Discussion \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 all Boeing Model 737-300, -400, and -500 series airplanes. That NPRM was published in the Federal Register on August 24, 2007 (72 FR 48597). That NPRM proposed to require revising the FAA-approved maintenance inspection program to include inspections that will give no less than the required damage tolerance rating for each structural significant item (SSI), doing repetitive inspections to detect cracks of all SSIs, and repairing cracked structure. \n\nComments \n\n\tWe gave the public the opportunity to participate in developing this AD. We considered the comments received from the four commenters. \n\nRequests To Allow Alternative Inspections for Previously Repaired/ Altered Structure \n\n\tBoeing, Southwest Airlines, and United Airlines request that the NPRM be revised to include a provision for alternative inspections when a repair area prohibits operators from doing the inspections specified in paragraph (h) of the NPRM. The commenters request that the initial alternative inspection be done within 12 months after the repair is discovered during the initial inspection required by paragraph (h). Two of the commenters point out that there is a similar provision in paragraph (e) of AD 98-11-04 R1, amendment 39-10984 (64 FR 987, January 7, 1999). The commenters state that including such a provision will assist operators. \n\n\tWe agree. We have added a new paragraph (i) to this AD (and reidentified subsequent paragraphs) that provides alternative inspections to those in paragraph (h) of this AD. \n\nRequest To Allow Compliance With the Repair Assessment Program (RAP) \n\n\tSouthwest and United request that the RAP be considered an alternative method of compliance (AMOC) for the supplemental structural inspection document (SSID) inspections of any repaired or modified SSI specified in paragraph (h) of the NPRM. United States that the FAA approved the RAP as an AMOC for those areas of the fuselage covered by repairs for Models 737-100, -200, and -200C series airplanes. Southwest states that multiple requirements for an individual repaired or modified area will create confusion, and that eventually the alternate inspection procedures will either be duplicated or only approved for one program. \n\n\tWe partially agree. We agree with the commenters that some of the inspection areas subject to the requirements of this AD also may be included in the RAP. The owner/operator of an affected airplane or Boeing, on behalf of the owner/operator, will need to perform an evaluation of each of these areas of the airplane to determine if the actions performed in accordance with the RAP meet the requirements of the SSID inspection program. Our understanding is that Boeing is looking into this evaluation; however, we have not received any data supporting a request for an AMOC. Once the evaluation has been completed, the owner/operator or Boeing may submit the data to substantiate that those actions performed in accordance with the RAP would provide an acceptable level of safety, under the provisions of paragraph (l) of this AD. We have made no change to the AD in this regard. \n\nRequest To Delegate Approval of Structure Affected by Winglet Modifications \n\n\tSouthwest requests that the NPRM be revised to allow an Authorized Representative (AR) for the Boeing Commercial Airplanes Delegation Option Authorization Organization to approve AMOCs for modified or altered structure such as winglets. Without such a provision, Southwest states that operators of airplanes on which winglets have been installed in accordance with a supplemental type certificate (STC) will need to seek AMOCs directly from the FAA. Southwest believes that such a provision would reduce the workload for operators and the FAA. \n\n\tWe do not agree. At this time, we cannot authorize Boeing ARs to approve repair data or AMOCs for non-Boeing type design products such as STCs for which Boeing does not have access to the design data. We have made no change to the AD in this regard. \n\nRequest To Approve NPRM as a Method of Compliance With Aging Airplane Safety Final Rule (AASFR) \n\n\tSouthwest and United request that the NPRM be approved as a method of compliance for the AASFR for the relevant SSIs. \n\n\tWe partially agree. We agree with the commenters that compliance with this AD would be an acceptable means of compliance with the AASFR for the baseline structure of Model 737-300, -400, and -500 series airplanes. The Costs of Compliance section of the NPRM included such a statement, which is restated in this final rule. In addition, the Supplemental Inspections section of the AASFR states, "The FAA will accept a SSID program for the baseline structure of an airplane developed by the OEM and approved by the FAA. If a SSID does not consider repairs, alterations, and modifications (RAMs), asrequired by this rule, the FAA would not accept it as a means to comply with this portion of the rule." Therefore, we find that no change to the final rule is necessary. \n\nRequest To Allow Zonal and Surveillance Inspections \n\n\tBritish Airways requests that zonal and surveillance inspections be considered acceptable for the general visual inspection specified in Boeing Document D6-82669, "Supplemental Structural Inspection Document Models 737-300/400/500 Airplanes," Original Release, dated May 2007 (hereafter "the SSID") (referred to in the NPRM as the appropriate source of service information for the proposed actions). \n\n\tWe do not agree. Each operator's maintenance inspection program defines inspection terminology. That maintenance inspection program might be defined by different revisions of the Maintenance Steering Group (MSG) procedures or other procedures accepted by the operator's Certificate Management Office. Because inspection definitions have changed over time, each operator must confirm that the maintenance inspections procedures (e.g., surveillance or general visual inspections) it performs are equivalent to those specified in section 5.0 of the SSID to take damage tolerance rating (DTR) credit for the SSID program. In addition, while zonal inspection programs include general visual inspections of an area, including the structure in that area, the zonal program might not include the same general visual inspection required by the SSID such as the specific structural detail, the frequency to do the inspection, and the requirement to do the inspection in the direction specified. Therefore, we have made no change to the AD in this regard. \n\nRequest To Extend Compliance Time of Reporting Requirement \n\n\tSouthwest and United also request that the compliance time for the reporting requirement in Section 6.0, "SSI Discrepancy Reporting," of the SSID be revised from 5 to 30 days. The commenters state that 5 days is insufficient time for reviewing documentation from various maintenance bases. \n\n\tWe do not agree. In developing an appropriate compliance time for this action, we considered the urgency associated with cracks involving an SSI or related structure in close vicinity to the SSI as well as the recommendations of the manufacturer. In consideration of these items, we have determined that a 5-day compliance time for reporting discrepant inspection findings will enable the manufacturer to obtain better insight into the nature, cause, and extent of the cracking, and eventually to develop a final action to address the unsafe condition. However, according to the provisions of paragraph (l) of this AD, we might approve requests to adjust the compliance time if the request includes data that prove that the new compliance time would provide an acceptable level of safety. \n\nRequest To Identify Differences Between the AD and the SSID \n\n\tBritish Airways requests that all differences between the AD and the SSID be identified. British Airways states that such differences were identified in other SSID ADs. \n\n\tWe partially agree. We agree with the commenter to identify differences between the AD and the SSID and did so in the Differences Between the Proposed AD and Service Information section of the NPRM. However, we find that no change to the final rule is necessary, since that section of the NPRM does not reappear in the final rule. \n\nRequest To Clarify a Certain Section of the Preamble of the NPRM \n\n\tBoeing requests that the Issuance of Advisory Circular (AC) section in the preamble of the NPRM be clarified. Boeing states that AC No. 91- 56, "Supplemental Structural Inspection Program for Large Transport Category Airplanes," dated May 6, 2001, applies to airplanes certified under the fail-safe and fatigue requirements of Civil Air Regulations (CAR) 4b or part 25 of the Federal Aviation Regulations (14 CFR part 25), not damage tolerance structural requirements as stated in the Issuance of AC section of the NPRM. \n\n\tWe agree with Boeing that the identified section could be clarified. However, no change has been made to the final rule since the identified sections of the NPRM do not reappear in the final rule. \n\nExplanation of Change to Reported Incidents \n\n\tWe have revised the AD to specify that this AD results from a report of incidents involving fatigue cracking only. \n\nExplanation of Change to Costs of Compliance \n\n\tThe requirements for the baseline structure of Model 737-300, -400, and -500 series airplanes are currently described in 14 CFR 121.1109(c)(1) and 129.109(b)(1), not in 14 CFR 121.370(a) and 129.16 as indicated in the third paragraph of the Cost of Compliance section of the NPRM. Therefore, we have revised the Costs of Compliance section of the AD accordingly. \n\nExplanation of Editorial Changes \n\n\tWe have revised references to the title of Boeing Document D6-82669 from "Supplemental Structural Inspection Document," to "Supplemental Structural Inspection Document Models 737-300/400/500 Airplanes" in this AD. \n\nConclusion \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 changes described previously. We also determined that these changes will not increase the economic burden on any operator or increase the scope of the AD. \n\nCosts of Compliance \n\n\tThere are about 1,961 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\t\t\t\t\t\tEstimated Costs\n\n\n\nAction\n\nWork hours\nAverage labor rate per hour\n\nCost \nNumber of U.S.-registered airplanes\n\nFleet cost \nRevision of maintenance inspection program\n1,200 per operator (26 U.S. operators)\n$80\n$96,000 per operator\n599\n$2,496,000\nInspections\n600 per airplane\n$80\n$48,000, per airplane, per inspection cycle\n599\n$28,752,000 per inspection cycle\n\n\n\tThe number of inspection workhours, as indicated above, is presented as if the accomplishment of the actions in this AD are to be conducted as "stand alone" actions. However, in actual practice, these actions for the most part will be done coincidentally or in combination with normally scheduled airplane inspections and other maintenance program tasks. Therefore, the actual number of necessary additional inspection work hours will be minimal in many instances. Additionally, any costs associated with special airplane scheduling will be minimal. \n\n\tFurther, compliance with this AD will be a means of compliance with the AASFR for the baseline structure of Model 737-300, -400, and -500 series airplanes. The AASFR requires certain operators to incorporate damage tolerance inspections into their maintenance inspection programs. These requirements are described in 14 CFR 121.1109(c)(1) and 129.109(b)(1). Accomplishment of the actions required by this AD will meet the requirements of these CFR sections for the baselinestructure. The costs for accomplishing the inspection portion of this AD were accounted for in the regulatory evaluation of the AASFR final rule. \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\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\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\tYou can find our regulatory evaluation and the estimated costs of compliance in the AD Docket. \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 AD: