Discussion \n\n\n\tWe issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to include an airworthiness directive (AD) that would apply to The Boeing Company Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes. That NPRM was published in the Federal Register on September 27, 2010 (75 FR 59167). That NPRM proposed to require modifying the inner walls of the thrust reverser (TR), inspecting for damage of the upper and lower inner wall insulation blankets, measuring the electrical conductivity on the aluminum upper compression pads 2 and 3 as applicable, inspecting for discrepancies of the TR inner wall, and corrective actions if necessary. That NPRM also proposed to require, for certain airplanes, doing various concurrent actions (including replacing the inner wall blanket insulation, installing updated full-authority digital electronic control software, and modifying the TR inner wall and insulation blankets). \n\nCommentsWe gave the public the opportunity to participate in developing this AD. The following presents the comments received on the proposal (75 FR 59167, September 27, 2010) and the FAA's response to each comment. \n\nRequest To Withdraw NPRM (75 FR 59167, September 27, 2010) \n\n\n\tDespite fully supporting the implementation of the actions of the NPRM (75 FR 59167, September 27, 2010), Boeing stated that it does not consider thermal overheat on the TR inner walls on the affected airplanes to be a safety issue. The structural integrity of the inner wall may deteriorate due to pre-cooler air ingress behind the blankets, but the Boeing Safety Review Board determined that this does not constitute a safety hazard to the airplane or to persons on the ground. Boeing identified support data for this determination, which included a safety assessment, full-scale test demonstration, and structural analysis. \n\tWe infer that Boeing wants us to withdraw the NPRM (75 FR 59167, September 27, 2010), because there is no unsafe condition. We disagree. The thermal overheat could affect the structural capability of the inner wall of the thrust reverser such that, if a pneumatic duct bursts, the inner wall could fail, causing uncontrollable asymmetric thrust during a rejected takeoff, or causing large parts to hit the fuselage or empennage in flight. \n\n((Page 20512)) \n\nRequest To Remove Model 737-900ER \n\n\n\tBoeing requested that we remove Model 737-900ER series airplanes from the applicability of the NPRM (75 FR 59167, September 27, 2010), since configuration control prevents the intermix of the affected TRs on these airplanes. \n\tWe agree. The TR inner walls on Model 737-900ER series airplanes have not been identified as having a thermal overheat issue. We have therefore removed these airplanes from the applicability of this AD. \n\nComments on EASA Proposed AD (75 FR 59167, September 27, 2010) \n\n\n\tThe European Aviation Safety Agency (EASA), which is the Technical Agent for the Member States of the European Community, issued Proposed AD 10-087, dated September 30, 2010 (which has since been issued as EASA AD 2010-0244R1, dated May 17, 2011, and corresponds to FAA NPRM (75 FR 59167, September 27, 2010)). Boeing reported that it had requested certain changes (to the compliance time and applicability) to the EASA Proposed AD, and provided a list of the specific requests including changing the compliance time and eliminating language regarding certain ''specific airplane(s).'' \n\tWe find these comments to be addressed to EASA Proposed AD 10-087 and do not apply to the FAA proposed AD (75 FR 59167, September 27, 2010). The applicability is the same in the EASA and FAA ADs, and accounts for Boeing's comment concerning Model 737-900ER series airplanes. The compliance time of the EASA AD is different from that of the FAA AD, based on differing AD processes and publication schedules. We have not changed the final rule regarding this issue. \n\nRequest To Revise CostEstimate \n\n\n\tContinental Airlines (CAL) disagreed with the fleet cost estimates for the actions specified in Boeing Service Bulletin 737-78-1088, dated May 12, 2010, as proposed in paragraph (i) of the NPRM (75 FR 59167, September 27, 2010). CAL explained that inner wall delamination requires the repair to be performed in an autoclave, which requires disassembly of the TR. (There are two half TR sections per engine.) CAL stated that returning each TR half to service after disassembly and inspection of components could cost from $16,000 to $56,000, depending on the hours and cycles on the TR. \n\tWe disagree with the request to revise the cost estimate. The economic analysis of the NPRM (75 FR 59167, September 27, 2010) did not consider the cost of conditional actions, such as repairing damage detected during a required inspection. The economic analysis of this AD is limited to the cost of actions that are required of every operator. Such conditional repairs would be required--regardless of AD direction--to correct an unsafe condition identified in an airplane and to ensure that the airplane is operated in an airworthy condition, as required by the Federal Aviation Regulations. We would have no way of determining these on-condition costs, which would depend on the TR condition and vary from operator to operator. We have not changed the final rule regarding this issue. \n\nRequest To Extend Compliance Time for Modification \n\n\n\tThree commenters requested that we revise the 24-month compliance time for the modification specified in paragraph (g) of the NPRM (75 FR 59167, September 27, 2010). \n\tCAL requested that we extend the compliance time to 30 months, when 116 of its airplanes will also require the inspection specified in paragraph (i) of the proposed AD (75 FR 59167, September 27, 2010). This compliance time extension would reduce CAL's modifications on its fleet from 5 airplanes to 4 airplanes per month. CAL added that, even with this extended compliance time, it would be difficult to modify 4 (16 TR halves) per month because of the limited number of spare TR halves available. \n\tSouthwest Airlines (SWA) reported that it would need to modify 39 of its 946 TRs each month to meet a 24-month compliance time, and therefore suggested a stepped compliance time schedule, ranging from 12 months to 48 months, based on the service life of the TR. \n\tAmerican Airlines (AAL) stated that the 24-month compliance time will have a significant impact on its ''light'' C check. \n\tWe disagree to extend the compliance time for paragraph (g) of this AD. In developing an appropriate compliance time for these actions, we considered the urgency associated with the subject unsafe condition, the practical aspect of accomplishing the required modification and the normal scheduled maintenance times for most affected operators. In consideration of these items and of parts availability, we have determined that the proposed 24-month compliance time for the modification will ensure an acceptable level of safety. According to the provisions of paragraph (o) of this AD, however, we may approve requests to adjust the compliance time if the request includes data substantiating that the new compliance time would provide an acceptable level of safety. We have not changed the final rule regarding this issue. \n\nRequest To Clarify Service Information \n\n\n\tAAL requested that the service instructions for Boeing Service Bulletin 737-78-1082, dated March 25, 2010; and Boeing Service Bulletin 737-78-1088, dated May 12, 2010; be revised to incorporate general findings and clarifications. AAL asserted that not addressing these issues could adversely affect accomplishment of these service bulletins. \n\tWe agree that additional clarification would be beneficial in the identified areas of Boeing Service Bulletin 737-78-1082, dated March 25, 2010; and Boeing Service Bulletin 737-78-1088, dated May 12, 2010. Such minor clarifications, however, are not necessary forcompliance with this AD. We have provided AAL's comments to Boeing for review and incorporation, as necessary, into future revisions of those service bulletins, which might be approved as a global alternative method of compliance with this AD if we can substantiate that the revision provides an acceptable level of safety. We have not changed the final rule regarding this issue. \n\nRequest for Optional Repair \n\n\n\tSWA requested that we revise paragraph (i) of the NPRM (75 FR 59167, September 27, 2010) to allow cold-bonding methods for repairing damaged areas, in addition to the autoclave procedures specified in Boeing Service Bulletin 737-78-1088, dated May 12, 2010. That service bulletin permits curing repaired areas only as specified in the Boeing 737-700 Structural Repair Manual (SRM), which specifies the autoclave procedures. According to SWA, this would require operators to pull TRs for repair at an approved overhaul facility, thereby increasing the turn time for repairs since only five Boeing-approved overhaul facilities have autoclave capabilities. \n\tWe disagree with the request to allow the cold-bonding procedure. Boeing and the FAA are unaware of any cold-bonding methods that would be applicable to the composite TR inner wall on the affected airplanes. Current SRM repair methods for composite structure involve either autoclave or vacuum bag/heat blanket cure methods. But Boeing Service Bulletin 737-78-1088, dated May 12, 2010, and the \n\n((Page 20513)) \n\nalternative Boeing Service Bulletin 737-78-1079, Revision 2, dated June 7, 2010, limit the SRM repairs for the TR inner wall to autoclave cures. For the areas being repaired on the inner wall, the additional plies required to make a structurally adequate vacuum bag-cured repair are excessive and would make the inner wall unusable. We therefore find it appropriate for those service bulletins to specify autoclave curing only. In addition, Boeing has evaluated the potential number of repairs that would be done at overhaul facilities with autoclave capabilities, and does not foresee a problem addressing the corrective actions on the affected airplanes within the compliance times. We have not changed the final rule regarding this issue. \n\nRequest To Revise Compliance Time for Certain Inspections \n\n\n\tSWA requested that we revise paragraph (i) of the proposed AD (75 FR 59167, September 27, 2010), which proposed certain inspections in accordance with Boeing Service Bulletin 737-78-1088, dated May 12, 2010. SWA recommended a minimum of 48 months, ''per (this service bulletin),'' for these actions. (The compliance time in the proposed AD ranged from 30 to 96 months.) \n\tWe disagree with the request. As stated previously, when we developed the compliance time for this AD action we considered the safety implications of the identified unsafe condition, the average utilization rate of the affected fleet, the practical aspects of performing the inspections on the fleet during regular maintenance periods, and the availability of replacement parts. We have determined that the proposed compliance times are appropriate. We have not changed the final rule regarding this issue. \n\nRequest for Clarification of Certain Procedures \n\n\n\tAAL described difficulty in accomplishing the actions specified in Boeing Service Bulletin 737-78-1079, Revision 2, dated June 7, 2010 (paragraph (n) of the NPRM (75 FR 59167, September 27, 2010)): Seals must be installed individually, the fire seal can tear and need replacement, and the roller edge of the insulation blanket interferes with the upper fire seal support flange insulations. AAL received some additional installation instructions from Boeing and recommended that they be included in this service bulletin. \n\tWe agree that additional clarification may be beneficial, but we find that accomplishing the actions specified in the Accomplishment Instructions of Boeing Service Bulletin 737-78-1079, Revision 2, dated June 7, 2010, will addressthe identified unsafe condition. We have provided AAL's comments to Boeing for review and incorporation, as necessary, into a future service bulletin revision, which might be approved as an alternative method of compliance with this AD if we can substantiate that the revision provides an acceptable level of safety. We have not changed the final rule regarding this issue. \n\nAdditional Changes to NPRM (75 FR 59167, September 27, 2010) \n\n\n\tParagraph (n) of this AD specifies the optional accomplishment of certain actions in accordance with Boeing Service Bulletin 737-78-1079, Revision 2, dated June 7, 2010. That service bulletin incorrectly notes that removal of a compression pad assembly is not necessary if the adjacent inner wall area does not show signs of heat damage, because the compression pad assembly is made from titanium. This AD requires removing the affected compression pads and inspecting the underlying structures as part of this optional action, regardless whether a pad assembly is made of titanium or aluminum alloy. Boeing has indicated that the incorrect notes may be removed in a future revision of that service bulletin; if so, we may approve the revised service bulletin as a global AMOC with the requirements of this AD. \n\tWe have revised or added certain headers in this AD. We have also revised the wording in paragraphs (l) and (n) of this AD; this change has not changed the intent of those paragraphs. \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 the AD with the change described previously. We also determined that this change will not increase the economic burden on any operator or increase the scope of the AD. \n\nCosts of Compliance \n\n\n\tWe estimate that this AD affects 710 airplanes of U.S. registry. The following table provides the estimated costs for U.S. operators to comply with this AD, at an average labor rate of $85 per hour.Estimated Costs -------------------------------------------------------------------------------------------------------------------------------------------------------- \n\tNumber of U.S.- \n\tActions (service bulletin) Work hours Parts Cost per product registered airplanes Fleet cost -------------------------------------------------------------------------------------------------------------------------------------------------------- Modification (Boeing Service 14 per engine......... $2,065 or $3,702...... $4,445 or $6,082..... 710.................. Up to $4,318,220. \n\tBulletin 737-78-1082, dated March \n\t25, 2010). Insulation replacement (Boeing 18 per engine......... $0.................... $3,060............... 15................... $45,900. \n\tService Bulletin 737-78-1063, \n\tRevision 2, dated October 7, 1994). Software update (CFM CFM56-7B 1..................... $0.................... $85.................. Up to 710............ Up to $60,350. \n\tService Bulletin 73-0135, dated \n\tMarch 30, 2007). Inspections (Boeing Service 35.................... $0.................... $2,975............... 710.................. $2,112,250. \n\tBulletin 737-78-1088, dated May \n\t12, 2010). Modifications (Boeing Service 110................... $0.................... $9,350............... 306.................. $2,861,100. \n\tBulletin 737-78-1069, Revision 4, \n\tdated June 16, 2005). Inspections and modification 37 per engine......... $2,070 or $3,391...... $8,360 or $9,681..... Optional action...... Optional action. \n\t(Boeing Service Bulletin 737-78- \n\t1079, Revision 2, dated June 7, \n\t2010) (if done as an option to \n\tBoeing Service Bulletin 737-78- \n\t1088 and Boeing Service Bulletin \n\t737-78-1082). --------------------------------------------------------------------------------------------------------------------------------------------------------((Page 20514)) \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.