The FAA proposed to amend 14 CFR part 39 with an AD for certain Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. That action, published in the Federal Register on August 6, 2004 (69 FR 47808), proposed to require repetitive inspections of the intercostal webs, attachment clips, and stringer splice channels for cracks; and corrective action if necessary. \n\nComments \n\n\tWe provided the public the opportunity to participate in the development of this AD. We have considered the comments that have been submitted on the proposed AD. \n\nQualified Support for the Proposed AD \n\n\tOne commenter, an operator, stated that the proposed AD is acceptable provided that the service bulletin referenced in the proposed AD is corrected to reflect the proper work instructions and to reference accurate figures for accomplishment. \n\n\tThe FAA cannot respond to the generality of the commenter's statement. However, other commenters have requested clarificationon certain aspects of the work instructions and requested certain revision of the "Costs of Compliance" section of this AD. Those comments are specified and responded to in the appropriate paragraphs below. \n\nRequest for Clarification in Paragraph (k) of the Proposed AD \n\n\tTwo commenters request that paragraph (k) be revised to clarify that the reference to using Figure 201 instead of Figure 202 of the service bulletin only applies to Model 737-400 series airplanes. \n\n\tWe agree that paragraph (k) of the AD should be clarified and have revised the AD accordingly. \n\nRequest To Withdraw the Proposed AD \n\n\tOne commenter, an operator, states that the Maintenance Planning Document (MPD) is the logical document to accomplish the main objectives of the inspections specified in the proposed AD. The commenter suggests that it makes more sense to revise MPD Task S53-22- A-2, rather than to issue an AD. We infer that the commenter is requesting that the proposed AD be withdrawn. \n\n\tWedo not agree. We are obligated by part 39 of the Federal Aviation Regulations (FAR) to appropriately address any identified unsafe condition that is likely to exist on other airplanes. The MPD is appropriate for addressing routine maintenance of critical structural components. However, operators may submit their specific and particular MPD task cards for consideration as an alternative method of compliance (AMOC) if they wish, in accordance with paragraph (n) of the AD. No change is necessary to the AD in this regard. \n\nRequest for More Information Regarding Paragraph (k) of the Proposed AD \n\n\tOne commenter, an operator, requests that inspection specifics be added to paragraph (k) of the proposed AD for the stringer-16L (S-16L) area in the post-repair configuration. The commenter does not believe that Boeing Special Attention Service Bulletin 737-53-1204, dated June 19, 2003, referenced in the proposed AD as the appropriate source of service information, provides sufficient inspection specifics in Figure 1. \n\n\tThe FAA does not agree that further inspection specifics are necessary to clarify paragraph (k) of the AD. Figure \n1 does not specifically show the repair/modification configurations at S-16L, and is simply intended to show typical crack locations and to identify the structural components that require inspection. Since the general inspection details provided in Figure 1 are applicable to both pre- and post-repair/modification configurations, no change to the AD is necessary in this regard. \n\nRequest To Allow "Credit" for Certain Repairs \n\n\tOne commenter, an operator, requests that repairs on the affected intercostals that are installed prior to the effective date of the AD be addressed. The operator states that the proposed AD specifies that certain repairs must be approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA; or per data meeting the type certification basis of the airplane approved by a Boeing Designated Engineering Representative (DER) who has been authorized by the Manager, Seattle ACO, to make such findings. The operator requests that the final rule allow credit for any repairs previously approved by the Seattle ACO. \n\n\tWe do not agree that previous repairs need to be addressed other than through the normal process as stated above by the commenter. Other than the repairs already identified at S-16L, we are not aware of any specific pre-existing repair configurations that should be addressed in the final rule. The Manager, Seattle ACO, can approve design data for previously installed or newly installed repair configurations prior to the issuance of this AD. However, approval as an AMOC with the AD cannot be given until the date the final rule is effective. No change is necessary to the AD in this regard. \n\nRequests To Address Previously Accomplished Modification/Repairs \n\n\tTwo commenters, both operators, request that provision for "previous or newly accomplished" installations of the repairbe added to paragraph (k) of the proposed AD. \n\n\tWe acknowledge the commenters' concern and partially agree. Paragraph (k) of the AD, as worded in the AD, simply permits deferring the repetitive inspections if the installation of the repair as a preventative modification or corrective action is accomplished in accordance with Part 1 of the Work Instructions of the service bulletin. We did not specify that installation of the repair must be performed either before or after the effective date of the AD, since, in this case, it does not matter when it is accomplished. No change is necessary to the AD in this regard. \n\nRequests To Extend Initial Compliance Time of Paragraph (g) of the Proposed AD \n\n\tSeveral commenters request that the grace period specified in paragraph (g), "4,500 flight cycles after the effective date of the AD," be extended to 7,500 or 8,000 flight cycles. One commenter states that the most critical area (STR 16L between Body Station (BS) 348.2 and BS 360) can only be inspected correctly by accessing additional areas, which may include removing lavatories or galleys. The commenters contend that extending the grace period of the initial compliance time would allow most operators to accomplish the requirements of paragraph (g) during normal scheduled maintenance. \n\n\tWe do not agree that the grace period should be extended beyond 4,500 flight cycles. We have determined that the grace period of 4,500 is appropriate and adequate to maintain an acceptable level of safety. The grace period represents more than two years of average operation, during which time most operators will have accomplished regularly schedule maintenance. The commenter has provided no technical data to show that extending the grace period compliance time to 7,500 or 8,000 flight cycles would continue to provide an acceptable level of safety. However, under the provisions of paragraph (n) of the AD, we may consider requests for adjustments to the grace period for the initial compliance time if sufficient data are submitted to substantiate that such an extension of the grace period would provide an acceptable level of safety. No change is necessary to the AD in this regard. \n\nRequest To Extend Initial Compliance Time of Paragraph (g) of the Proposed AD for Certain Areas \n\n\tOne commenter requests that, for areas that are non-critical, the compliance time be extended from 15,000 total flight cycles to 25,000 total flight cycles. The commenter notes that it is not clear why the non-critical areas have the same initial threshold as the critical area (S-16L). The commenter contends that the compliance time should be extended for those areas other than S-16L. \n\n\tWe do not agree. We have received recent service reports of cracked structure occurring at locations other than S-16L as early as 18,910 total flight cycles. No change is necessary to the AD in this regard. \n\nRequest for Credit for Similar Inspections \n\n\tOne commenter, an operator, notes that certain inspections similar to the inspections in the proposed AD are already required under the Corrosion Prevention and Control Program (CPCP). Although the commenter acknowledges that the intensity and type of inspection is not identical to the inspections specified in the proposed AD, the commenter requests that some relief of the compliance time should be considered if the CPCP inspections have been performed recently. \n\n\tWe do not agree to extend the compliance time. In developing this AD, we considered the inspections of the baseline CPCP program, but also noted that certain operators may be using different CPCP programs. However, under the provisions of paragraph (n) of the AD, we may approve requests for adjustments to the compliance time if data are submitted to substantiate that such an adjustment would provide an acceptable level of safety. No change is necessary to the AD in this regard. \n\nRequest To Clarify the Failure Mechanism \n\n\tOne commenter, an operator, requests that we explain why the FAA and the manufacturer disagree on the potential failure mechanism. The commenter points out that the manufacturer does not indicate that the fatigue cracking could result in loss of the forward entry door, only that incorporation of the service bulletin would prevent possible decompression and unscheduled down time. \n\n\tWe acknowledge that the manufacturer's service bulletin does not specifically advise that "loss of the forward entry door" could occur as a result of the identified unsafe condition. However, the manufacturer and the FAA agree that several potential failure scenarios, such as loss of the forward entry door, could occur. Both the manufacturer and the FAA agree that an unsafe condition has been identified and is likely to exist or to develop in other airplanes. Therefore, the actions specified in the AD are necessary to detect and correct fatigue cracking of the intercostals of the forward entry door. No change is necessary to the AD in this regard. \n\nRequest To Revise the Costs of Compliance \n\n\tSeveral commenters request that the "Costs of Compliance" section be revised to reflect the number of work hours required for access. \n\n\tWe do not agree that the cost estimate provided in the proposed AD should be revised. Based on the best data available, the manufacturer provided the number of work hours (two) necessary to do the required actions. \n\nThis 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. No change is necessary to the AD in this regard. \n\nRequest To Replace Parts Without FAA Approval \n\n\tOne commenter, an operator, asks that allowance be made in paragraph (m) for the replacement of parts without the need to contact the FAA for approval. \n\n\tWe do not agree with the commenter's request. Since the service bulletin referenced in this AD does not provide specific instructions for repair (replacing the parts), operators must perform the repair in accordance with a method approved as specified in paragraph (m) of this AD. \n\nExplanation of Change Made to This AD \n\n\tSince the issuance of the proposed AD, Boeing has received a Delegation Option Authorization (DOA). We have revised this AD to delegate the authority to approve an alternative method of compliance for any repair required by this AD to the Authorized Representative for the Boeing DOA Organization rather than the Designated Engineering Representative. \n\nConclusion \n\n\tWe have carefully reviewed the available data, including the comments that have been submitted, and determined 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\nCosts of Compliance \n\n\tThis AD will affect about 3,113 airplanes worldwide and 876 airplanes of U.S. registry. The required actions will take about 2 work hours per airplane, at an average labor rate of $65 per work hour. Based on these figures, the estimated cost of the AD for U.S. operators is $113,880, or $130 per airplane, per inspection cycle. \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 theauthority 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 Order12866; \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. 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 FAA amends Sec. 39.13 by adding the following new airworthinessdirective (AD):