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 44241). That action proposed to require an inspection of the aileron/elevator power control units (PCU) and the rudder PCU to determine if reworked PCU manifold cylinder bores containing chrome plating are installed, and replacement of the cylinder bores with bores that have been reworked using the oversize method or the steel sleeve method, if necessary. \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 Support for the Proposal \n\n\tOne commenter supports the proposed rule. \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, which read: "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 to enhance 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\nRequests to Withdraw the Proposal: No Supporting Data \n\n\tOne commenter contends that the proposal is not justified since it cannot be supported by data. The commenter does not consider that theproposal contributes to improving the safety aspects of Model 737 airplanes. The commenter states that the CDR team's report does not indicate that there is any evidence to tie the referenced service documents to any in-service problems or accidents. The commenter adds that the FAA has not indicated it has reviewed any routine component tear-down reports that would support the proposed actions. The commenter concludes that the FAA does not understand the enormity of the proposed action. A second commenter states that it has incorporated the repair on several PCU's and has not witnessed a single failure of the chrome plating of the cylinder bore. The FAA infers from these remarks that the commenters request the proposed rule be withdrawn. \n\n\tIn support of its request to withdraw the proposal, the Air Transport Association (ATA) of America, points out that any performance degradation that might result from chrome plate separation would be determined readily by flight crews priorto departure. The ATA also indicates that if the plating repair were suspect, an incident involving separation would have been identified early in the service life of the units. The ATA contends that service experience and tests conducted by the National Transportation Safety Board (NTSB) confirm that there is no justification to consider this issue as an existing airworthiness concern. \n\n\tThe FAA does not concur with the requests of these commenters to withdraw the proposal. The FAA has received at least five reports from operators of Model 737 series airplanes indicating that the chrome plating separated from reworked cylinder bores of the aileron/elevator PCU's. A number of aileron/elevator and rudder PCU's were repaired using chrome plating on the aluminum cylinder bores. Separation of the chrome plating could result in contamination of hydraulic fluid from chrome plating chips. Such contamination can result in a reduced rate of movement of the elevator, aileron, or rudder,and consequent reduced controllability of the airplane. The FAA's position is that this condition is a potential unsafe condition that must be corrected in order to ensure the safety of the affected fleet. \n\n\tThe FAA acknowledges that in some cases (depending on when chrome plate separation occurred), prior to departure, flight crews could determine any performance degradation that might result from chrome plate separation. However, if the separation occurred during flight, the flight crew may be unaware of the occurrence and, when the flight crew's workload is heavy, the crew's ability to control the airplane may be reduced. The FAA considers that this presents a potential unsafe condition that must be corrected. \n\n\tNo commenter presented evidence that would indicate chrome separation occurs early in the life of the unit. Further, the FAA assumes the NTSB testing discussed by one commenter refers to chip shear testing accomplished by the NTSB on the rudder PCU. While it appears that a chrome chip should be able to be sheared by the hydraulic action of the PCU and not cause a PCU jam, chrome chips can still contaminate the interior of the rudder, aileron, and elevator PCU's, which could cause sluggish flight control operation. \n\nRequest to Withdraw the Proposal: Revise Component Maintenance/Overhaul Manuals \n\n\tOne commenter recommends revising the proposal to require a Component Maintenance Manual (CMM) revision to require inspection at the next shop visit to identify any units with chrome bores and rework those in accordance with the service letter in order to eliminate what the commenter finds to be an enormous financial and operational impact that would be imposed on operators if the proposed rule is issued. \n\n\tOne commenter, Boeing, indicates that separation of chrome plating from aluminum cylinder bores has not been a significant in-service problem; in fact, there have been no reports of chrome plating separation since 1985. Boeing asserts thatsince no direct safety hazard related to chrome plating has been established, the proposal should be withdrawn and, instead, removal of chrome plating should be required through a revision to the PCU overhaul manuals for the rudder and aileron/elevator. \n\n\tThe FAA does not concur with the commenters' requests to withdraw this AD and revise the component maintenance or overhaul manuals instead. The FAA considers issuance of an AD necessary in this instance since an unsafe condition has been identified, and AD's are the means by which accomplishment of procedures and adherence to specific compliance times are made mandatory to correct that unsafe condition. While the FAA has no objection to Boeing revising the component maintenance or overhaul manuals to provide a procedure for an inspection to identify any units with chrome bores and rework of those units in accordance with the service letter, or for removal of chrome plating, such revision will not affect the requirements of this AD. \n\nRequests to Limit Applicability to Certain PCU's \n\n\tIn lieu of withdrawing the proposal, several commenters suggest limiting the applicability of the proposed AD to certain PCU's: \n\n\tOne commenter states that no PCU's containing chrome plated manifold cylinder bores were produced or reworked after April 1, 1985 (the date of issuance of Boeing Service Letter 737-SL-27-30); therefore, only those PCU's manufactured or reworked prior to that date should be subject to the requirements of the proposal. \n\n\tTwo other commenters state that, as of June 5, 1985, the chrome plating procedure was removed from the PCU overhaul manuals. One of these commenters asserts that any PCU's manufactured after that date would not have been reworked or manufactured with chrome plating in the cylinder bores; therefore, those PCU's should be excluded from the applicability of the proposal.\n\n\tAnother commenter states that the applicability of the proposal should be limited only to those units manufactured prior to 1980. The commenter does not provide justification for selecting 1980 as a cutoff date. This commenter adds that a records search should be allowed to confirm that no chrome plate repairs to the bore have been accomplished.\n\n\tOne commenter states that inspection should be required only for valve bodies made from 2024 aluminum, since the repair procedures were changed in 1987 and there have been no problems since the original issue of the service letter. The only reported problems have been with the old-style valve bodies made from 2024 aluminum. (Production of the 7075 aluminum valve body started in the late 1970's.) \n\n\tBoeing suggests that only those PCU's manufactured prior to June 1984 should be included in the applicability of the proposal. This date is one year prior to the revisions of the overhaul manual, which eliminated chrome plating as a repair procedure. (Since the average overhaul interval is approximately 15,000 flight hours, it is conservativeto assume that no overhauls would occur within one year of manufacture.) Boeing indicates that only serial numbers prior to 1252A (for the rudder PCU) and those prior to 5360A (for the aileron/elevator PCU) would need to be inspected. In addition, Boeing recommends that any aileron/elevator actuators having a part number that includes an "ss" should be eliminated from the applicability of the proposal since those PCU's have a steel sleeve (i.e., those PCU's could not have chrome plating on aluminum). \n\n\tOne commenter states that the proposed rule should include an exemption for PCU's that have been inspected previously and found to have no chrome, or units on which the steel sleeve repair has been incorporated. \n\n\tThe FAA concurs that the applicability of the final rule should be revised. At the time the proposal was issued, the part and serial numbers of PCU's that have been overhauled or repaired were not available; therefore, the FAA was unable to include them in the applicability of the proposal. However, Boeing has provided this information in its comments to the proposed rule. The FAA has revised the applicability of the final rule by specifying the part and serial numbers of affected PCU's. In effect, such revision limits the applicability of this AD to those PCU's manufactured prior to June 1984, and specifically excludes those aileron and elevator PCU's having a part number that contains an "ss" (i.e., those that have been reworked with a steel sleeve). In addition, paragraph (a) of the final rule has been revised to specify the part and serial numbers of affected PCU's. \n\n\tAs for allowing a records search to confirm that no chrome plate repairs to the bore have been accomplished, the FAA finds that no change to the final rule is necessary. The applicability of this final rule specifies that the AD applies only to certain Model 737 series airplanes that are equipped with an aileron, elevator, or rudder PCU having a particular part and serial number. This AD does not preclude an operator from performing a records search to determine if an airplane in its fleet is subject to the requirements of this AD. \n\nRequest to Limit Applicability of Proposal to Rudder PCU's \n\n\tOne commenter requests that the requirements of the proposal be limited to rudder PCU's only, rather than aileron, elevator, and rudder PCU's. The commenter states that the adoption of the proposed rule would overburden existing competent repair facilities and would expose the airlines and the public to a hazardous condition far greater than any condition that exists presently from the identified unsafe condition. This commenter believes the proposal implies that a simple inspection can determine the status of chrome plated bores on the affected units; however, the commenter indicates that, due to the fact that the chrome sleeving process was used in both manufacturing and repair of the units, that implication is incorrect. The commenter states that limiting the inspection to rudder PCU's -- especially those manufactured before chrome sleeving was discontinued -- would reduce the risk of unintended consequences resulting from the overburdening of competent repair facilities. \n\n\tThe FAA does not agree that the applicability of the AD should be revised to include only rudder PCU's. The FAA has determined that sluggish ailerons and elevators pose a potential unsafe condition similar to that of the rudder. Therefore, the FAA finds that it is appropriate to address this potential unsafe condition as it applies to aileron and elevator PCU's, as well as rudder PCU's. \n\nRequests to Extend Compliance Time \n\n\tThe ATA, on behalf of several of its members, requests that the proposed compliance time be extended from 18 months to at least five years. \n\n\tOne ATA member states that it would be physically impossible and unnecessary for operators to accomplish the required actions within the proposed compliance time of 18 months. This commenter indicates that a review of its maintenance records for the past 15 years showed that it has never repaired either an aileron or rudder cylinder due to a worn bore. \n\n\tOne ATA member states that it is important that ample time be provided to accomplish the intent of the proposal because such accomplishment requires the removal of PCU's from airplanes and partial disassembly of PCU's in order to determine the type of rework of the cylinder bore, which requires scheduling, airplane downtime, unit turnaround time, and availability of spare PCU's. Another ATA member states that removal of all PCU's would require industry to process almost 750 PCU's per month for the next 18 months. \n\n\tOne commenter asks that the compliance time be extended to the next shop overhaul in light of the fact that there is no documented unsafe condition or in-service concern. \n\n\tBoeing suggests an extension of the compliance time to five years or 15,000 flight hours so that the majority of PCU's can be inspected as a part of normal maintenance actions. Boeing adds that the possibility of maintenance errors during PCU replacement will result in a net degradation in airplane safety as a result of the inspection schedule established by the proposed rule. \n\n\tThe FAA concurs with the commenters' requests to extend the compliance time. In light of the information presented by the commenters, the FAA finds that extending the compliance time to within five years or 15,000 flight hours after the effective date of the AD, or at the next time the PCU is sent to a repair facility (whichever of these times occurs first), will not adversely affect safety. In addition, this revised compliance time will allow the inspection to be performed at a base during regularly scheduled maintenance where special equipment and trained maintenance personnel will be available if necessary. Paragraph (a) of the final rule has been revised to specify this revised compliance time. \n\nRequest to Revise Cost Impact Information \n\n\tTwo commenters believe that the cost impact information presented in the proposal has been underestimated greatly, and that the proposal places an unreasonable financial burden on operators. \n\n\tOne of these commenters indicates that the inspection of the PCU cylinder bores requires removal of PCU's from the airplane and disassembly of those units. The commenter presents the following cost estimates: \n\n\t 28 work hours for removal and reinstallation of five PCU's per airplane;\n\t 20 work hours (per PCU) for disassembly, inspection, assembly, and testing; and \n\t $1,100 (per PCU) for parts required for reassembly of the units after inspection. Using these figures, the commenter estimates the cost impact on U.S. operators to be approximately $14.3 million, or $13,180 per airplane. \n\n\tThe other commenter states that the proposal does not account for any return-to-service checks after the units have been changed, overhaul costs once the units are in the shop,or costs of frequent repetitive checks and short compliance periods for changing the units. \n\n\tThe FAA infers from these remarks that the commenters request the cost impact information, below, be revised. \n\n\tThe FAA does not concur. The cost impact information, below, describes only the "direct" costs of the specific actions required by this AD. The number of work hours necessary to accomplish the required action (i.e., the inspection), specified as 5 in the cost impact information, was provided to the FAA by the manufacturer based on the best data available to date. This number represents the time necessary to perform only the action (inspection) that is actually required by this AD. The FAA recognizes that, in accomplishing the requirements of any AD, operators may incur "incidental" costs in addition to the "direct" costs. The cost analysis in AD rulemaking actions, however, typically does not include incidental costs, such as the time required to gain access and close up;planning time; or time necessitated by other administrative actions. Because incidental costs may vary significantly from operator to operator, they are almost impossible to calculate. \n\n\tAdditionally, the revised compliance time presented in this AD should coincide with normal overhaul schedules (within 5 years or 15,000 flight hours). The FAA estimates that the PCU inspections (and any "on condition" replacements) will be accomplished during normal overhauls, which will minimize the economic impact on operators and accomplish the safety objectives addressed in this AD. \n\nRequest to Clarify Inspection of Reworked or Overhauled PCU's \n\n\tOne commenter requests that the FAA revise the proposal to clarify that only reworked or overhauled PCU's must be inspected. The commenter suggests the following wording for the Summary section of the preamble to the proposal: " ... This proposal would require an inspection of reworked aileron/elevator PCU's and rudder PCU's to determine ...."The commenter asks that paragraph (a) of the proposal be revised to state clearly that "reworked" or "overhauled" PCU's must be inspected. The commenter considers that foreign operators and airworthiness authorities may misinterpret the intent of this AD as proposed.\n\n\tThe FAA concurs with the commenter's request. The intent of this final rule is that operators inspect only reworked or overhauled PCU's to determine if chrome plating is applied in the cylinder bores. The Summary section of the preamble to the final rule has been revised to clarify this intent. Additionally, paragraph (a) of the final rule has been revised to specify that only reworked or overhauled PCU's must be inspected. \n\nRequest to Address Cause of Chrome Plating Separation \n\n\tOne commenter contends that chrome plating on aluminum is successful (the component will have the required integrity for airworthiness) when the base metal has been subjected to the proper pretreatment (cleaned), proper chrome plating, and properly executed post plating operations such as finish grinding and nondestructive testing. The commenter states that poor maintenance techniques, environmental factors, or overlooked design parameters present a more relevant and detrimental concern to flight safety than a properly applied chrome on aluminum repair. The commenter believes that removal of the chrome repair is treating the effect without fully understanding the cause and addressing the underlying issues. \n\n\tThe FAA infers from these remarks that the commenter requests that the FAA address the cause for chrome plating separation, rather than remove the chrome plating repair as an option. \n\n\tThe FAA agrees that if the repair is done properly, it could provide satisfactory service. In any event, the FAA is aware of instances of failure of the repair. The FAA finds that issuance of this AD is necessary to ensure that all chrome plating repairs are removed from the affected Model 737 fleet so that an acceptable level of safety for these airplanes is attained. However, the FAA would consider a request for approval of an alternative method of compliance, provided that a satisfactory repeatable repair procedure using chrome plating can be developed.\n \nRequest to Revise Preamble of Proposal \n\n\tBoeing requests that the wording of the "Reports Received by FAA" section of the preamble to the proposed rule be revised. The commenter notes that this section of the preamble states that chrome plating chips could block or jam the rudder PCU control valve and thereby cause partial or full rudder deflection. Boeing indicates that this statement is incorrect. Boeing remarks that testing conducted by the NTSB systems group showed that a chrome chip in the control valve could be sheared by a force of six pounds. This force would be provided by the rudder feel and centering unit (in combination with system friction) before any significant rudder deflected had occurred. \n\n\tThe commenter also states that this section of the preamble refers only to the rudder PCU, but also should refer to the aileron and elevator PCU's. \n\n\tThe FAA concurs with the remarks submitted by the commenter. However, the section of the preamble to the proposed rule referenced by the commenter does not reappear in this final rule. Therefore, no change to the final rule is necessary. \n\nRequest to Correct PCU Part Number \n\n\tOne commenter requests that the part number referenced in the proposal for the rudder PCU be corrected to 65-44861. The FAA concurs with this request. The FAA has revised paragraphs (a) and (c) of this final rule to correct this inadvertent typographical error. \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 2,675 Model 737 series airplanes of the affected design in the worldwide fleet. The FAA estimates that 1,091 airplanes of U.S. registry will be affected by this AD, that it will take approximately 5 work hours per airplane to accomplish the required inspection, and that the average labor rate is $60 per work hour. Based on these figures, the cost impact of the AD on U.S. operators is estimated to be $327,300, or $300 per airplane. \n\n\tThe cost impact figure discussed above is 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\n\tShould an operator be required to accomplish the necessary replacement, it would take approximately 18 work hours per airplane to accomplish, at an average labor rate of $60 per work hour. Required parts will cost approximately $15,800 per airplane. Based on these figures, the cost impact of any necessary replacement action is estimated to be $16,880 per airplane. \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: \nPART 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: