A proposal to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) to include an airworthiness directive (AD) that is applicable to certain Boeing Model 747 series airplanes was published in the Federal Register on February 24, 1994 (59 FR 8875). That action proposed to require inspections to detect fatigue-related skin cracks and corrosion of the skin lap joints in the fuselage upper lobe, and repair, if necessary. That action also proposed to require modification of certain lap joints and inspections of modified lap joints. \n\n\tInterested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received. \n\n\tOne commenter supports the proposal. \n\n\tOne commenter requests that proposed inspection requirements be revised. This commenter requests that, if the visual inspection (proposed in paragraph (a)(1) of the AD) reveals any evidence of corrosion, operators should be allowed to perform a low frequency eddy current (LFEC) inspection of the entire length of the lap joint to determine if corrosion exceeds 10% of the skin thickness. If this LFEC inspection is accomplished, the commenter also requests that the visual inspection repetitive interval specified in paragraph (a)(1) be extended from the proposed 2,000 landings to 3,000 landings so that the interval corresponds with other major structural inspection intervals. The FAA cannot concur with this request, since the commenter provided neither specific inspection instructions nor data to justify the effectiveness of the proposed procedures. Under the provisions of paragraph (i) of the final rule, however, operators may apply for the approval of an alternative methods of compliance with the AD, or adjustment of the compliance time, if sufficient justification is presented to the FAA. \n\n\tTwo commenters request that proposed paragraph (a) be clarified with regard to the inspections and inspection interval required after a lap joint has been modified. These commenters request that, for lap joints that have been modified by either the full or partial modification procedures described in the referenced Boeing service bulletin, operators be allowed a period of time during which no inspections are required for those lap joints. The referenced Boeing service bulletin, as well as proposed paragraph (f), state that post-modification inspections are to start at 10,000 cycles, 7,000 cycles, or 5,000 cycles after modification of the skin lap joints, depending upon the type of modification that has been accomplished. However, as worded in paragraph (a) of the proposal, the same inspections meant to detect cracks and corrosion in unmodified skin lap joints would be required to be performed on modified skin lap joints until the inspections specified in proposed paragraph (f) begin. The FAA concurs that clarification of this requirement is necessary. It was not the FAA's intent to require that the initial-type of inspections of unmodified lap joints be performed on modified lap joints. Accordingly, paragraphs (a)(1) and (a)(3) have been revised to indicate that the inspections of the specified lap joints are to continue at repetitive intervals only until the lap joints are modified in accordance with the requirements of paragraph (e). \n\n\tSeveral commenters request that the compliance time for accomplishing the "full" modification on certain lap joints, as specified in paragraph (b)(2), be extended from the proposed 15 months to 18 months. Additionally, these commenters request that the compliance time for a similar requirement specified in paragraph (c)(2) be extended from the proposed 30 months to 36 months. The commenters state that, by extending these compliance times, operators would be able to accomplish the modification during a scheduled maintenance interval ("C" check), and thereby eliminate the need to schedule special times for the accomplishment of this modification, at additional expense. The FAA concurs. It was the FAA's intent that the modification be performed at a main base during regularly scheduled maintenance where special equipment and trained maintenance personnel will be available if necessary. Accordingly, the final rule has been revised to specify compliance times as requested by the commenters. The FAA has determined that this extension will not adversely affect safety since the affected lap joints will be subject to repetitive inspections in the interim.\n\n\tTwo commenters request that paragraph (e) be revised to extend the compliance time for accomplishment of the "full" modification of the specified skin lap joints from the proposed 1,000 cycles (for airplanes that have accumulated nearly or more than 20,000 total flight cycles) to 4 years after the effective date of the final rule. These commenters consider the proposed compliance time to be too short and not consistent with compliance times provided for similar actions in other AD's pertaining to aging aircraft issues. The FAA does not concur. The 1,000 flight cycle compliance time is consistent with the mandatory modification of a lap joint that has been found to have corrosion or cracking, as required by paragraph (b)(2) of this AD. Cold bonded lap joints such as these have a history of numerous findings of fatigue cracks and corrosion. In light of this, and the fact that fatigue cracking is cycle-related, the FAA has determined that modification of these lap joints cannot be delayed any further than 1,000 flight cycles, especially on those airplanes that already have accumulated 20,000 flight cycles, without compromising safety. \n\n\tOne commenter requests that the applicability of proposed paragraphs (f)(1), (f)(2), and (f)(3) be revised to clarify that it is the "lap joint," rather than the "airplane," on which the particular modification has been accomplished. The FAA concurs and has revised wording of the final rule to clarify this point. \n\n\tOne commenter requests that all references in the rule to "landings" be changed to "cycles." The FAA concurs that the reference should be revised. The FAA has reviewed the applicable service information related to this action and finds that the majority of references to recommended compliance times are stated in "flight cycles." Although the FAA normally uses the terms "landings" and "flight cycles" interchangeably, the FAA considers that it is appropriate in this case to change the verbiage of the AD to match that of the cited service bulletins. Therefore, in this final rule, the FAA has substituted the term "flight cycles" for the term "landings." \n\n\tAnother commenter, a non-U.S. operator, requests that the proposed rule be revised to include a provision specifying that pressurization cycles of 2.0 psi or less need not be counted as a flight cycle when determining the number of flight cycles relative to the proposed compliance thresholds. This commenter states that a pressurization cycle of 2.0 psi or less is a typical pressure used during flight training, and causes little fatigue damage of the kind addressed by this AD action. The FAA does not concur. The FAA does not consider it appropriate to include various provisions in an AD applicable to a single operator's unique use of an affected airplane. Paragraph (i) of this AD provides for the approval of alternative methods of compliance to address these types of unique circumstances. Further, this commenter does not compile data for each of its airplanes so that an individual airplane's pressurization cycles could be determined; instead, it uses a fleet average to calculate the equivalent number of pressurization cycles. The FAA does not consider it appropriate to use approximations for determining compliance with this AD. \n\n\tAfter careful review of the available data, including the comments noted above, the FAA has determined that air safety and the public interest require the adoption of the rule with the changes previously described. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD.\n \n\tThere are approximately 200 Model 747 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 116 airplanes of U.S. registry will be affected by this proposed AD. \n\n\tThe required inspections will entail 100 work hours per airplane per inspection, at an average labor rate of $55 per work hour. (This figure does not include the time necessary for gaining access and closing up.) Based on these figures, the total cost impact of this requirement on U.S. operators is estimated to be $638,000, or $5,500 per airplane, per inspection. \n\n\tThe required "full" modification will entail approximately 96 work hours for each 200-inch length of uncracked and uncorroded lap joint, at an average labor rate of $55 per work hour. (This figure does not include thetime necessary for gaining access and closing up.) There are 100 lap joint sections per airplane, each with a length of 200 inches. The cost of required parts is expected to be negligible. Based on these figures, the total cost impact of this requirement on U.S. operators is estimated to be $61,248,000, or $528,000 per airplane. \n\n\tThe initial high frequency eddy current (HFEC) inspection following modification will entail approximately 56 work hours, at an average labor rate of $55 per work hour. Based on these figures, the total cost impact of this requirement AD on U.S. operators is estimated to be $357,280, or $3,080 per airplane. \n\n\tBased upon the figures discussed above, the total cost impact of this AD on U.S. operators is expected to be $62,243,280, or $536,580 per airplane. This total cost figure includes the inspections and modification, for the first year of the average five-year inspection cycle. Additionally, the total cost figure indicated is based on assumptionsthat no operator has yet accomplished any of the requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted. \n\n\tThe FAA recognizes that the obligation to maintain aircraft in an airworthy condition is vital, but sometimes expensive. Because AD's require specific actions to address specific unsafe conditions, they appear to impose costs that would not otherwise be borne by operators. However, because of the general obligation of operators to maintain aircraft in an airworthy condition, this appearance is deceptive. Attributing those costs solely to the issuance of this AD is unrealistic because, in the interest of maintaining safe aircraft, most prudent operators would accomplish the required actions even if they were not required to do so by the AD. \n\n\tA full cost-benefit analysis has not been accomplished for this AD. As a matter of law, in order to be airworthy, an aircraft must conform to its type design andbe in a condition for safe operation. The type design is approved only after the FAA makes a determination that it complies with all applicable airworthiness requirements. In adopting and maintaining those requirements, the FAA has already made the determination that they establish a level of safety that is cost- beneficial. When the FAA, as in this AD, makes a finding of an unsafe condition, this means that this cost-beneficial level of safety is no longer being achieved and that the required actions are necessary to restore that level of safety. Because this level of safety has already been determined to be cost-beneficial, a full cost-benefit analysis for this AD would be redundant and unnecessary. \n\n\tThe regulations adopted herein will not have substantial direct effects 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. Therefore, in accordance withExecutive Order 12612, it is determined that this final rule does not have sufficient federalism implications to warrant the preparation of a Federalism Assessment. \n\n\tFor the reasons discussed above, I certify that this action (1) is not a "significant regulatory action" under Executive Order 12866; (2) is not a "significant rule" under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (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. A final evaluation has been prepared for this action and it is contained in the Rules Docket. A copy of it may be obtained from the Rules Docket at the location provided under the caption "ADDRESSES."\n\n List of Subjects in 14 CFR Part 39 \n\nAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety. \n\nAdoption of the Amendment \n\nAccordingly, pursuant to the authority delegated tome by the Administrator, the Federal Aviation Administration amends part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows:\n\n PART 39 - AIRWORTHINESS DIRECTIVES \n\n1.\tThe authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. App. 1354(a), 1421 and 1423; 49 U.S.C. 106(g); and 14 CFR 11.89. § 39.13 - (Amended) \n\n2.\tSection 39.13 is amended by adding the following new airworthiness directive: