Examining the Docket\n\n You may examine the 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 \nADDRESSES section.\n\nDiscussion\n\n The FAA issued a notice of proposed rulemaking (NPRM) to amend 14 CFR part 39 to include an AD that would apply to all Boeing Model 737-100, -200, -200C, -300, -400, and -500 series airplanes. That NPRM was published in the Federal Register on February 15, 2005 (70 FR 7693). (A correction of the NPRM was published in the Federal Register on May 19, 2005 (70 FR 28988).) That NPRM proposed to require repetitive inspections of the stiffeners at left buttock line (LBL) and right buttock line (RBL) 6.15 for cracks; and replacement of both stiffeners with new, improved stiffeners if any stiffener is found cracked. That NPRM also proposed to allow replacement of both stiffeners at LBL and RBL 6.15 with new, improved stiffeners, which would terminate the repetitive inspections.\n\nComments\n\n We 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\nSupport for the Proposed AD\n\n Two commenters support the proposed AD.\n\nRequest To Use Operator Equivalent Procedures\n\n One commenter requests that we revise paragraph (h) of the proposed AD to allow the use of FAA-approved, operator equivalent procedures for draining and gaining access to the center fuel tanks. The commenter states that Parts II and III of Boeing Alert Service Bulletin 737-57A1269, Revision 1, dated September 16, 2004, contain work steps for de-fueling and purging the center tanks. The commenter further states that mandating how the fuel tanks are purged does not directly affectthe means of correcting the unsafe condition addressed in the proposed AD.\n\n We agree that the procedures specified in Parts II and III are intended for gaining access to the center tanks. We also agree that using an equivalent procedure to gain access would not adversely affect the means of correcting the unsafe condition, which is to detect and correct cracks in the stiffeners at LBL and RBL 6.15. Therefore, we have revised the first sentence of paragraph (h) of this AD to require accomplishing the applicable actions in Part IV through Part IX of the \nservice bulletin.\n\nRequest for Credit for Previous Inspections\n\n One commenter requests that inspections performed in accordance with Boeing All Operator Telex (AOT) M-7200-01-00426, dated February 19, 2001, be considered acceptable for compliance with the inspections specified in paragraph (g) of the proposed AD. The commenter states that both Boeing AOT M-7200-01-00426 and Boeing Alert Service Bulletin 737-57A1269, Revision 1, dated September 16, 2004, provide instructions for doing a detailed inspection of the stiffeners at LBL and RBL 6.15 on the rear spar of the wing center section within the same compliance time.\n\n We agree that the detailed inspections specified in Boeing AOT M-7200-01-00426 are acceptable for compliance with the corresponding requirements of this AD. We have added a new paragraph (k) to this AD to give credit and re-lettered the subsequent paragraphs accordingly.\n\nRequest To Use New Stiffeners of the Existing Type Design\n\n Two commenters request that we revise paragraph (h) of the proposed AD to add the option of replacing a cracked stiffener with a new stiffener of the existing type design (made from 7075-T6511 aluminum extrusion). One commenter states that since we have determined a repetitive inspection interval of 4,500 flight cycles provides an adequate level of safety for detecting cracks in the existing stiffeners, then replacement with new stiffeners of the existing type \ndesign should also provide an adequate level of safety if the repetitive inspections are continued. Although there is an ample supply of new stiffeners of the existing type design available, both commenters are concerned that there is an insufficient supply of new, improved stiffeners (made from 2024-T351 aluminum alloy plate) to comply with the proposed replacements.\n\n We do not agree. The manufacturer has confirmed that there is a sufficient supply of new, improved stiffeners available to comply with this AD. Therefore, no change to this AD is necessary in this regard.\n\nRequest To Replace a Stiffener, Only if Cracked\n\n Two commenters request that we revise paragraph (h) of the proposed AD to state that, if only one of the two stiffeners is found cracked, operators would be required to replace only the cracked stiffener instead of both stiffeners. As justification, the first commenter states that it frequently finds only one cracked stiffener during inspections of the stiffeners at LBL and RBL 6.15. Both commenters believe that the proposed AD should allow the option of replacing only the cracked stiffener provided that the repetitive inspections for cracking are continued until both stiffeners are eventually replaced. The second commenter supports this change because replacing both stiffeners requires additional labor and material. The commenter also states that the aggressive initial inspection threshold will force operators to inspect affected airplanes outside of a heavy maintenance visit. The commenter asserts that the additional impact of replacing \nboth stiffeners will strain available resources.\n\n We do not agree because the new, improved stiffeners are much more rigid than the stiffeners of the existing type design. Replacing only one of the two stiffeners will lead to changes in the loading of the structure and premature fatigue of the new, improved stiffener. However, we acknowledge that replacing a cracked stiffener with a new stiffener of the existing type design will not adversely affect the relative stiffness of the two keel beam stiffeners, since they would be \nthe same type design. If service information containing repair instructions and subsequent inspection requirements for replacing a cracked stiffener with a new stiffener of the existing type design is developed, under the provisions of paragraph (l) of this AD, we may consider requests for approval of an AMOC. Sufficient data must be submitted to substantiate that such a design change would provide an acceptable level of safety. Therefore, no change to this AD is \nnecessary in this regard.\n\nRequest To Allow Temporary Repairs\n\n Three commenters request that we revise the proposed AD to allow operators to make temporary repairs until cracked stiffeners can be replaced. One commenter suggests adding an interim repair plan to the proposed AD to give operators time to schedule the terminating action (replacement of cracked stiffenerswith new, improved stiffeners). The \ncommenter proposes that an interim repair plan could consist of stop-drilling small cracks where possible and reducing the repetitive inspection intervals to monitor crack growth, until the terminating action could be accomplished. The commenter is concerned it will not be able to comply with the requirements of the proposed AD because of the \nshort initial inspection threshold and number of affected airplanes in its fleet.\n\n The second commenter states that repairs done in accordance with Boeing AOT M-7200-01-00426, dated February 19, 2001, and Repair Sketch LOR-760 will take less time than replacement of the stiffeners, especially since most of its affected airplanes will be inspected outside of a heavy maintenance visit. The third commenter asks if we would consider the two temporary repairs, which do not require access into the center tank, as an AMOC to the proposed AD.\n\n We partially agree. The FAA is working with the manufacturerto establish appropriate inspection and replacement requirements for this interim repair. Once this evaluation is concluded we may, under the provisions of paragraph (l) of this AD, we approve the subject interim repairs as an AMOC. We do not consider that delaying this final rule is \nwarranted.\n\nRequest To Revise Compliance Time\n\n One commenter requests that we extend the compliance time of the initial inspection, from 180 days to 15 months for airplanes that have accumulated less than 30,000 total flight cycles, to allow affected operators to perform the inspection during a regularly scheduled maintenance interval. The proposed AD reported that cracked stiffeners were found on two airplanes with over 40,000 total flight cycles and on a third airplane with just over 20,000 total flight cycles. The \ncommenter believes that the data are not consistent enough to warrant a short compliance time for airplanes that have accumulated fewer than 40,000 total flight cycles. To comply with the proposed AD, the operator states that it would have to inspect more than 1 airplane per week, since the proposed AD affects the majority of its fleet. The operator also states that its operations would be negatively impacted if several of its airplanes required the terminating action, estimated \nat 250 work hours.\n\n We do not agree with the commenter's request. The commenter provides no technical justification for revising this inspection interval. Furthermore, since issuing the proposed AD, we have received numerous additional reports of cracked stiffeners. Eight of those reports included airplanes that have accumulated fewer than 30,000 total flight cycles. We have determined that the compliance time, as proposed, represents the maximum interval of time allowable for the \naffected airplanes to continue to operate safely before the inspection is accomplished.\n\nRequest To Revise "Cost of Compliance"\n\n One commenter estimates that the proposed inspection would takeabout 4 work hours, not 1 work hour as we specified in the proposed AD. We infer that the commenter would like us to revise the "Cost of Compliance" section.\n\n We do not agree, since the commenter has not provided justification for the increase in work hours. Our cost estimate is based on information that the manufacturer has provided to us, and we point out that the cost impact figures discussed in AD rulemaking actions represent only the time necessary to perform the specific actions actually required by the AD. These figures typically do not include incidental costs, such as the time required to gain access and close \nup, planning time, or time necessitated by other administrative actions. Therefore no change to this AD is necessary in this regard.\n\nExplanation of Changes Made to This AD\n\n We have simplified paragraph (h) of this AD by referring to the "Alternative Methods of Compliance (AMOCs)" paragraph of this AD for repair methods. Also, we have revised the "Alternative Methods of Compliance (AMOCs)" paragraph in this AD to clarify the delegation authority for Authorized Representatives for the Boeing Commercial \nAirplanes Delegation Option Authorization.\n\nConclusion\n\n We 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 This AD affects about 3,132 airplanes worldwide. The following table provides the estimated costs, at an average labor rate of $65 per hour, for U.S. operators to comply with this AD.\n\n Estimated Costs\n\n\n\n\n\n\n\n\nAction \nWork hours \nParts \nCost per airplane \nNumber of \nU.S.-registered airplanes \nFleet cost \nInspection, per inspection cycle \n1None \n$65, per inspection cycle \n1,384 \n$89,960, per inspection cycle. \n\n\nAuthority for This Rulemaking\n\n Title 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 We 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 We 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 For the reasons discussed above, I certify that this AD:\n\n (1) Is not a "significant regulatory action" under Executive Order 12866;\n\n (2) Is not a "significant rule" under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and\n\n (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 We 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 Air 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 Authority: 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 airworthiness directive (AD):