Examining the Docket \n\n\tYou may examine the airworthiness directive (AD) docket on the Internet at http://dms.dot.gov or in person at the Docket Management Facility office between 9 a.m. and 5 p.m., Monday through Friday, except Federal holidays. The Docket Management Facility office (telephone (800) 647-5227) is located on the plaza level of the Nassif Building at the street address stated in the ADDRESSES section. \n\nDiscussion \n\n\tThe FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to include an AD that would apply to certain Boeing Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. That NPRM was published in the Federal Register on April 18, 2006 (71 FR 19835). That NPRM proposed to require repetitive inspections to detect cracks in the vertical beam webs of the body station (BS) 178 bulkhead, and corrective actions if necessary. That NPRM also proposed to require a terminating modification for the repetitive inspections. \n\nComments \n\n\tWe provided the public the opportunity to participate in the development of this AD. We have considered the comments received. \n\nRequest To Extend Compliance Time Threshold \n\n\tContinental Airlines (Continental) requests that the threshold for the compliance times specified in Table 1 of the NPRM be aligned with the compliance times specified in ADs 2000-05-29, amendment 39-11639 (65 FR 14834, March 20, 2000), and 2001-02-01, amendment 39-12085 (66 FR 7576, January 24, 2001). Continental states that this will reduce the economic impact on operators from doing early inspections and will encourage operators to terminate those ADs at 20,000 total flight cycles as opposed to doing repetitive inspections. \n\n\tWe do not agree. Continental provided no technical justification for revising the inspection threshold. In developing an appropriate compliance time for this action, we considered the safety implications and normal maintenance schedules for the timely accomplishment of the inspections. In consideration of these items, as well as the reports of numerous cracks in the vertical beam webs in service, we have determined that the compliance times specified in Table 1 of this AD will ensure an acceptable level of safety and allow the inspections to be done during scheduled maintenance intervals for most affected operators. However, according to the provisions of paragraph (m) of the AD, we may approve requests to adjust the compliance time if the request includes data that substantiate that the new compliance time would provide an acceptable level of safety. \n\nRequest To Include an Additional Grace Period \n\n\tThe Air Transport Association (ATA), on behalf of one of its members, United Airlines (United), requests that the compliance time specified in paragraph (f)(2) of the NPRM be revised to reflect the intention of Boeing Service Bulletin 737-53A1225, Revision 1, dated April 14, 2005 (referred to in the NPRM as the appropriate source of service information for accomplishing the repetitive inspections and terminating preventative modification). United proposes that all airplanes should have a minimum of 4,500 flight cycles after the effective date of the AD to do the initial inspection required by paragraph (f) of the NPRM. United also states that Boeing Service Bulletin 737-53A1225, dated October 19, 2000, specifies an interval of 12,000 flight cycles for the repetitive high frequency eddy current (HFEC) inspections. Without a grace period, United points out that operators doing those inspections would be grounded as of the effective date of the AD. \n\n\tWe agree and have revised paragraph (f)(2) of this AD to provide a grace period of 4,500 flight cycles after the effective date of this AD. \n\nRequest To Include Certain Airplanes in Compliance Time Table \n\n\tBoeing requests that we revise Table 1, "Compliance Times,'' of the NPRM to address airplanes inspected in accordance with Boeing Service Bulletin 737-53A1225, Revision 1. \n\n\tWe do not agree. Operators are given credit for actions previously done by means of the phrase in paragraph (e) of this AD that states, "unless the actions have already been done.'' Therefore, in the case of this AD, if the required inspection specified in Boeing Service Bulletin 737-53A1225, Revision 1, has been done before the effective date of this AD, this AD does not require that it be repeated. In addition, if the required inspection specified in Boeing Service Bulletin 737-53A1225, Revision 1, has not been done before the effective date of this AD, this AD requires that inspection to be done at the applicable time specified in Table 1. We have made no change to the final rule in this regard. \n\nRequests To Allow the Use of Boeing BOECOM M-7200-01-00546 \n\n\tKLM Engineering & Maintenance (KLM), Southwest Airlines (Southwest), and United request that the procedures specified in Boeing BOECOM M-7200-01-00546, dated March 1, 2001 (referred to in paragraph (j) of the NPRM) be allowed to be used after the effective date of the AD as an acceptable method of compliance with the preventative modification specified in paragraph (i) of the NPRM. Southwest states that BOECOM M-7200-01-00546 describes procedures for fabricating replacement parts, which would result in a significant cost savings to operators. United states that it has modified the majority of its fleet using instructions equivalent to those contained BOECOM M-7200-01- 00546. KLM states that it has modified a majority of its fleet using Boeing Service Bulletin 737-53A1173, Revision 4, dated September 19, 2002 (Revision 3 of Boeing Service Bulletin 737-53A1173 is referred to in paragraph (k) of the NPRM as the appropriate source of service information for accomplishing the preventative modification), together with the instructions specified in BOECOM M-7200-01-00546. United and KLM would like to continue modifying their fleets using the same instructions. In addition, Boeing requests that the description of acceptable actions in paragraph (j) of the NPRM be revised to include procedures done in accordance with Boeing BOECOM M-7200-01-00546 and approved by Boeing and the FAA after March 1, 2001.\n \n\tWe partially agree. We agree that doing the replacement or modification specified in Boeing BOECOM M-7200-01-00546, dated March 1, 2001, may be an acceptable means of compliance with the requirements of paragraph (j) of this AD. However, it is not likely that replacement or modification in accordance with BOECOM M-7200-01-00546 can be done without deviations that require further FAA approval. It has been our experience that work done in accordance with BOECOM M-7200-01-00546 has nearly always required deviations. As noted in BOECOM M-7200-01-00546, to obtain approval for using the BOECOM, the operator must provide an Authorized Representative (AR) for the Boeing Commercial Airplanes Delegation Option Authorization Organization with the airplane identification, the details of the proposed replacement, and any deviations. Therefore, we have determined that operators who use the BOECOM procedures after the effective date of this AD must get them approved as an alternative method of compliance (AMOC) in accordance with the procedures specified in paragraph (m) of this AD. We have made no change to the final rule in this regard. \n\nRequest To Remove Option To Repair \n\n\tBoeing requests that the word "repair'' in paragraph (i) of the NPRM and in the "Relevant Service Information'' section of the NPRM be deleted. Boeing did not provide a justification. \n\n\tWe agree. We have re-reviewed Boeing Service Bulletin 737-53A1225, Revision 1. In several places in Parts II through IV of the Accomplishment Instructions, the service bulletin states, "Repair or change the vertical beam * * * Refer to Figure 25 * * *.'' Figure 25 refers to "replacement'' procedures; however, it does not refer to a repair procedure. Therefore, we have deleted "repair or'' in paragraph (i) of this AD. We have made no change to the AD in regard to the "Relevant Service Information'' section, because that section of the NPRM does not reappear in the final rule. \n\nRequest To Allow Repair Plans Approved Previously \n\n\tSouthwest requests that paragraph (j) of the NPRM be revised to allow certain repair plans approved by an AR for the Boeing Commercial Airplanes Delegation Option Authorization Organization or a Boeing Designated Engineering Representative (DER) before the release of Boeing BOECOM M-7200-01-00546, dated March 1, 2001, as an acceptable method of compliance with the preventative modification specified in paragraph (i) of the NPRM. Southwest states that it has installed thicker vertical beam webs with such approval on some of its airplanes before the issuance of Boeing BOECOM M-7200-01-00546, dated March 1, 2001. \n\n\tWe do not agree with Southwest to revise paragraph (j) of this AD. Southwest did not provide sufficient data for us to determine if these earlier repairs are equivalent to those specified in Boeing BOECOM M- 7200-01-00546, dated March 1, 2001. It is possible that the review and approval of earlier repairs may not have taken into account the latest information that was used to develop the BOECOM. However, if a particular repair is shown to be equivalent to that specified in the BOECOM, paragraph (m) of the AD provides operators the opportunity to apply for an AMOC to address this type of repair. \n\nRequest for Clarification \n\n\tSouthwest requests that paragraph (j) of the NPRM be revised to clarify that it is not necessary to replace certain stiffeners per step 4 of Boeing BOECOM M-7200-01-00546, if the existing holes can be oversized and a new identical fastener can be installed with an acceptable edge distance. Step 4 indicates that certain stiffeners must be replaced because they are offset by the thickness of the new webs. Southwest believes that the intent of that step is to eliminate detrimental fastener over-sizing and short edge distances that can result from the offset. \n\n\tWe do not agree with Southwest to revise paragraph (j) of this AD. Southwest did not provide any specific limits nor define any acceptable combinations of maximum over-sizing of fasteners and/or minimum fastener edge distance. Therefore, we are unable to provide approval at this time. However, under the provisions of paragraph (m) of this AD, we may consider requests for approval of an AMOC if sufficient data are submitted to substantiate that such a design change would provide an acceptable level of safety. \n\nRequest To Delete Concurrent Requirements \n\n\tDelta Air Lines (Delta) requests that the concurrent requirements of paragraphs (k) and (l) of the NPRM be deleted, and to continue to allow the requirements specified in paragraph (c) of ADs 2000-05-29 and 2001-02-01 to be done separately. Delta notes that the "Effect of Accomplishing Concurrent Requirements'' section in the preamble of the NPRM states, "We realize that the concurrent requirements of this proposed AD will force some operators to do the preventative modifications required by AD 2001-02-01 early and to do the optional preventative modification specified in AD 2000-05-29. However, accomplishing the applicable preventative modifications together is necessary to avoid repeated disassembly and re-assembly of common parts, which increases the likelihood of additional assembly errors.'' Delta states that the timing of doing the preventative modification is an economic and operational decision, which is properly at the discretion of the operators, not a subject for an AD. \n\n\tWe partially agree. We do not agree with Delta that the concurrent requirements be deleted. We determined that mandating the previous optional preventative modification specified in AD 2000-05-29 in this AD will better ensure long-term continued operational safety of the affected airplanes by removing the source of the problem, rather than by repetitive inspections.Long-term inspections may not provide the degree of safety necessary for the affected airplanes. This, coupled with our understanding of the human factor errors associated with numerous repetitive inspections, has led us to consider placing less emphasis on special procedures and more emphasis on design improvements. The preventative modification required by paragraph (l) of this AD is consistent with these considerations. Additionally, accomplishing the modifications concurrently provides the most effective installation of these modifications and will avoid repeated disassembly and re-assembly of common parts of critical structure, which increases the likelihood of additional assembly errors. Boeing also has provided us with data supporting our determination. \n\n\tWe somewhat agree with Delta to allow the requirements specified in paragraph (c) of ADs 2000-05-29 and 2001-02-01 to be done separately. It is acceptable to do the preventative modifications required by AD 2001-02-01 beforethe requirements of paragraph (i) of this AD. However, paragraphs (k) and (l) of the NPRM state, "Concurrently with the requirements of paragraph (i) of this AD * * *.'' Therefore, we have revised those paragraphs to clarify that the concurrent requirements must be done "before or concurrently with the requirements of paragraph (i) of this AD.'' For clarification purposes, we also removed the phrase "unless already done before the effective date of this AD'' from paragraph (k) of this AD. \n\nRequest To Supersede AD 2000-05-29 \n\n\tThe ATA, on behalf of one of its members, Delta, requests that AD 2000-05-29 be superseded or revised to avoid conflicting requirements. Delta states that this should be done if its request in the "Request To Delete Concurrent Requirements'' section of this AD is not feasible. \n\n\tWe do not agree. Paragraph (k) of this AD mandates the previously optional preventative modification specified in paragraph (c) of AD 2000-05-29. A mandatory requirement takes precedence over an optional action. Therefore, we find that no conflict exists between the requirements of this AD and AD 2000-05-29. \n\n\tIn addition, we considered superseding ADs 2000-05-29 and AD 2001- 02-01 when developing the NPRM. We determined that doing so would have made this AD more complex and would have increased the consequent workload associated with revising maintenance record entries, because this AD does not affect all requirements of those ADs. This AD only affects paragraph (c) of those ADs. Therefore, we determined that a less burdensome approach for operators was not to supersede those existing ADs. \n\nRequest To Address Certain Airplanes \n\n\tIf the concurrent requirements of the NPRM are kept, Delta further requests that Boeing be tasked to address airplanes on which the replacement of the forward pressure bulkhead web has been done and on which the modification of the vertical beam has not been done. \n\n\tWe do not agree. We have determined that the procedures specified in the Accomplishment Instructions of Boeing Service Bulletin 737- 53A1225, Revision 1, dated April 14, 2005, adequately address all affected airplanes. Although the information mentioned by Delta may be helpful, the procedures specified in the service bulletin are adequate. Therefore, we find it inappropriate to task Boeing to revise the service bulletin and to delay the issuance of this AD. However, if additional data are presented that would justify additional actions, we may consider further rulemaking on this issue. \n\nRequests To Allow AMOCs Approved Previously \n\n\tSouthwest requests that paragraphs (k) and (l) of the NPRM be revised to allow AMOCs approved previously in accordance with ADs 2000- 05-29 and 2001-02-01, respectively. Southwest wants to avoid any issues as to whether or not those AMOCs must be resubmitted to us for approval. \n\n\tContinental requests that paragraph (k) of the NPRM be revised to refer to Boeing Service Bulletin 737-53A1173, Revision 4, dated September 19, 2002. Continental states that Revision 4 included several corrections and work flow improvements. \n\n\tWe partially agree with both Southwest and Continental. We agree that approved AMOCs to paragraph (c) of ADs 2000-05-29 and 2001-02-01 that are done before or concurrently with the requirements of paragraph (i) of this AD are acceptable as AMOCs for the corresponding provisions of paragraphs (k) and (l) of this AD, respectively. Boeing Service Bulletin 737-53A1173, Revision 4, is one of those AMOCs. We do not agree with the commenters that the paragraphs (k) and (l) should be revised in regard to AMOCs. The appropriate paragraph to revise is paragraph (m) of this AD, which is the AMOC paragraph. Therefore, we have revised paragraph (m) accordingly. \n\nRequest To Revise AMOC Paragraph \n\n\tBoeing requests that paragraph (m)(3) of the NPRM be changed to allow AR approval of modifications as well as repairs. \n\n\tWe agree and have revised paragraph (m)(3) of this AD accordingly. \n\nRequests To Revise Costs of Compliance \n\n\tThe ATA, on behalf of two of its members, U.S. Airways and United, requests that the Costs of Compliance section in the preamble of the NPRM account for the work required to gain access, reassemble, complete post-modification checkouts, close access, etc. associated with the proposed inspection and preventative modification. U.S. Airways states that these actions represent an increase of almost 40 percent above and beyond the 240 work hours specified in the NPRM. United states that the proposed inspection and preventative modification are not normally accessed at any routine maintenance visit. \n\n\tWe do not agree. The Costs of Compliance section describes only the direct costs of the specific actions required by this AD. Based on the best data available, the manufacturer provided the number of work hours (240 for preventative modification; 4 for each inspection) necessary to do the required actions. This number represents the time necessary to perform only the actions actually required by this AD. We recognize that, in doing the actions required by an AD, operators may incur incidental costs in addition to the direct costs. The cost analysis in AD rulemaking actions, however, typically does not include incidental costs such as the time required to gain access and close up, time necessary for planning, or time necessitated by other administrative actions. Those incidental costs, which may vary significantly among operators, are almost impossible to calculate. Therefore, we have made no change to this AD in this regard. \n\nRequest To Correct Typographical Error \n\n\tBoeing requests that a typographical error be fixed in paragraph (h) of the NPRM. The reference to "paragraph (1) of this AD'' should be changed to "paragraph (m) of this AD.'' \n\n\tWe agree and have changed paragraph (h) of this AD accordingly. \n\nConclusion \n\n\tWe have carefully reviewed the available data, including the comments received, anddetermined that air safety and the public interest require adopting the AD with the changes described previously. We have determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD. \n\n\tCosts of Compliance \n\n\tThere are about 3,132 airplanes of the affected design in the worldwide fleet. The following table provides the estimated costs for U.S. operators to comply with this AD. \n\n\tEstimated Costs\n\n\nAction\nWork hours\nAverage labor rate per hour\nParts\nCost per airplane\nNumber of U.S.-registered airplanes\nFleet cost\nInspection, per inspection cycle\n4\n$80\nNone\n$320, per inspection cycle\n1,172\n$375,040, per inspection cycle\nPreventative modification\n240\n$80\nBetween $960 and $13,620, depending on kit purchased\nBetween $20,160 and $32,820, depending on configuration\n1,172 (720 airplanes have had the preventative modification incorporated)\nBetween $9,112,320 and $14,834,640\n\nAuthority for This Rulemaking \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\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\tWe have determined that this 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\n\tFor the reasons discussed above, I certify that this AD: \n\n\t(1) Is not a "significant regulatory action'' under Executive Order 12866; \n\n\t(2) Is not a "significant rule'' under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and \n\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\n\tWe prepared a regulatory evaluation of the estimated costs to comply with this AD and placed it in the AD docket. See the ADDRESSES section for a location to examine the regulatory evaluation. \n\nList of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety. \n\nAdoption of the Amendment \n\nAccordingly, under the authority delegated to me by the Administrator, the FAA amends 14 CFR part 39 as follows: \n\nPART 39--AIRWORTHINESS DIRECTIVES \n\n1. The authority citation for part 39 continues to read as follows: \n\n\tAuthority: 49 U.S.C. 106(g), 40113, 44701. \n\nSec. 39.13 (Amended) \n\n2. The Federal Aviation Administration (FAA) amends Sec. 39.13 by adding the following new airworthiness directive (AD):