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 all Boeing Model 737-100, -200, -300, -400, and -500 series airplanes was published in the Federal Register on August 28, 1996 (61 FR 44234). That action proposed to require operational tests of the standby rudder power control unit (PCU) to ensure correct operation of the rudder, and correction of any discrepancy found; and repetitive inspections to detect galling on the input shaft and bearing of the standby PCU, and replacement of the standby rudder actuator with a serviceable actuator, if necessary. That action also proposed to require eventual replacement of the input bearing of the standby PCU with an improved bearing, which would constitute terminating action for the inspections to detect galling. \n\nActions Since the Issuance of the Proposal \n\n\tSince the issuance of the proposal, the FAA has reviewedand approved Dowty Aerospace Los Angeles Service Bulletin 1150-27-04, dated December 5, 1996, which describes procedures to replace the input shaft assembly and related hardware with a new, improved input shaft. The new input shaft uses radial bearings, which will prevent galling on the input shaft and bearing. Paragraph (b) of this final rule has been revised to reference the Dowty Aerospace service bulletin as an appropriate source of service information for accomplishment of the replacement. \n\n\tIn addition, since the issuance of the proposal, the manufacturer has advised the FAA that the replacement of the input bearing of the standby PCU with an improved bearing has been incorporated on airplanes having line numbers 2815 and subsequent. Therefore, the FAA has revised the applicability of this final rule to include only airplanes having line numbers 1 through 2814 inclusive. \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\nRequest to Revise Statement of Findings of Critical Design Review Team \n\n\tOne commenter requests the second paragraph of the Discussion section that appeared in the preamble to the proposed rule be revised to accurately reflect the findings of the Critical Design Review (CDR) team. The commenter asks that the FAA delete the one sentence in that paragraph that reads: "The recommendations of the team include various changes to the design of the flight control systems of these airplanes, as well as correction of certain design deficiencies." The commenter suggests that the following sentences should be added: "The team did not find any design issues that could lead to a definite cause of the accidents that gave rise to this effort. The recommendations of the team include various changes to the design of the flight control systems of these airplanes, as well as incorporation of certain design improvements in order toenhance its already acceptable level of safety." \n\n\tThe FAA does not find that a revision to this final rule in the manner suggested by the commenter is necessary, since the Discussion section of a proposed rule does not reappear in a final rule. The FAA acknowledges that the CDR team did not find any design issue that could lead to a definite cause of the accidents that gave rise to this effort. However, as a result of having conducted the CDR of the flight control systems on Boeing Model 737 series airplanes, the team indicated that there are a number of recommendations that should be addressed by the FAA for each of the various models of the Model 737. In reviewing these recommendations, the FAA has concluded that they address unsafe conditions that must be corrected through the issuance of AD's. Therefore, the FAA does not concur that these design changes merely "enhance (the Model 737's) already acceptable level of safety." \n\nRequest to Delete Operational Test RequirementSeveral commenters request that the requirement to perform the operational tests to cycle hydraulic fluid through the standby rudder PCU and to ensure correct operation of the rudder when the standby hydraulic system is powered) be deleted from the proposal. These commenters point out that the Boeing Service Letter referenced in paragraph (a) of the proposal does not provide a description of procedures to perform the operational tests and does not include instructions to correct any discrepancies found. Another commenter requests that paragraph (a) be deleted from the proposal because it is not a technically sound approach to eliminating galling. This commenter states that the only way to prevent galling is to upgrade the input bearing of the standby hydraulic system. \n\n\tThe FAA concurs that replacement of the input bearing of the standby hydraulic system with a new, improved (upgraded) input bearing is a technically sound approach to eliminate galling. The requirement to replace the input bearing with a new, improved input bearing within 3 years, as specified in the proposed AD, supports that approach. Accordingly, this final rule has been revised to delete the proposed requirement for operational tests. The FAA finds that, until the replacement of the input bearing is required, repetitive inspections to detect galling of the input shaft and bearing, and replacement with a serviceable standby rudder actuator, if necessary (as specified in the proposed AD), will positively address the unsafe condition. \n\nRequest to Extend the Compliance Time for Operational Tests \n\n\tSeveral commenters request that the compliance time be extended for the operational tests discussed previously. The commenters request that the compliance time for the repetitive operational tests be extended from the proposed "at intervals not to exceed 250 hours time-in- service" to "at intervals not to exceed 800 hours time-in-service." The commenters state that the recent FAA MSG-3analysis on the hydraulic fluid compound revealed that the appropriate interval for the operational test is every 800 hours time-in-service. \n\n\tAs explained previously, the FAA has removed the requirement for operational tests from the final rule; however, this final rule is considered to be interim action. The FAA may consider further rulemaking to require operational tests of the standby system and correction of any discrepancies. The FAA will consider the results of the previously discussed MSG-3 analysis in determining an appropriate compliance time for future proposed operational tests. \n\nRequest to Extend the Compliance Times for Inspections for Galling \n\n\tSeveral commenters request that the compliance time for the initial and repetitive inspections for galling be extended from 3,000 hours time-in-service to "18 months or 4,500 hours time-in- service" for the proposed inspections to detect galling on the input shaft and bearing of the standby rudder PCU. The commenters state that 18 months or 4,500 hours time-in-service closely corresponds to a "C" check, which allows operators to schedule maintenance at a heavy maintenance base without impacting safety. One commenter suggests that the initial inspection and repetitive interval inspections should be extended to 46,000 flight hours. (The FAA infers that the "46,000" flight hours is a typographical error and that the commenter actually requests an extension to 4,600 flight hours.) \n\n\tThe FAA concurs with the commenters' request to revise the compliance time to 18 months or 4,500 hours time-in-service (whichever occurs later) since the last inspection. The FAA finds that this extension of the compliance time will not adversely affect safety, and will more closely correspond to the operators' scheduled "C" checks. The FAA has revised paragraph (a) of this final rule accordingly. \n\nRequests to Revise the Compliance Time for Replacement of the Input Bearing \n\n\tOne commenter (the airplane manufacturer) requests that the proposed compliance time for replacement of the input bearing be changed from 3 to 4 years after the effective date of the AD. The commenter states that the inspection should be accomplished at least once in 4 years, and the inspection should detect any units that are galled. Another commenter requests that the replacement be required by August 1, 1997. This commenter states that the National Transportation Safety Board (NTSB) suggests that date in a recommendation to the FAA. \n\n\tThe FAA does not concur that the compliance time should be revised. In developing an appropriate compliance time for this action, the FAA considered not only the degree of urgency associated with addressing the subject unsafe condition, but the availability of required parts and the practical aspect of accomplishing the required replacement within an interval of time that parallels normal scheduled maintenance for the majority of affected operators. The manufacturer has advised that an ample number of required parts will be available for modification of the U.S. fleet within the compliance period. However, under the provisions of this final rule, the FAA may approve requests for adjustments to the compliance time if data are submitted to substantiate that such adjustments would provide an acceptable level of safety. \n\nRequest to Delete (or Make Optional) the Replacement Requirement \n\n\tSeveral commenters request that the proposed replacement requirement be deleted to provide more time to review the retrofit program. One commenter suggests that the requirement should be optional, as long as the inspection to detect galling on the PCU input shaft is carried out repetitively every 46,000 flight hours. The commenter does not provide a justification for the recommended 46,000 flight hours. (The FAA infers that the "46,000" flight hours is a typographical error and that the commenter actually requests a compliance time of 4,600 flight hours.) \n\n\tThe FAA doesnot concur with the commenters' requests. Although the repetitive inspections required by this final rule may detect galled units before the galling progresses to a level that would affect the flight control system, the inspections do not ensure that galling will not occur. The replacement of the input bearing with a new, improved bearing, as described in the Dowty Aerospace service bulletin discussed previously, will positively address the subject unsafe condition and provide an acceptable level of safety. \n\n\tThe FAA has determined that long term continued operational safety will be better assured by modifications or design changes to remove the source of the problem, rather than by repetitive inspections. Long term inspections may not be providing the degree of safety assurance necessary for the transport airplane fleet. This, coupled with a better understanding of the human factors associated with numerous repetitive inspections, has led the FAA to consider placing less emphasis on special procedures and more emphasis on design improvements. The replacement requirement is in consonance with these considerations. \n\nRequest to Revise the Cost Estimate \n\n\tOne commenter, the airplane manufacturer, requests that the cost estimate for the proposed inspections be revised from $60 to $120 per airplane, per inspection cycle. The FAA acknowledges that the correct cost estimate is $120 per airplane, per inspection cycle, and has revised the cost impact information, below, 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 significantly increase the economic burden on any operator nor increase the scope of the AD.\n\nCost Impact \n\n\tThere are approximately 2,830 Model 737 series airplanes of the affected design inthe worldwide fleet. The FAA estimates that 1,037 airplanes of U.S. registry would be affected by this proposed AD.\n\tThe FAA estimates that it will take approximately 2 work hours per airplane to accomplish the required inspections, at an average labor rate of $60 per work hour. Based on these figures, the cost impact of the required inspections on U.S. operators is estimated to be $124,440, or $120 per airplane, per inspection cycle. \n\n\tThe FAA estimates that it will take approximately 2 work hours per airplane to accomplish the required replacement, at an average labor rate of $60 per work hour. The cost of the replacement parts is estimated to be $793 per airplane. Based on these figures, the cost impact of the required replacement is estimated to be $946,781, or $913 per airplane. \n\n\tThe cost impact figures discussed above are based on assumptions that no operator has yet accomplished any of the proposed 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 of power 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. \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: