. \n\tThe FAA agrees with the commenter that STC ST00830SE does not affect the accomplishment of the manufacturer's service instructions. Therefore, the installation of STC ST00830SE does not affect the ability to accomplish the actions required by this AD. The FAA has not changed this AD as a result of this comment. \n\nConclusion \n\n\n\tThe FAA reviewed therelevant 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 has 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 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 737-24A1148, Revision 2, dated September 14, 2020. This service information describes procedures for repetitive records checks to determine exposure to certain deicing fluids or repetitive inspections for corrosion of the electrical connectors, and corrective actions if necessary. 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 346 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 ---------------------------------------------------------------------------------------------------------------- Repetitive records check....... 1 work-hour x $85 per $0 $85 per inspection Up to $29,410 per \n\thour = $85 per cycle. inspection cycle. \n\tinspection cycle. Repetitive detailed inspection. 3 work-hours x $85 per $0 $255 perUp to $88,230 per \n\thour = $255 per inspection cycle. inspection cycle. \n\tinspection cycle. ---------------------------------------------------------------------------------------------------------------- \n\n\n\tThe FAA estimates the following costs to do any necessary repairs or replacements that would be required based on the results of the inspection. The FAA has no way of determining the number of aircraft that might need these repairs or replacements: \n\n\n\tOn-Condition Costs ---------------------------------------------------------------------------------------------------------------- \n\tAction Labor cost Parts cost Cost per product ---------------------------------------------------------------------------------------------------------------- Cleaning or replacement.......... Up to 5 work-hours x $85 Up to $831.............. Up to $1,256. \n\tper hour = Up to $425. ---------------------------------------------------------------------------------------------------------------- \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\nRegulatory Findings \n\n\n\tThis AD willnot 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.