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 certain Boeing Model 737-600, -700, -700C, -800, and -900 series airplanes. That NPRM was published in the Federal Register on September 28, 2007 (72 FR 55118) (An extension of the comment period for that NPRM was published in the Federal Register on November 14, 2007 (72 FR 64009)). That NPRM proposed to require various repetitive inspections to detect cracks along the chemically milled steps of the fuselage skin or missing or loose fasteners in the area of the preventative modification or repairs, replacement of the time-limited repair with the permanent repair if applicable, and applicable corrective actions if necessary, which would end certain repetitive inspections. \n\nComments \n\n\tWe gave the public the opportunity to participate in developing this AD. We considered the comments received. \n\nRequest To Provide Exception for Previously Installed Repairs \n\n\tSouthwest Airlines notes that the proposed AD does not state how to do the inspection in an area that has a previously installed repair. Southwest Airlines states that AD 2004-18-06, amendment 39-13784 (69 FR 54206, September 8, 2004), which addresses chemically milled steps of the fuselage skin for Boeing Model 737-200, -200C, -300, -400, and -500 series airplanes, contains an exception that addresses the issue of a previously installed repair. Southwest Airlines asks that we include a similar exception in this AD. \n\n\tWe agree that the NPRM needs to be clarified regarding procedures for previously installed repairs, and have added new paragraphs (j) and (k) to this AD to explain the exceptions. We note that the exception to the procedures required by paragraph (g) of this AD is similar to the exception in AD 2004-18-06, except that for this AD, post-preventive modifications and repair supplemental inspections are required for repairs installed in accordance with Boeing Special Attention Service Bulletin 737-53-1232, dated April 2, 2007 (cited as the appropriate source of service information for accomplishing the actions in the NPRM). We have also re-identified subsequent paragraphs accordingly. \n\nRequest To Allow Optional Eddy Current Inspection Method \n\n\n\tContinental Airlines (Continental) requests that we allow the use of the eddy current inspection procedures given in the Boeing 737 Non- Destructive Test (NDT) Manual, Part 6, Subjects 53-30-25 (c-scan eddy current inspection), as an alternative to Subjects 53-30-19 and 53-30- 23 listed in Boeing Special Attention Service Bulletin 737-53-1232, dated April 2, 2007. Continental notes that the eddy current procedure in Subject 53-30-25 was approved as an alternative method of compliance (AMOC) for AD 2005-13-27, amendment 39-14164 (70 FR 36821, June 27, 2005), which mandates a similar fuselage skin inspection for Boeing Model 737-300, -400, and -500 series airplanes. \n\n\tWe agree that the NDT method Continental specifies provides an acceptable means to find cracking in the internal surface of the fuselage skin at the edge of a sub-surface doubler. Therefore, we have revised this AD to include a new paragraph (l)(4) to the AMOC paragraph (paragraph (j) of the NPRM). Paragraph (l)(4) states that Boeing Model 737 NDT Manual, Part 6, Subject 53-30-25, is an AMOC for Subjects 53- 30-19 and 53-30-23. \n\nRequest To Clarify Paragraph (g) of the NPRM \n\n\tBoeing requests that we clarify the wording in paragraph (g) of the NPRM to indicate which corrective actions are required and when. Boeing specifically states that the word "applicable" is missing from paragraph (g) of the NPRM, and requests that the paragraph state "accomplishing all of the applicable actions specified in the Accomplishment Instructions of the service bulletin." Boeing explains that, without the word "applicable," the AD would require accomplishment of all actionswithin the Accomplishment Instructions, even those that do not apply under certain conditions. \n\n\tWe agree to clarify paragraph (g) for the stated reasons. We have revised paragraph (g) of this AD to include the word "applicable" in the requested place. \n\nRequest To Improve Detail in Service Bulletin \n\n\tThe Air Transport Association (ATA) on behalf of its member Delta Airlines, requests that we encourage Boeing to improve the level of detail in Boeing Special Attention Service Bulletin 737-53-1232, dated April 2, 2007, specifically Part V of the Accomplishment Instructions, "Preventative Modification." The commenters explain that the current data and figures for the modification are vague and could lead to considerable variation among operators in interpretation and installation. The commenters also state that, as a minimum, Boeing should issue a set of engineering drawings for typical modification parts for each affected group of airplanes, and incorporate them into a revision of the service bulletin. \n\n\tWe disagree that the level of detail in Part V of the service bulletin is insufficient. As shown in Part V and its associated figures, modification doublers and fillers are to be centered in the skin pocket with their width determined by the existing fastener spacing common to the lap splice. Adding engineering drawings to the information already in the service bulletin could result in confusion due to variations in fastener spacing common to the lap joints. We have not changed the AD in this regard. \n\nRequest To Extend Repetitive Interval To Match C-Check Interval \n\n\tThe ATA, on behalf of its member Alaska Airlines, requests that we extend the repetitive inspection intervals proposed in the NPRM and express them in terms of C-check intervals. The commenters explain that the current repetitive inspection intervals are not sufficient to bridge successive C-checks, and will thus make it necessary to have a frequent and possibly repetitive inspection in the line environment. The commenters further state that the preventive modification proposed in the NPRM would lengthen the repetitive inspection interval from 1,500 flight cycles to either 4,000 or 6,000 flight cycles. In the commenters' opinion, this action does not justify the cost or manpower for doing the preventive modification. \n\n\tWe do not agree with the commenter's request to extend the repetitive intervals. We have determined that the proposed compliance time represents the maximum interval of time allowable for the affected airplanes to continue to safely operate before the modification is done. We determined the inspection intervals in this AD using damage tolerance methods to ensure that damage can be detected before it becomes critical on the structure. Also, compliance intervals cannot be based on nonspecific intervals such as a C-check. Since maintenance schedules vary among operators, there would be no assurance that corrective action would be done within the timeframefor safe operation of the airplane. Further, in developing appropriate compliance times for this AD, we considered the urgency associated with the subject unsafe condition, and the practical aspect of accomplishing the required actions within a period of time that corresponds to the normal scheduled maintenance for most affected operators. The repetitive intervals following preventative modification were part of these considerations. We have not changed the AD in this regard. \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 871 airplanes of the affected design in the worldwide fleet. This AD affects about 378 airplanes of U.S. registry. The inspections take between 11 and 25 work hours per airplane depending on the airplane configuration, at an average labor rate of $80 per work hour. Based on these figures, the estimated cost of the AD for U.S. operators is between $332,640 and $756,000, or between $880 and $2,000 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 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: