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 737-300, -400, and -500 series airplanes was published in the Federal Register on January 7, 1997 (62 FR 947). That action proposed to require interchanging the location of the hydraulic fuse and the flow limiter of the standby hydraulic system of the leading edge so that the hydraulic fuse is positioned upstream of the flow limiter. That action also proposed to require replacing the existing hydraulic fuses in the standby hydraulic system with new fuses that are not affected by low temperature operation. \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\nSupport for the Proposal \n\n\tOne commenter supports the proposed rule. \n\nRequests to Revise the Compliance Times of the Proposed Interchange and Replacement Actions \n\n\tThe Air Transport Association (ATA) of America states that one commenter generally supports the proposed action; however, this commenter requests an amended compliance time of 18 months in lieu of 4,000 flight hours specified in paragraph (b) of the proposed AD. The commenter states that such an extension is needed because of an expected large demand for these fuses. A second commenter requests changing the compliance time to 6,000 flight hours or 2 years, whichever occurs first, because the hydraulic fuse manufacturer is unable to support a compliance time of 4,000 flight hours. Another commenter also requests a change in the compliance time to 6,000 flight hours. \n\n\tThe FAA concurs partially with these requests and acknowledges that parts availability and scheduling may present problems. The FAA does not concur with the request to extend the compliance time from 4,000 flight hours to 6,000 flight hours, or the request to change it to 6,000 flight hours or 2 years, whichever occurs first. However, the FAA has considered the need to allow additional time to obtain the number of fuses required for the fleet and to avoid scheduling problems for the replacement of discrepant fuses. Therefore, the FAA has revised paragraph (b) of the final rule to read: "Within 18 months or 4,000 flight hours after the effective date of this AD, whichever occurs later. . . ." In addition, for the same reasons, the FAA has revised the compliance time of paragraph (a) of the final rule, which is identical to paragraph (b). The FAA has determined that extending these compliance times will not adversely affect safety. \n\nRequests to Clarify the Summary Section of the Preamble \n\n\tTwo commenters request a number of revisions and additions to clarify the technical content of the "Summary" Section of the NPRM. \n\n\tIn that section, one commenter requests that the third sentence be changed from ". . . and excessive fusing volumesof some of the fuses, can adversely affect . . ." to ". . . and excessive fusing volumes of some of the fuses in extreme cold environment, can adversely affect . . . ." The FAA concurs with this request and has changed the final rule accordingly. \n\n\tTwo commenters request that the statement of unsafe condition be changed from ". . . in the loss of all hydraulic system pressure and consequent severely reduced controllability of the airplane" to ". . . in the loss of all standby hydraulic system pressure and may reduce the controllability of the airplane during certain flight phases." The FAA concurs partially with these changes. The FAA has determined that the word "standby" and the phrase "during certain flight phases" add clarity and has revised the final rule accordingly. However, the FAA does not concur with the proposed addition of "may reduce the controllability" to the sentence, because the FAA considers that "could result in" is more accurate. \n\nRequests to Clarify Additional Sections of the Preamble \n\n\t1. "Discussion" Section. In the first paragraph of this section, one commenter requests that the second sentence be changed from "Results of that performance test . . ." to "In the existing configuration, the standby leading edge flow limiter is upstream of the standby leading edge fuse. The results of the performance test revealed that this configuration of the flow limiter and fuse assembly adversely affects the operation of the fuse." \n\n\tIn the second paragraph of this section, one commenter requests deleting the second sentence and changing the third sentence from ". . . are not affected by this condition . . ." to ". . . are not affected by this condition because steady state temperatures keep the fluid warm." \n\n\tIn the third paragraph of this section, two commenters request changing the second sentence from "The hydraulic fuse is designed to prevent total loss of the hydraulics systems after a certain volume of fluid passes through the fuse within a specified time following the development of a leak downstream of the fuse . . ." to "Hydraulic fuses are designed to prevent total loss of the hydraulics system after a certain volume of fluid (continually/continuously) passes through the fuse following the development of a leak downstream of the fuse." \n\n\t2. Explanation of Relevant Service Information. In the second paragraph of this section, two commenters request changing the first sentence from ". . . new fuses that are not affected by low temperature operation" to ". . . new fuses that function in low temperatures." These commenters also request changing the second sentence from ". . . as a result of fluid depletion if a leak occurs downstream of the fuses" to ". . . as a result of a fuse failing to set following a leak downstream of the fuses." \n\n\t3. Explanation of Requirements of Proposed Rule. In the first paragraph of this section, two commenters request changing the second sentence from ". . . new fusesthat are not affected by low temperature operation" to ". . . new fuses that function at/in low temperatures." \n\n\tAlthough the FAA acknowledges that the commenters' suggested wording in these sections of the preamble adds technical clarity, the FAA has determined that these changes are not relevant because these sections do not appear in the final rule. \n\nRequests to Clarify the Body of the AD \n\n\tOne commenter requests changing paragraph (b) to read: "For airplanes listed in Boeing Service Bulletin 737-29-1071 (line numbers 2001 through 2791) . . ." The FAA does not concur with this request for two reasons. First, the line number "2001" is incorrect, and the correct number (1001) is shown in the applicability of the proposed AD. Second, because the line numbers are included in the applicability of the AD, it is unnecessary to include them elsewhere in the AD. \n\n\tTwo commenters request changing paragraph (b) to read ". . . with new fuses that are not adversely affected during low temperature operation . . . ." The FAA has determined that this change adds clarity and has changed the wording of the final rule accordingly. \n\nConclusion \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\nCost Impact \n\n\tThere are approximately 1,791 Boeing Model 737-300, -400, and -500 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 596 airplanes of U.S. registry will be affected by this AD. \n\n\tThe FAA estimates that it will take approximately 2 work hours per airplane to accomplish the required interchange of the hydraulic fuse and the flow limiter, and that the average labor rate is $60 per work hour. The cost for required parts will be minimal. Based on these figures, the cost impact of the required interchange on U.S. operators is estimated to be $71,520, or $120 per airplane. \n\n\tThe FAA also estimates that it will take approximately 4 work hours per airplane to accomplish the required replacement, at an average labor rate of $60 per work hour. Required parts will be provided by the manufacturer at no cost to operators. Based on these figures, the cost impact of the required replacement on U.S. operators is estimated to be $143,040, or $240 per airplane. \n\n\tThe cost impact figures discussed above are based on assumptions that 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\nRegulatory Impact \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 ofpower and responsibilities among the various levels of government. Therefore, in accordance with Executive 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\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\n\tAccordingly, pursuant to the authority delegated to me 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\n\t1. The authority citation for part 39 continues to read as follows: Authority: 49 U.S.C. 106(g), 40113, 44701. § 39.13 (Amended) \n\t2. Section 39.13 is amended by adding the following new airworthiness directive: